Polymers in Cementitious Materials

Polymers in Cementitious Materials

Michelle Miller

Polymers in Cementitious Materials

Michelle Miller

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2005 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2005, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 1-85957-491-2

Typeset, printed and bound by Rapra Technology Limited Cover printed by Livesey Limited, Shropshire, UK

Contents

Contents Preface ............................................................................................................ xi 1

History of Cementitious and Polymer Technology and their Unison ............... 1 1.1

Introduction ........................................................................................... 1

1.2

The Introduction of Polymer into Cementitious Materials...................... 2 1.2.1

1.3

1.4

1.5

The Basic Advantages of Mixing Polymers with Cementitious Materials .............................................................. 3

The History of Cement .......................................................................... 4 1.3.1

Basic Principles of the Cement Manufacture............................... 5

1.3.2

A Summary of the Hydration of Portland Cement ..................... 8

History of Polymers.............................................................................. 11 1.4.1

The Introduction of Polymers into Cementitious Materials ...... 12

1.4.2

Brief Summary of the Physical Properties of Polymer Modified Cementitious Materials ............................................. 15

Concluding Statement .......................................................................... 16

References ..................................................................................................... 17 2

Common Polymers used in the Formation of Concrete and Cementitious Products .................................................................................. 23 2.1

Introduction ......................................................................................... 23

2.2

Summary of the Manufacture and Chemical Composition of a Number of Polymers Commonly used to Modify Concrete and Cementitious Materials.................................................................. 24 2.2.1

Cellulose .................................................................................. 24

2.2.2

Natural Rubber (NR) ............................................................... 25

2.2.3

Polyvinyl Acetates (PVA) .......................................................... 27

2.2.4

Copolymers of Vinyl Acetate/Ethylene ..................................... 28

2.2.5

Acrylic Polymers....................................................................... 30

2.2.6

Styrene-Butadiene Rubber ........................................................ 32 iii

Polymers in Cementitious Materials

2.2.7

Chloroprene ............................................................................. 34

2.2.8

Polyvinylidene Chloride............................................................ 35

2.2.9

Nitrile Butadiene Rubber (NBR) .............................................. 36

2.2.10 Epoxy Resins ............................................................................ 37 2.2.11 Polyurethanes ........................................................................... 47 References ..................................................................................................... 51 3

Polymer Concrete ......................................................................................... 59 3.1

Introduction ........................................................................................ 59

3.2

Chemical Composition of Polymer Concrete ........................................ 60

3.3

iv

3.2.1

Advantages of Thermosetting Resins ........................................ 60

3.2.2

Thermosetting Resins Typically Used in the Manufacture of Polymer Concrete............................................ 61

Brief Introduction to Mixing and Application of Polymer Concrete .... 65 3.3.1

Summary .................................................................................. 65

3.3.2

Mixing .................................................................................... 65

3.3.3

Aggregates ................................................................................ 65

3.3.4

Workability and Shrinkage Characteristics ............................... 66

3.3.5

Voids ........................................................................................ 67

3.3.6

Priming of the Substrate ........................................................... 67

3.3.7

Mixing ..................................................................................... 68

3.4

Application Techniques ........................................................................ 68

3.5

Decision Model .................................................................................... 69

3.6

Repairing Polymer Composites............................................................. 69

3.7

Physical Properties of Polymer Concrete............................................... 70 3.7.1

Typical Strength Characteristics Observed for Polymer Concrete ..................................................................... 70

3.7.2

Bond Adhesion of Polymer Concrete to a Substrate and the Effect of Expansion and Contraction ........................... 71

Contents

3.7.3

3.8

3.9

Permeability of the Polymer-Aggregate Matrix and Performance when Subjected to Varying Atmospheric and Environmental Conditions ............................ 72

Common Uses of Polymer Concrete ..................................................... 72 3.8.1

General Road and Bridge Repair .............................................. 73

3.8.2

Overlays and Coatings ............................................................. 73

3.8.3

Resin Grouts and Mortars ........................................................ 74

3.8.4

Sealants .................................................................................... 75

3.8.5

Castable Systems ...................................................................... 76

Polymer Impregnated Concrete ............................................................ 77 3.9.1

Concrete Preparation and Impregnation Process ...................... 78

3.9.2

Physical Properties.................................................................... 79

3.9.3

Common Uses of Polymer Impregnated Concrete..................... 80

3.9.4

Disadvantages Associated with Polymer Impregnated Concrete ................................................................................... 81

References ..................................................................................................... 81 4

Polymer Portland Cement Concrete .............................................................. 85 4.1

Introduction ......................................................................................... 85

4.2

Brief Summary of the Advantages of Incorporating Polymer into Cementitious Materials ................................................... 86

4.3

Physical Requirements of Latexes in Order to be Suitable as Cement Additives ............................................................................ 86

4.4

Polymer Emulsion ................................................................................ 87 4.4.1

4.5

Manufacture of Latex Emulsion ............................................... 88

Redispersible Polymer Powders ............................................................ 91 4.5.1

Introduction ............................................................................. 91

4.5.2

Conception and Development .................................................. 92

4.5.3

Spray Drying Procedure ........................................................... 92

4.5.4

Particle Size of Redispersible Polymer Powders ....................... 94

4.5.5

Advantages of Adding a Redispersible Polymer Powder ........... 94 v

Polymers in Cementitious Materials

4.5.6

Rehydrating Redispersible Polymer Powders ........................... 95

4.5.7

A Summary of the Various Redispersible Polymer Powders Commercially Available ............................................. 96

4.5.8

Typical Chemical Composition of Modern Day Redispersible Polymer Powders ................................................ 96

4.6

The Formation of the Polymer Modified Cementitious Matrix ............. 97

4.7

Introduction to the Benefits of Modifying Cementitious Materials with Polymer Dispersions .................................................... 99

4.8

4.7.1

Acrylics .................................................................................. 100

4.7.2

Styrene Acrylics ...................................................................... 100

4.7.3

Vinyl Acetate Ethylene Co-Polymers ...................................... 101

4.7.4

Styrene Butadiene Rubber Co-Polymer ................................... 102

4.7.5

Epoxies .................................................................................. 102

A Summary of the Effect of Polymer Modifying Cementitious Materials ...................................................................... 103 4.8.1

Water Demand ....................................................................... 103

4.8.2

Bleed Water ........................................................................... 104

4.8.3

Workability ............................................................................ 104

4.8.4

Stability ................................................................................. 104

4.8.5

Voids ...................................................................................... 105

4.8.6

Drying Characteristics ............................................................ 105

4.8.7

Strength Characteristics .......................................................... 106

4.8.8

Influence of Application Method upon the Strength Characteristics of Polymer Modified Cementitious Materials . 108

4.8.9

Abrasion and Impact Resistance ............................................. 108

4.8.10 Durability and Modulus of Elasticity...................................... 109 4.8.11 Permeability of Polymer Modified Cementitious Materials ..... 109 4.8.12 Carbonation ........................................................................... 110 4.8.13 Chemical and Acid Resistance ................................................ 111 4.9

Common Polymer Modified Construction Materials .......................... 111 4.9.1

vi

Bridge Decking ....................................................................... 111

Contents

4.9.2

Mortars and Renders.............................................................. 112

4.9.3

Flooring ................................................................................. 113

4.9.4

Primers .................................................................................. 113

4.9.5

Ancillary Construction Products............................................. 114

References ................................................................................................... 115 5

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials ................................................................................ 121 5.1

Introduction ....................................................................................... 121

5.2

Natural Fibres .................................................................................... 121

5.3

Synthetic Fibres .................................................................................. 122 5.3.1

Fibre-Reinforced Composites Incorporating Carbon Fibres and Glass Fibres ........................................................... 122

5.3.2

Polymer-Based Fibres.............................................................. 123

5.4

Long-Term Effect of Incorporating Synthetic Fibres into Cementitious Matrices upon their Physical Performance .................... 123

5.5

Introduction of Fibres into Cementitious Materials ............................ 124 5.5.1

The Effect of Introducing Fibres into a Cementitious Medium upon the Physical Properties..................................... 124

5.5.2

Slump Characteristics ............................................................. 125

5.5.3

Structural Shrinkage and Effect of Structural Deterioration on the Cementitious Matrix ............................. 126

5.5.4

Microcracking and Deformation within the Cementitious Matrix .............................................................. 126

5.5.5

Other Factors Influenced by the Inclusion of Fibres ............... 127

5.5.6

The Introduction of a Latex into Fibre Reinforced Concrete ................................................................................. 127

5.5.7

The Influence on Corrosion of Incorporating Fibres into a Cementitious Matrix .............................................................. 128

5.6

Comparison of Steel and Polymer-Based Fibres upon the Physical Properties of Cementitious Materials .................................... 128

5.7

Typical Applications of Fibre Reinforced Materials ............................ 129

vii

Polymers in Cementitious Materials

5.7.1

Construction .......................................................................... 129

5.7.2

Introduction of Fibres into Cementitious Overlays ................. 129

5.7.3

Comparison of Different Types of Reinforcement upon the Long-Term Performance of Cementitious Overlays ........................................................... 129

5.7.4

Introduction of Synthetic Fibres into Cementitious Repair Mortar ........................................................................ 130

5.7.5

Fibre-Reinforced Resin Composites ........................................ 131

References ................................................................................................... 134 6

Adhesives and Coatings .............................................................................. 137 6.1

Introduction ....................................................................................... 137

6.2

Types of Adhesives Available .............................................................. 138

6.3

viii

6.2.1

Solvent-Based Adhesives ......................................................... 138

6.2.2

Solvent-Free and Water-Based Adhesives ................................ 138

6.2.3

Hot Melt Adhesives ................................................................ 139

Brief Summary of Composition, Properties and General Uses of Adhesives ............................................................................... 140 6.3.1

Epoxy Resins .......................................................................... 140

6.3.2

Polyurethane .......................................................................... 141

6.3.3

Acrylic .................................................................................... 141

6.3.4

Urea-Formaldehyde/Phenolic Resins ....................................... 142

6.3.5

Other Polymers used in the Manufacture of Adhesives ........... 142

6.3.6

The Effect of Incorporating Rubber into Adhesives ................ 142

6.4

The Use of Adhesives within the Construction Industry ..................... 143

6.5

Automotive and Aerospace Applications ............................................ 144

6.6

Resin Coatings ................................................................................... 145 6.6.1

Introduction ........................................................................... 145

6.6.2

Solvent-Based Coatings .......................................................... 146

6.6.3

Conversion from Solvent-Based to Water-Based Compounds ....................................................... 147

Contents

6.6.4

Solvent-Free and Water-Based Coatings .................................. 147

6.6.5

Common Applications............................................................ 152

References ................................................................................................... 154 7

Summary of the Applications and Benefits of Utilising Polymers in Construction ........................................................................................... 159 References ................................................................................................... 166

8

Glossary ..................................................................................................... 169

Abbreviations .................................................................................................... 171 Index ................................................................................................................. 173

ix

Polymers in Cementitious Materials

x

Preface

Preface

I initially became interested in polymer chemistry and the benefits of combining polymers and cement during my days of product formulation within the construction industry. An industry which increasingly required products which were cost effective, easy to use, enabling fast track application whilst achieving a high physical performance. Products used to renovate or refurbish concrete structures, providing a durable hard wearing surface and/or decorative finish typically require these key properties. Today, if you look around the vast majority of industrial and commercial buildings, polymer-based materials or polymer-containing cementitious materials will have been utilised in one form or another. Whether it be self-levelling floor compounds onto which a decorative finish has been applied, tile adhesives, resin floors and coatings to provide hygienic, easily cleanable surfaces with a degree of chemical resistance, numerous examples will typically be found. The product's ease of use, reduces the necessity for skilled labour and potential for product failure whilst, fast track application reduces the time period over which a particular area is out of action. Using a combination of additives and polymers within the cementitious mix enables these requirements to be achieved. I quickly became aware of the key advantages of incorporating a polymer as either a powder or liquid into the cementitious mix, particularly in terms of workability, abrasion and impact resistance. So much so, that in many respects the polymer component was classed as a vital ingredients in the majority of cementitious product I formulated. The exact type and quantity added would however depend upon the final product application, performance and physical properties required, (i.e., glass transition temperature (Tg) stability, chemical and hydrolysis resistance). Chapter four provides a summary of the types of polymers utilised directly with cement and how incorporating these polymers influences the final physical properties of the modified cementitious material. A brief description of the types of polymer-modified cementitious materials and their uses has also been provided. Working within the adhesive and tooling industry I also gained a broad understanding of epoxy and polyurethane technology. Chapters three and five focus more of these types of polymers highlighting the physical properties of this polymers and key uses within the

xi

Polymers in Cementitious Materials construction industry. Methyl methacrylates and polyesters are also covered within the text of these chapters but to a lesser extent. This book is intended to provide an insight into the uses of polymers within the construction industry. Principally, this text charts the transition from the conception of polymer modified cementitious materials through to the vast array of polymer-based or polymer modified material utilised in modern day construction. Advances in polymer technology perpetuated the development of a new range of synthetic polymers which subsequently contributed to the formulation of more innovative construction materials. The text not only covers the uses of polymers in direct combination with cement but polymer concrete, impregnation of polymers into the concrete substrate and other polymer based products (i.e., coatings and adhesives). Both natural and synthetic polymers are reviewed. Detailed explanations of the methods of polymer manufacture and analytical analysis have not been included in this text as the objective is to provide an overview of the application of the polymer within construction products. This book is therefore meant to provide an introduction to the union of the polymer and construction industries. Although there is a wealth of literature associated with polymer chemistry, the actual uses of polymer products within the construction industry and particularly in conjunction with cement is more limited. I am indebted to my colleagues within the construction, adhesives, polymer and coatings industries for the technical information, data and knowledge you provided. Without this information from your particularly area of industrial expertise certain parts of this text would certainly of been less detailed. I would particularly like to acknowledge, Reg Dennis for his in-depth knowledge and papers provided. To Nigel Coxon and Elotex AG for your courtesy in providing and allowing me to use the scanning electron microscope picture seen in Figure 4.1 Thank you also for the technical information and literature provided on redisperisble polymer powders. The same debt of gratitude should also go to Damien Baxendale and Lyn Palmer of Dow Chemical Company Limited for the redispersible polymer information provided. Thank you to Frances Powers of Rapra Technology for your advice, guidance and patience, and also to John Holmes, who created the Index. But the greatest thanks must go to my husband, David for his constructive comments, patience and understanding during the writing of this book. Michelle Miller September 2005

xii

History of Cementitious and Polymer Technology and their Unison

1

History of Cementitious and Polymer Technology and their Unison Author

1.1 Introduction In a recent review published by The Concrete Society it was stated that cement and concrete should be regarded as one of the primary construction materials for use in the new millennium [1]. Portland cement is by far one of the most extensively used materials on earth today. In 1996, analysis determined the total worldwide production of Portland cement to be 109 tons [2]. However, in 1999, this value had escalated to 1010 tons. The definition of cement is a ‘material of various constituents used for binding or setting to produce a hardened object’ [3]. The construction of increasingly tall, more intricate and ornate structures such as the Millennium Dome and the Öresund Fixed Link required cementitious materials to be stronger and more flexible. The development of objects as diverse as concrete submarines illustrate the versatility and physical properties of modern cement materials [1, 4]. However, in order to achieve the structural and technological demands of modern construction the inclusion of chemical additives into cementitious materials is required [5]. This includes the addition of fillers to give improved strength, accelerators to decrease reaction time and polymer-based materials to make the final material less brittle. This book seeks to explore the use of polymers as additives to cementitious materials. Since their initial introduction into construction materials, the benefits of utilising polymeric materials to alter and improve the working characteristics of cement has been extensively studied. Polymer materials have been used by man since ancient times; natural polymers such as silk were used for clothing and decoration. The discovery of modern polymers such as rubber and cellulose has had a significant effect on modern life [6, 7]. Synthetic polymers have been one of the most important discoveries of the chemical industry; it is these synthetic polymers which are most commonly utilised as cement additives. Synthetic polymers can be designed to enhance specific properties of a cementitious material, and can be produced on a larger commercial scale.

1

Polymers in Cementitious Materials

1.2 The Introduction of Polymer into Cementitious Materials The introduction of polymers into cementitious materials in order to improve the bond adhesion, flexibility and workability of the resultant composite first occurred in the 1930s where natural rubber was utilised [9-11]. However, the main difficulty with using this naturally occurring polymer material was the inability to significantly alter the chemical structure and thus tailor the physical properties to those required for a particular application. Advances in polymer technology over several decades resulted in the manufacture of synthetic polymers with properties, which improved on those of the natural polymers available, i.e., polyvinyl acetate (PVA), styrene butadiene rubber (SBR), vinyl acrylics (VA), methyl methacrylate (MMA) and epoxy resins, etc., [8, 10, 12]. These synthetic polymers are now commonly used in conjunction with concrete or cementbased materials to produce a variety of polymer-modified cementitious products. The resulting physical and chemical properties of the final product are dependent upon the nature of the polymer material and the actual quantity used in relation to the cement phase [8, 10, 13]. They can be collated into four main categories: polymer-concrete, polymer impregnated concrete, polymer Portland cement concrete and fibre-reinforced concrete, all of which will be covered in some detail in later sections [13-16]. In polymer concrete a monomer or pre-polymer such as MMA, polyester/styrene or epoxy resins were principally used to bind the aggregate together to produce a hardened composite [13, 15-18]. Hence no cementitious phase is actual used to form this material [13, 15, 17, 18]. The second type of polymer modified cementitious material in this series involved the impregnation of the concrete with a polymer possessing a low viscosity. This is normally a monomer, which is subsequently polymerised with the concrete structure [13, 15-18]. One of the most commonly used methods of impregnation involves subjecting the hardened concrete to a vacuum in order to eliminate the presence of air from the voids within the structure [17]. The pressure is then reduced from 0.101 MPa (standard atmospheric pressure) to 0.099 MPa and the polymeric material is induced into the cementitious matrix. A common combination used in this process is MMA at a concentration of  95% by weight along with  5% by weight of trimethlolpropanetrimethyacrylate (TMPTM). Isobutylonitrile was also normally used in conjunction with the above chemicals to initiate polymerisation of the monomers in the cementitious matrix. Polymer dispersions labelled as emulsions or latexes are commonly added to cementitious materials during the mixing process to produce polymer-modified composites [8, 11]. Typically natural rubbers or SBR are mixed with a cementitious compound to produce a fluid underlayment. These types of cementitious underlayment are used to level out any variations in height on a floor and provide a smooth flat surface on to which tiles or other flooring compounds can be applied. Alternatively they are incorporated into the actual cement-based product as a redispersible polymer powder such as polyvinyl acetate ethylene

2

History of Cementitious and Polymer Technology and their Unison (VAE), SBR or vinyl ester of vinyl acetate, all of which enter solution upon contact with water and subsequently repolymerise [12, 19, 20]. Such systems are commonly added to improve the flexural strength, prevent structural cracking and enhance the bond adhesion of the resultant cement-based material to the substrate, i.e., floor screed, repair a mortar’s durability, etc., [8, 10, 12, 16, 18, 20]. As well as polymer emulsions and powders, fibres are also utilised to a great extent in cement-based materials [14, 18, 21]. Polypropylene and polyethylene fibres are commonly incorporated in lightweight repair mortars to prevent slumping and allow the mortar to be applied to significant thickness, i.e., 50-75 mm on vertical and overhang situations [18]. Polymer emulsions are also applied in conjunction with cement-based systems as primers. Commonly an acrylic or styrene butadiene rubber emulsion is applied (normally two to three coats depending upon the porosity of the concrete) onto the prepared surface of a concrete substrate before the application of a cementitious or epoxy floor screed [22-25]. The application of such a polymer emulsion seals the surface of the concrete. If this process is not carried out, air contained within the pores of the concrete will enter the newly applied screed as the substrate gets effectively wet. If air enters the screed the surface tends to contain a vast amount of pinholes, (i.e., holes left after the air bubbles have burst), which subsequently effect the appearance of the screed. Epoxy resin and polyurethane coatings are also commonly applied to concrete surfaces, i.e., floors and walls in order to provide a more chemically resistant or easily cleanable surface which are required in a vast number of industries, i.e., food manufacture [24, 26].

1.2.1 The Basic Advantages of Mixing Polymers with Cementitious Materials The primary reason for introducing polymer materials into concrete and cement-based compounds is to modify the physical characteristics of the resultant cementitious systems [8, 10, 18, 27]. Research showed that mixing polymer emulsions such as SBR and acrylics with cementitious materials reduced the risk of cracking in the hardened structure due to structural movement or as a consequence of expansion/contraction induced by changes in ambient temperature [10]. An improvement in the workability and bond adhesion was seen along with an increase in flexural and tensile strengths. Polymer modified cementitious materials also showed an increase in durability and water resistance hence the addition of the polymer into the cementitious system enabled its use in more complex structures and situations [8, 10, 12, 16, 28-31]. The initial use of polymers in concrete systems was achieved using a liquid additive, which was added to the cementitious material during mixing. However, in certain circumstances it has become more advantageous nowadays to include the polymer in the actual cementitious powder, which polymerises on contact with water during the mixing process [5, 19, 20]. This allows the quantity of polymer throughout the mix to be controlled more precisely,

3

Polymers in Cementitious Materials reducing the errors associated with the mixing of the dispersion directly with the cementbased product. This can have the advantage of lowering the cost of manufacturing and packaging. The problems associated with placing concrete underwater such as loss of the cement component due to it being washed away from the mixture during placement have been addressed by the addition of polymer additives. By supplementing concrete with polymers such as polyacrylamide and cellulose ethers, i.e., methyl hydroxyl, ethyl hydroxyl cellulose, the placement of such a material underwater has been successfully undertaken. Segregation of the concrete constituents along with uneven settling can be prevented by the cohesive effects of such polymers [32]. The objective of this review is to give an insight into the diverse uses of polymers in cementitious products and the properties and benefits that they bestow on the final material. The specific uses of polymers and the materials produced in combination with concrete or cement will be discussed in some detail in later chapters. However, it is useful to understand the history behind the development of both cement-based materials and polymers and how the two became so intertwined. These topics will be discussed initially.

1.3 The History of Cement Throughout the centuries, man has utilised the materials most readily available to build dwellings, whether it be stone, wood, lime-based mortar or wattle and daub [33-35]. As the knowledge of construction materials became more advanced, substances such as lime and volcanic clay were combined in order to provide stronger more durable materials [34-35]. Cementitious materials were being manufactured and utilised albeit in a primitive form as early as the ‘Greek and Roman Empire’. The Greeks realised the advantage of mixing hydrated lime called ‘slake’ with sand and a volcanic material. When combined with water the hydrated lime would react with the volcanic material. Analysis has shown that a similar hydration precipitation process to that witnessed for cement occurred, consequently resulting in a hardened compound. It is believed that the Romans subsequently adopted this technology. This volcanic material is classified as a ‘pozzolana’. The name was derived from the Italian city of Pozzoli where this type of material naturally occurred [34, 35]. It would appear that such advances in mortar technology were not communicated and utilised throughout the Middle Ages [35]. Instead, lime was carbonated producing a much poorer quality material. Improvements in the quality and performance of mortar utilising limestone continued throughout the 15th-17th centuries. However, no significant advances in building

4

History of Cementitious and Polymer Technology and their Unison technology occurred until 1756 when John Smeaton realised that a superior mortar could be obtained by blending limestone with a clay material [34, 36]. In some ways this mixture formed the basis of a primitive cement. The actual formulation of Portland cement occurred in 1824 and is nominally attributed to Joseph Aspdin. Limestone and clay were initially ground down to a fine powder; the desired proportions of each were blended together and subsequently calcined in a lime kiln. This procedure resulted in a basic form of cement which when hydrated with the desired quantity of water, hardened to form a solid structure suitable for use in the construction of buildings. Although a significant improvement in the quality was achieved compared to the previous lime-containing mortars it was still sub-standard in comparison to the modern Portland cement manufactured today. In subsequent years, a number of modifications to the original formulation occurred including the addition of gypsum, which prevented rapid setting of the Portland cement a phenomenon known as ‘flash setting’ [34, 37, 38]. This improvement has enabled the mortar to be worked for a longer period of time to achieve the desired shape or structure. Advances in the techniques adopted for grinding and calcining of the raw materials have also occurred improving the final product quality [34, 35].

1.3.1 Basic Principles of the Cement Manufacture Modern day Portland cement is principally manufactured using a combination of calcium ‘calcareous’ and silica/alumina ‘argillaceous’ containing materials [34, 38]. These raw compounds are finely ground together in the required proportions to achieve the necessary chemical composition. To achieve the final cement product these raw materials have to be heated to high temperatures in order to produce the desired cementitious composition. This heating process occurs in a kiln, which slowly rotates to ensure all material passing through is subjected to the same heating conditions. The raw materials enter the kiln either as a dry powder or alternatively as a slurry (i.e., dry powder mixed with water) [34, 35, 38, 39]. However, the water required to form a slurry has to be removed as part of manufacturing process [35, 38]. Adopting this method therefore increased the amount of energy required during production. Manufacturing cement without the necessity of forming a slurry is therefore the most commonly used method adopted today. Within the kiln, different temperature regions exist and as the mixture passes through each of these zones a series of chemical reactions occur [39, 40]. Initially any water present in the argillaceous and calcareous components is removed. When the temperature reaches 500-505 ºC, dehydroxylation of the silica and alumina containing material occurs which subsequently breaks down into its ionic constituents, i.e., SiO2, Al2O3 and Fe2O3 [39]. At around 850 ºC calcination of the calcareous component proceeds to yield CaO. These ionic constituents then combine in a series of reactions to produce the four main phases along with the other minor components present in cement. These phases are tricalcium silicate

5

Polymers in Cementitious Materials (C3S), dicalcium silicate (C2S), calcium aluminate (C3A) and the calcium aluminoferrite (C4AF) [34, 37]. The formation of dicalcium silicate, tricalcium aluminate and ferrite phases occurs from 850 ºC. Synthesis of the main tricalcium silicate phase is initiated between 1300-1450 ºC and is due to the combination of the dicalcium silicate phase with the available unreacted silica and subsequently results in the molten cementitious material known as the clinker [40] (Figure 1.1). The resulting mixture is subsequently cooled causing the material to harden before being combined with gypsum, normally 2-3% (w/w) and then ground in a ball mill to a fine powder. The inclusion of gypsum prevents the components of cement reacting too quick upon contact with water and hence resulting in poor strength of the final cured cementitious material [35, 37]. The calcium rich ‘calcareous’ component used in the manufacture of Portland cement is available in many forms, i.e., limestone, cement rock, chalk, alkali waste, marine shells and marl [34, 35, 38, 41]. A waste product produced during the manufacturing processes used in the alkali and ammonium cement-based industries as well as a precipitated form of calcium carbonate can also be utilised. The silica rich ‘argillaceous’ component is normally slate, blast furnace slag, fly ash, clay or shale [38]. The two materials previously mentioned generally contain mineral phases, which are structurally related to either kaolinite, a double layered clay material (Al2 (OH)4Si2O5) and pyroillite (Al2(OH)2Si4O10) which possesses a triple layered structure. Normally the composition of the clay materials utilised in Portland cement manufacture consists of 55-60% (w/w) SiO2, 15-25% (w/w) Al2O3 and 5-10% (w/w) Fe2O3 along with minor components such as magnesia (MgO), alkalis and water [38, 41]. The composition of initial calcareous and argillaceous components has to be carefully monitored to prevent levels of certain components, i.e., magnesia (MgO) exceeding 4% (w/w) and phosphorus pentoxide (P2O5) exceeding 0.3% (w/w) [38]. When such compounds surpass these quantities they have detrimental effects on the phase compositions and hence the physical performance of the resultant Portland cement. It is important that the magnesium carbonate (MgCO3) concentration of the calcium rich compound is restricted to less than 5% (British standard BS 12: 1978) [34, 38]. The hydration of the latter results in the formation of MgO, which is an expansive material and results in volume expansion of the cementitious matrix. This subsequently weakens the structure of the final material making it unsound. It is also desirable to limit the quantity of other compounds within the raw materials which have been proven to inhibit the formation of the major phase produced during the manufacture of clinker, i.e., tricalcium silicate [39]. The setting characteristics of the resultant Portland cement have also been affected, i.e., retarded by the presence of such

6

History of Cementitious and Polymer Technology and their Unison

Figure 1.1 Summary of Portland cement formation

compounds. If the levels of phosphorus pentoxide (P2O5) exceed 0.3%, the formation of the dicalcium silicate is favoured over the formation of the primary reactive phase, i.e., tricalcium silicate [34, 38]. The reactions of the individual components formed within the cement clinker upon contact with water have been analysed. This research has established that the rate of reaction of dicalcium silicate is twenty times slower than that of tricalcium silicate and is primarily responsible for continued microstructure development after 28 days of ambient temperature curing [42]. After 28 days of curing at this temperature analysis has shown that practically all the tricalcium silicate has been consumed. Subsequently reducing the quantity of tricalcium silicate formed within the clinker will adversely effect the early strength development of hardened cementitious structure. It is also advantageous to limit the proportion of titanium oxide and calcium sulfate to less than 4% (w/w) [39]. Excessive amounts of these compounds have been observed to produce significantly weaker cementitious materials due to preferential formation of the dicalcium silicate and volume expansion of the mortar respectively. A variety of fuels

7

Polymers in Cementitious Materials have been utilised to produce the energy required to manufacture the cement clinker. The more traditional sources of energy include diesel, coal and natural gas. However, in today’s industrial climate minimising costs incurred during production is a key factor for any manufacturing process. Subsequently alternative sources of fuel have been sought by companies manufacturing cement. These alternatives include waste fuel, and solvent residues for the chemical industry and tyres [34, 38].

1.3.2 A Summary of the Hydration of Portland Cement Cement principally comprises of four compounds, tri and dicalcium silicate, tricalcium aluminate and ferrite. Le Chatelier discovered that when these compounds reacted individually with water certain hydrates formed. These hydrates are similar to those formed as a result of cement reacting with water under the same reaction conditions [35, 36, 43]. Principally, when cement is combined with water, hydration of the cement takes place and a series of dissolution, precipitation and diffusion controlled processes occur. The vast majority of these processes take place within the first 24 hours of curing [34, 36, 37, 42, 44]. Such processes result in the development of compounds of varying chemical composition, i.e., calcium silicate hydrates, calcium aluminates and calcium monoaluminosulfate that make up the cementitious microstructure. The physical properties of the hardened structure are dependent upon the type of phases formed during the curing process [2, 34, 35, 37, 44] (see Figure 1.2). When the cement grains first make contact with water, the dissolution of certain ions (Ca2+, SiO44-, alkalis and SO42-) from the phases present within the clinker occurs [42]. The rate of dissolution will depend upon the solubility of the ionic species present; the aqueous phase subsequently becomes rich in such ionic components. The tricalcium aluminate phase reacts within the first hour of curing, resulting in the development of ettringite [45]. Within the first few hours of curing the tricalcium silicate undergoes initial hydration resulting in the formation of a gel like substance rich in ions such as Al2O3, Ca2+, SiO44and SO42- [37, 42]. The formation of this gelatinous material appears to restrict the flow of ions to and from the surface of the cement grain [37, 42, 44, 45]. A period of relative inactivity follows over the next 2-3 hours which has been termed the ‘induction period’ [37, 45]. Research has indicated that during this time the concentration of certain ionic constituents, i.e., calcium (Ca2+) and hydroxide ions (OH-) increases to a sufficient level to favour the nucleation of calcium hydroxide Ca(OH)2 [37, 42]. This subsequently encourages further dissolution of Ca2+, OH- ions into solution in order to compensate for those being consumed in the development of calcium hydroxide. Analysis has also indicated the formation of calcium silicate hydrates is initiated once more at this point in the curing process.

8

History of Cementitious and Polymer Technology and their Unison A number of theories have been put forward as why the induction period occurs [37]. One theory states that the precipitation of the initial gelatinous hydrate results in a silica rich layer [37, 42, 44]. As calcium and hydroxide ions enter into the surrounding aqueous phase, Ca2+ can adsorb onto this silica rich surface subsequently rendering the surface positively charged [37, 42]. The high Ca2+ concentration within these interfacial regions is believed to hinder further hydration until the point where the concentration of Ca2+ and OH- reaches supersaturation and the nucleation of calcium hydroxide Ca(OH)2 is favoured within the aqueous phase. However, further studies of this reaction have indicated that the development of Ca(OH)2 is not the singular controlling factor but nucleation of calcium silicate hydrates also determines when termination of the induction period occurs. Another explanation attributes the beginning of the induction period to the development of the

Figure 1.2 Summary of the hydration of Portland cement

9

Polymers in Cementitious Materials initial gelatinous membrane. The passage of ionic species is restricted until the osmotic pressure inside the membrane reaches a maximum and the latter bursts [2, 37]. A sudden release of active species into the aqueous phase consequently promotes the development of calcium silicate hydrates once more. Over the next 12 hours of the curing process (at room temperature), rapid nucleation of calcium silicate hydrates (C-S-H) is favoured. However as the time of ambient curing tends towards 24 hours, the development of C-S-H phases is dependent upon the ability of the reactive ionic species to diffuse through the hydrate shell already deposited around the cement grain [37, 42]. The precipitation of the reactive components is no longer the predominant factor which affects the formation of the cementitious microstructure [44, 45]. Hydration of the tricalcium and dicalcium silicate phases which are the most predominant in Portland cement primarily contribute to the strength development of the hardened cementitious structure [34, 37, 42]. Admixtures which can contain a variety of additives are often included in Portland cement [35, 46]. These additives can be used to extend the setting characteristics in order to improve the workability or alternatively accelerate the curing process and hence strength development. Additives are also used to reduce air entrapment within the slurry preventing the inclusion of air bubbles within the mixture or to improve the flow of the product. This reduces the occurrence of air voids (macropores) within the hardened cementitious material, which can subsequently produce points of weakness with the matrix and reduce the overall strength [47]. It is advantageous to extend the workability and the time which the cement-based system remains fluid when working in hot climates > 25 °C by incorporating chemical retarders into the mix [46]. Suitable compounds, which extend the setting characteristics of Portland cement, include citric acid, tartaric acid, trisodium citrate, sodium gluconate and calcium chloride [48]. The chemical retarder chosen will ultimately depend on the application of the resultant cementitious material and the initial set required. Indeed, the use of calcium chloride is not recommended when the modified mortar is going to be in contact with steel reinforcing bars. Leakage of any chloride ions still remaining in the mortar can result in corrosion of any steel present [35]. However, it can be prudent to accelerate the hydration characteristics by adding a compound which promotes phase formation and microstructural development, i.e., calcium silicate hydrates. The addition of a chemical accelerator may be required when operating in cold conditions (5 °C) or when a rapid setting system is required. Sodium, potassium and calcium hydroxide, sodium, potassium or lithium carbonate and lithium salts can be utilised as accelerators [46, 48].

10

History of Cementitious and Polymer Technology and their Unison

1.4 History of Polymers The definition of a polymer is ‘a substance which comprises a series of repeating units denoted as monomers which are connected to each other’ [49]. The actual word originates from the Greek language and is a combination of ‘poly’ the word meaning many and ‘mer’ referring to the term for parts [3, 12, 49]. Polymers are commonly used in a vast array of manufacturing processes and industries to produce an immense variety of materials and finish products, i.e., plastic, polyvinyl chloride (PVC), tyres, artificial fabrics such as polyamide, etc., [33, 50]. The main advances in polymer technology primarily occurred during the 20th century with the formulation and manufacture of synthetic polymers such as polyamide 66, polyester, Rayon, PVA, nitrile butadiene and SBR. The earliest recorded use of a polymeric material is detailed in The Holy Bible where it was used to make dirty water fit to drink [51]. Polymers frequently exist in nature and can be found in substances such as soil, glass, trees, and the cocoon of the silk worm. Natural polymers have been used for centuries in the manufacture of clothing and everyday items required by man such as tools, arrows and pots [33, 52]. Polymers can also be inorganic as well as organic in origin [52]. Indeed the initial synthetic polymers developed by man were inorganic in nature. After structuring steatites (soapstone) to the desired shape, the Egyptians coated the surface with an alkaline silicate glass probably to harden and protect the surface like a glaze. Advances in polymer usage continued throughout the ages. Developments included the use of opaque glass to decorate and cover objects such as pots and vases. Sections of coloured glass have been dated as early as 3000 BC, whilst, the occurrence of transparent glass was not witnessed until approximately 1500 BC. Other naturally occurring polymer materials include silk and cellulose [7, 50, 53]. In fact cellulose is one of the most bountiful organic compounds found on earth, occurring in a number of forms such as cotton, linen as well as trees and vegetables. Even the building blocks of the human race, deoxyribonucleic acid (DNA), the molecules that contain the genetic information, which determines how we form and develop as human beings are polymers [6]. Proteins, which perform essential operations within all living things, are also polymeric in nature and composed of amino acids, which are interconnected to each other. Silk was originally harvested from the cocoon of the silk worm, however, this was a time consuming process and required expert handling in order to prevent damage to the material [54]. These factors made silk a rather expensive commodity. In order to address this problem a synthetic source of this material was required. Count Hilaire de Chardonnet,

11

Polymers in Cementitious Materials achieved this objective and in 1889, he caused a sensation when he exhibited his fabric termed ‘artificial silk’, a substance which was to be later called Rayon. Advances in polymer technology were slow until the 1930s when a number of scientists adopted and utilised the hypothesis of large molecule synthesis conceived by Staudinger [9, 33]. He in turn was rewarded for his revolutionary theories with the Noble Prize for Chemistry. The demand for fibres, plastics and coating escalated during the Second World War and hence rapid development of synthetic polymers occurred in order to meet demand [33]. One primary example is the synthesis of synthetic rubbers, i.e., SBR, as supplies of the natural product could not satisfy requirements [12, 55]. During this period (1939) the synthesis of an epoxy resin which possessed exceptional mechanical properties was patented [56]. Epoxy resins have been used extensively in the formulation of polymer-based mortars, grouts, concretes, adhesives, and coatings due to their high physical performance. During the 1940s, polyurethanes were also introduced into the commercial market place and the manufacturing process involved the reaction of a polyol with a diisocyanate, i.e., diphenylmethane diisocyanate [57, 58]. Throughout the 1940s and beyond, the development of synthetic polymers has advanced with the synthesis of polymers such as PVA, styrene acrylics, polychloroprene rubber, polyacrylic esters, vinyl acetate copolymers [8-10, 12, 21, 59]. Polymers are commonly used today to produce a vast variety of materials and products utilised within the construction industry a number of which are detailed in Table 1.1.

Table 1.1 Polymer composition of a variety of modern day products [10, 12, 16, 22, 24, 33, 50] Nature of polymer

Type of material

Phenolic, urea, melamine

Laminates, fibre board

Epoxy resins, PVC, urethane

Flooring

Vinyl acetate ethylene copolymers, acrylics, styrene butadiene rubber

Polymer modified cementitious flooring

Phenolic, polystyrene, urethane foam

Insulation and foams

1.4.1 The Introduction of Polymers into Cementitious Materials Polymers are incorporated into cementitious materials in a number of ways, i.e., by direct addition as part of the gauging liquid (latex emulsion) or pre-blended into the cementitious material (redispersible polymer powders). Alternatively they are added as fibres [or directly absorbed (impregnation)] of the desired resin (MMA) into the hardened composite [8,

12

History of Cementitious and Polymer Technology and their Unison 10, 12, 13, 16, 18, 24, 33]. As described earlier the use of polymers as cement additives can be classified into four main categories [10, 12-14, 16, 18, 24, 26, 30, 33]: 1. Polymer-Portland Cement Concrete (PPCC), 2. Polymer Concrete (PC), 3. Polymer Impregnated Concrete (PIC), 4. Reinforced Concrete.

1.4.1.1 Polymer-Portland Cement Concrete Modification of a cementitious compound with a polymer in order to modify the physical properties has been practised for over 80 years [8]. Initially throughout the 1920s and 1930s, natural rubber emulsions were used to alter and improve the performance of concrete [9, 10]. Although the addition of this polymer into a cementitious material had a number of advantages its resistance to weathering was not good and being natural in origin the physical properties could not be tailored to produce a particular requirement. During the 1940s synthetic polymers such as polyvinyl acetates became commercially available and found favour as alternatives to using natural rubber [10]. However, the disadvantage of using this polymer in conjugation with cement was its tendency to deteriorate and reemulsify when exposed to moisture. Consequently modified PVA, which are less susceptible to this type of deterioration, were developed. As the development of synthetic polymers progressed styrene acrylics (SA), polyacrylic ester polymers and copolymers (acrylics), SBR, acrylonitrile butadiene copolymers (NBR), vinyl acetate copolymers where the secondary monomer can be ethylene, acrylic ester, vinyl acetate (VAc) and polyvinylidiene dichloride were formulated [9-12, 16, 24, 29]. A variety of these latexes have been used as cement additives throughout the years [11] to form PPCC. The type of emulsion utilised as an additive depends upon the cost, the application and environment to which the resultant material will be exposed, i.e., an external surface exposed to moisture [29]. Some of these latexes are also available as redispersible polymer powders, which disperse to form an emulsion when combined with water once more. The actual quantity of polymer incorporated into the cement-based materials can vary from 1-5% by weight for redispersible polymers to a polymer/cement ratios of 0.1-0.20 for latex dispersions, which have a 25-50% solids content [12]. All of these when combined with concrete or cement-based powders improved the resultant physical properties by increasing the workability, durability along with the flexural and tensile strength of the resultant cement-based mortar [29]. The use of such latexes has

13

Polymers in Cementitious Materials increased towards the latter end of the 20th century to produce a wide variety of materials including latex modified mortars, adhesives, boat decking compounds, and bitumen to name just a few [11, 14, 22, 30].

1.4.1.2 Polymer Concrete and Polymer Impregnated Concrete Methyl methacrylates and polyurethanes, which were initially synthesised in the 1940s, have been used to produce a vast array of construction products [12, 30, 60]. Methyl methacrylate (MMA), a mixture of MMA/trimethylolpropane, along with polyesters combined with styrene have commonly been used to produce polymer concrete and polymer impregnated concrete [13, 17]. Vinyl chloride, acrylonitrile, chlorostyrene have also been used to impregnate concrete. Polymer concrete has been manufactured using epoxy resins and to a lesser extent using furans and polyester amides [7, 13, 30]. Epoxy resins were in fact originally synthesised as a dental material, however, subsequently experimentation indicated they could not be used effectively for this application [12]. Instead, they were found to have numerous physical properties such as bond adhesion, flexibility and high chemical resistance which made them ideal for use in the manufacture of a wide variety of materials including adhesives, plastics, coating and grouts [12, 61]. The use of polymers in combination with cement-based materials gathered popularity throughout the 1950-1960s with polymer concrete and polymer-impregnated concrete being regularly utilised within the construction industry [12, 18, 30]. Originally, polymer concrete was manufactured as a synthetic replacement for marble [30]. The advantages of using the resin additives were seen in the improved physical properties of the final product. Polymer concrete was then used as a bridge overlay and for concrete repairs. The impregnation of polymers into hardened concrete structures increased in magnitude during the 1970s in order to improve the strength characteristics, longevity and stiffness of the matrix. The use of polymer concrete escalated throughout the 1980s, the type of monomer combination utilised in the additive was dependent on the application, cure efficiency and the chemical and physical properties required of the resultant composite [13, 17].

1.4.1.3 Reinforced Concrete Fibres manufactured from steel or synthetic polymers such as polypropylene, polyethylene or polyester are commonly combined with a resin or cement binder [62]. Plastic settlement or segregation of aggregate contained within a cementitious material can be prevented by the addition of a small quantity of fibres [63]. These fibres can be synthetic (polypropylene) or natural in origin such as cellulose. The addition of fibres of this nature can also prevent

14

History of Cementitious and Polymer Technology and their Unison slumping of a cementitious material when applied to a vertical surface. Research has clearly shown that the introduction of fibres whether steel or polymer in origin are beneficial in terms of minimising shrinkage of the final cementitious microstructure [64]. An increase in surface toughness and reduction in the extent of microcracking within the cementitious matrix has also been identified [65, 66].

1.4.2 Brief Summary of the Physical Properties of Polymer Modified Cementitious Materials Cement, when hydrated with water, results in the formation of a number of calcium silicate hydrate phases, calcium aluminate hydrates (ettringite), calcium carbonate, and silica, which form the hardened structure [2, 12, 35-37, 42, 44, 67]. The quantity and nature of the hydrates which are formed govern the resultant physical performance of the composite, i.e., strength (compressive, flexural and tensile), stability and durability [35]. Typically, curing at ambient temperature over a 24-hour period produces a strong solid structure with a high compressive strength (20-30 N/mm2) depending on the nature of cementitious product (concrete, grout, self-smoothing compound) [68]. Polymers possess the ability to change their physical structure when exposed to different environments such as temperature and moisture [8, 69]. The advantage of such materials is their ability to withstand a greater degree of external force before breaking, hence enhanced flexural/tensile strength [69]. Thus combining polymers with cement-based products generally results in a cementitious composite (when cured) which possesses a high degree of compressive strength along with improved degree of flexibility within the matrix [9, 17, 18, 24, 29, 69, 70].

1.4.2.1 Workability Casein is a naturally occurring polymer and up until a few years ago was commonly incorporated in cementitious materials as a ‘flow aid’ to improve the workability and reduce the water requirements [67]. However, in recent years there has been concern about the biodegradability of casein and its ability to promote bacterial growth have limited its use in cementitious materials. These factors are obviously a concern in areas where food will be prepared or a high degree of hygiene is required such as in a hospital [25]. The addition of a latex dispersion into a cement-based mortar generally improves the workability. Research has attributed this improvement to the polymer particles acting as ball bearings [8]. The quantity of water required to achieve the desired consistency is also lower due to this effect [8, 11, 12, 69]. A reduction in the cement to water ratio aids strength development as long as there is sufficient water available to fully hydrate the cement phase present.

15

Polymers in Cementitious Materials

1.4.2.2 Viscosity, Segregation Control Cellulose materials are commonly used as cement additives to increase the viscosity of the cementitious material and prevent any heavy aggregate such as quartz, slag and pea gravel from segregating out from the slurry when mixed with water [46, 53]. In fact the polyacryl amides and cellulose ethers have also been regularly added to wet concrete to prevent the actual cementitious component washing out of the mixture when being placed underwater, i.e., gas and oil platforms and so on [32].

1.4.2.3 Strength Characteristics The flexural and tensile strength of cement-based materials is heightened by the incorporation of latex materials, vinyl acetates, PVA, vinyl ester of versatic acid, acrylics [16, 18, 19, 24, 30, 69]. Increasing the flexibility of the cured cementitious microstructure improves its resistance to stress and reduces its tendency to fracture when exposed to a load. Hence, this enables a thinner section of the polymer modified cementitious material to be applied in order to obtain the required stability and resistance to fracture when subjected to a load [26].

1.4.2.4 Adhesion Improvements in the bond adhesion of polymer modified cementitious materials to the chosen substrate have been identified in comparison to cementitious products containing no polymers [10, 16, 19, 29, 69]. Polymers possessing a high glass transition temperature are ideal as cement additives as they do not have a sticky consistency, which would in turn make the modified material viscous and more difficult to place and finish [59].

1.5 Concluding Statement Today, the incorporation of polymers into cement-based compounds is commonplace and the polymer modified materials are subsequently used for a vast array of applications. In compounds as diverse as self-smoothing floor screeds, fibres for lightweight repair mortars, tile adhesives and concrete primers [8, 10, 12, 25, 46, 69, 71, 72]. Within the following text the multitude of polymer uses within the cementitious materials and the construction industry will be discussed.

16

History of Cementitious and Polymer Technology and their Unison

References 1.

Science and Technology: Concrete Proposals in The Economist, 1999, July 22nd, p.87-88.

2.

L.D. Mitchell, M. Prica and J.D. Birchall, Journal of Materials Science, 1996, 31, 16, 4207.

3.

J. Daintith in A Concise Dictionary of Chemistry, Ed., J. Daintith, 1990, Oxford University Press, Oxford, UK, p.62.

4.

Peramin Adds Strength to Europe's Largest Infrastructural Project, Perstorp Construction Chemicals, Perstorp AB, Perstorp, Sweden.

5.

Polymer Modified Factory-Made Dry-Mix Mortars as Modern Building Materials, Wacker Polymer Systems GmbH & Co. KG, Munich, Germany, 1999.

6.

A.R. Rennie, Physics Education, 1997, 32, 3, 154.

7.

O. Moy, Cellulose Polymers, 1999.

8.

Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 189.

9.

ACI Committee 548, State-Of-The-Art Report on Polymer-Modified Concrete, Report No.548.3R-3, American Concrete Institute, Detroit, MI, USA.

10. V.R. Riley and I. Razl, Composites, 1974, 5, 1, 27. 11. D.G. Walters in Procceedings of ACI 1986 Fall Convention, American Concrete Institute, 1986, Baltimore, MD, USA, p.1-6. 12. J.D.N. Shaw, P.J. Brown, R. Dennis, P.C. Hewlett, R.A. Johnson, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Polymers In Concrete, Technical Polymers No 39, Concrete Society Working Party, The Concrete Society, Camberley, Surrey, UK. 13. E. Kirhikovali, Polymer Engineering and Science, 1981, 21, 8, 507. 14. Polymer Use With Concrete Increases, Adhesives Age, 1988, 31, 9, 42. 15. M. Steinberg, Polymer Plastics Technology and Engineering, 1974, 3, 2, 199. 16. J.A. Manson, Materials Science and Engineering, 1976, 25, 1, 41.

17

Polymers in Cementitious Materials 17. I. Berkovitch, Civil Engineering (London), 1984, August, 45. 18. C. Ellis, High Performance Plastics, 1989, 6, 6, 1. 19. D.G. Walters, Concrete International, 1992, 14, 4, 30. 20. A. Hoffman, Effect of Redispersible Powders on the Properties of Self-levelling Compounds, Handout from Annual Contract Flooring Association, 1999, Coventry, UK. 21. I.M. Ward in Integration of Fundamental Polymer Science and Technology, Eds., L.A. Kleintjens and P. Lemstra, 1986, Elsevier Applied Science Publishers Ltd, Barking, UK, p.634-647. 22. More Than Just Cement, Synthomer GmbH, Frankfurt am Main, Germany. 23. 'Synthomer 29Y' - Specification for Waterproofing Internal Surfaces, e.g., Concrete Water Tanks, Technical Information, 1996, Synthomer Limited, Harlow, Essex, UK. 24. T.P. O'Brien, W.B. Long, P.C. Hewlett, J.D.N. Shaw, L.J. Tabor and P.J. Winchcombe, Repair of Concrete Damaged by Reinforcement Corrosion, Technical Report No.26, 1984, The Concrete Society, London, UK. 25. D.M. Roberts in Industrial Contract Floors, 2000, p.11-12. 26. Civil Engineering (London), 1987, April, 39. 27. D.Z. Zurbriggen, Influence of Redispersible Powders on Shrinkage, Hydration Behaviour and Microstructure of Tile Adhesives, Elotex AG, Sempach Station, Switzerland, 1999. 28. D.G. Walters, ACI Materials Journal, 1990, 87, 4, 371. 29. D.G. Walters in Proceedings of the ACI Convention, 1997, Atlanta, GA, USA, p.1-5. 30. D.W. Fowler in the Proceedings of the 43rd Annual Conference and Focus '88, 1988, Cincinnati, OH, USA, Session 16-B/1 – Session 16-B/5. 31. Your Partner For Redispersible Powders, Elotex AG (Division of National Starch and Chemical Company), Sempach Station, Switzerland, 1998. 32. B. Staynes and B. Corbett, Civil Engineering (London), 1988, March, 28.

18

History of Cementitious and Polymer Technology and their Unison 33. D. Feldman, Polymer News, 1993, 18, 9, 261. 34. G.C. Bye, Portland Cement, Composition, Production and Properties, Institute of Ceramics, Pergamon Press, Oxford, UK, 1983. 35. A.M. Neville, Properties of Concrete, 3rd Edition, Pitman Publishing Inc., Marshfield, MA, USA, 1981. 36. H.F.W. Taylor, The Chemistry Of Cements, Lecture Series Series Number 2, The Royal Institute Of Chemistry, London, UK, 1966, p.1-2. 37. J. Bensted in Advances In Cement Technology, Ed., S.N. Ghosh, Pergamon Press, Oxford, UK, 1983, p.307-347. 38. C.P. Kerton and R.J. Murray in Structure and Performance of Cements, Ed., P. Barnes, Applied Science, London, UK, 1983, p.205-236. 39. T.K. Chatterjee in Advances in Cement Technology, Ed., S.N. Ghosh, Pergamon Press, Oxford, UK, 1983. 40. The Chemistry of Cements, Ed., H.F.W. Taylor, Academic Press, London, UK, 1990, p.60. 41. R.H. Bogue in The Chemistry of Portland Cement, 2nd Edition, Ed., R.H. Bogue, 1955, Reinhold Publishing, New York, NY, USA, p.38. 42. I. Jawed, J. Skalny and J.F. Young in Structure and Performance of Cements, Ed., P. Barnes, 1983, Applied Science, London, UK, p.237-312. 43. P.A. Atkins, Physical Chemistry, 4th Edition, 1990, Oxford University Press, Oxford, UK, p.216-219. 44. H.M. Jennings, B.J. Dagleish and P.L. Pratt, Journal of the American Ceramic Society, 1981, 64, 10, 567. 45. B.J. Dalgleish, P.L. Pratt and E. Toulson, Journal of Materials Science, 1982, 17, 8, 2199. 46. L. Holmberg and J. Engstrand, Peramin Cementitious Screed Handbook, Perstorp Construction Chemicals, Perstorp, Sweden, 1999. 47. S. Mindess, Journal of The American Ceramic Society, 1970, 53, 11, 621.

19

Polymers in Cementitious Materials 48. K.L. Scrivener and A. Capmas in Lea's Chemistry of Cement and Concrete, 4th Edition, Ed., P.C. Hewlett, Arnold, London, UK, Chapter 13, p.748. 49. M.P. Stevens, Polymer Chemistry: An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p.3. 50. R. Hussein and N.P. Cheremisinoff in Elastomer Technology Handbook, Ed., N.P. Cheremisinoff, CRC Press, Boca Raton, FL, USA, 1993, p.875-907. 51. B. Glad, An Introduction To Reactive Polymers, 1994. 52. C.E. Carraher, Jr., Polymer News, 1986, 11, 12, 366. 53. R. Donges, Non-Ionic Cellulose Ethers, Hoechst AG, Wiesbaden, Germany, 1989. 54. A Short History Of Manufactured Fibers, American Fiber Manufacturer's Association. 55. D.P. Tate, Butadiene Polymer, in Encyclopaedia Of Polymer Science And Engineering, Eds., A. Klingberg, J. Muldoon and A. Salvatore, John Wiley & Sons, New York, NY, USA, 1985, p.537. 56. Pitture e Vernici, 1991, LXVII, 3, 47. 57. M.S. Bhatnagar, Popular Plastics and Packaging, 1992, 37, 7, 43. 58. B.K. Howe in Proceedings of Rubberplas 84, Singapore, Malaysia, 1984, Volume 1, Paper No.2. 59. L.A. Kuhlmann, J.W. Young, Jr., and D. Moldovan, inventors; The Dow Chemical Company, assignee; US 5576378, 1996. 60. W.A. Whitaker III, Medical Plastics and Biomaterials Magazine, 1996, January, 12. 61. A. Blaga, Thermosetting Plastics, Canadian Building Digest No.159, 1974, Division of Building Research, National Research Council Canada, Ottawa, Canada. 62. P.N. Balaguru, ACI Materials Journal, 1994, 91, 3, 280. 63. Materials Engineering (Cleveland), 1991, 108, 1, 38.

20

History of Cementitious and Polymer Technology and their Unison 64. S.W. Shin, J.G. Oh and S.K. Ghosh, Shear Behaviour of Laboratory-Sized High Strength Concrete Beams Reinforced with Bars and Steel Fibers in Fiber Reinforced Concrete: Developments and Innovations, an American Concrete Institute (ACI) Convention, Boston MA, USA, 1991. 65. M.A. Taylor in Proceeding of the ASCE Structures Congress 89, Ed., J.F. Orofino, 1989, San Francisco, CA, USA, p.12-20. 66. F. Benaiche and B. Barr in Fibre Reinforced Cements and Concretes: Recent Developments and Innovations, Ed., R.N. Swarmy and B. Barr, Elsevier Applied Science, London, UK, 1989, p.411-419. 67. T.A. Bier and L. Amathieu, Calcium Aluminate Cement (CAC) In Building Chemistry Formulations, Lafarge Aluminates, Paris, France, 1997. 68. M. Roberts in Proceedings of Calcium Aluminate Cements 2001, Edinburgh, UK, Eds., R.J. Mangabhai and F.P. Glasser, 2001, Cambridge University Press, Cambridge, UK, p.605-614. 69. R.N. Swarmy, Cement and Concrete Composites, 1995, 17, 3, 175. 70. R.N. Swarmy, Cement and Concrete Composites, 1992, 14, 3, 155. 71. R. Harbron in Proceedings of Calcium Aluminate Cements 2001, Edinburgh, UK, Eds., R.J. Mangabhai and F.P. Glasser, 2001, Cambridge University Press, Cambridge, UK, p.597-604. 72. A. Blaga and J.J. Beaudoin, Polymer Modified Concrete, Canadian Building Digest No.241, Division of Building Research, National Research Council Canada, Ottawa, Canada, 1985.

21

Polymers in Cementitious Materials

22

Common Polymers Used in the Formation of Concrete and Cementitious Products

2

Common Polymers used in the Formation of Concrete and Cementitious Products Author

2.1 Introduction Lefeburne (1924) was one of the first people to patent a process which utilised a natural polymer to modify the physical properties of a cement-based mortar [1-3]. From this point, the use of polymers in combination with cementitious products has grown significantly [4, 5]. This has been due to developments in polymer technology and subsequent synthesis of more advanced latexes throughout the 20th century. Natural rubber dispersions were one of the first polymers to be used for this purpose between the 1920s and 1950s [3, 6]. Natural rubber was primarily incorporated into concrete and mortars because of the benefits it bestowed on the modified material such as improved durability and bond adhesion along with its economical price. However, the resultant polymer modified cementitious material was difficult to compress into a compacted form, which restricted its use in certain applications [3, 6, 7]. Another drawback of using natural polymeric materials in cementitious systems is the inability to adapt the physical properties of the polymer to favour a particular one [6]. It was during the 1930s that the first major developments in the manufacture of synthetic polymers were witnessed [3]. The first patent detailing the use of a synthetic latex in conjunction with cementitious systems was released in 1932 [3]. This was closely followed by the use of polyvinyl acetates (PVA) in 1933. The formulation and manufacture of synthetic polymers possessing physical properties such as a heightened degree of flexural/tensile strength, bond adhesion, durability and increased chemical resistance occurred. A reduction in the ability of water to penetrate the polymer modified cementitious structure was obtained by incorporating the polymer within the pores or the cementitious microstructure [8, 9]. Indeed improvements in the physical properties of cementitious materials such as flexural strength, tensile strength and bond adhesion were noted with the inclusion of polymer emulsions. Polymer modification of cementitious materials thus expanded the potential uses of these materials. Throughout the 1930s and 1940s, the formulation of synthetic polymers continued with the manufacture of polyacrylic esters and polychloroprene rubber (Neoprene) [3]. Whilst the synthesis and use of PVA dispersions

23

Polymers in Cementitious Materials as a cement additive escalated during the 1960-1970s where they were primarily used as an alternative to natural rubber [10]. A brief description of the natural and synthetic polymers (water-based, solvent-free and solvent systems) commonly used follows. This includes polymer materials used in the past as well in the present.

2.2 Summary of the Manufacture and Chemical Composition of a Number of Polymers Commonly used to Modify Concrete and Cementitious Materials

2.2.1 Cellulose Cellulose is one of the most abundant organic compounds found on this planet and is one of the main constituents of wood, cotton, numerous naturally occurring fibres and the leaves and barks of trees. Throughout the ages, fibres of cellulose have been processed to produce threads which were subsequently used (i.e., woven), to produce clothing [11, 12]. Cellulose comprises of a vast number of basic repeating units (i.e., glucose) attached to each other via hydrogen bonds. There are two possible isomeric structures glucose can adopt such as chair or boat configuration [11]. The glucose constituent of cellulose possesses a chair configuration (see Figure 2.1) as this structure experiences the least degree of bond strain relative to the other configuration which can be obtained (boat configuration) [12-15]. The glucose monomers are inter-connected to one another and a vast number of individual units, (i.e., thousands), may be present in one polymer molecule [11, 13]. Cellulose is observed to decompose before becoming molten [12]. This is inherently due to the hydrogen bonding between the glucose units. A variety of derivatives of cellulose which contain ester or ether constituents are commonly synthesised to produce a wide variety of materials including fibres. The most technologically important cellulose-based products

Figure 2.1 Chair configuration of glucose [12]

24

Common Polymers Used in the Formation of Concrete and Cementitious Products are cellulose nitrate and cellulose acetate. Cellulose nitrate was one of the first derivatives to be formulated and has been used in a variety of applications such as the manufacture of celluloid films and explosives. The use of this derivative has diminished nowadays due to the chemical instability and flammability and it has generally been replaced by cellulose acetate. As well as producing celluloid film, cellulose acetate is also is produced as fibres suitable for use in clothing and artificial silk due to the silky appearance of cellulose nitrate [11]. This naturally occurring polymer is commonly added to cementitious materials such as grouts and self-smoothing floor screeds to increase the viscosity of the slurry when mixed with water and to prevent segregation of any aggregate present [11, 16-20]. This enables a fluid system with a uniform consistency to be obtained.

2.2.2 Natural Rubber (NR)

2.2.2.1 Extraction and Processing This latex is extracted from the rubber tree (Hevea brasiliensis) and other trees, which naturally grow in south-east Asia. This is done by a process called ‘tapping’ [1, 2, 12]. This basically involves driving an implement into the truck of the tree to provide a channel for the rubber solution contained within to travel into the appropriate collection vessel. Depending upon the end requirement, the resultant emulsion is concentrated in order to achieve the desired solids content or dried to form a rubbery solid. To obtain the desired solids content, natural rubber normally undergoes a centrifugal process, which produces a product with nominally 60% solids. Alkali compounds such as ammonia are normally added, the latter is introduced to counteract the acid nature of the latex over a period of time [6, 10]. Subsequently the smell of ammonia is noted when these types of natural rubber latexes are mixed with cement-based materials [10].

2.2.2.2 Chemical Composition The chemical analysis of natural rubber revealed it to comprise of a dispersion of polyisoprene-polymerisation of the monomer that occurs within the tree [2]. Polyisoprene can exist in a number of structural forms. The vast majority of the polyisoprene in natural rubber adopts a cis 1,4 structure when harvested from Hevea brasiliensis (Figure 2.2). In the case of the other type of rubber which is extracted from the gutta-percha a trans 1,4 structure is favoured (Figure 2.3) [12]. The gutta-percha type of rubber is used commercially to a lesser extent, as it is more brittle as a consequence of the 1,4 trans structure it adopts being more rigid and a lower solubility than that extracted from Hevea brasiliensis [12, 21].

25

Polymers in Cementitious Materials

Figure 2.2 Cis 1,4 isoprene [22]

Figure 2.3 Trans 1,4 isoprene [22]

2.2.2.3 Physical Properties One of the unique properties of natural rubber is its ability to extend to several times its original length without destroying the actual molecular matrix [6]. The surrounding temperature influences the state of natural rubber, hence its flexibility increases as temperature increases. Natural rubbers can have their properties altered by a chemical process. They can be hardened by vulcanisation which involves combining the rubber with sulfur [21]. This process enhances the resistance of the resultant polymer to wear. Natural rubber possesses a low resistance to acids and chemicals and deterioration of the structure is observed when exposed to fats, oxygen and UV radiation [6]. Although antioxidants are added to natural rubber to prevent oxidation if this process proceeds the latex becomes soft with a sticky consistency [6, 10].

2.2.2.4 Common Uses of Natural Rubber Natural rubbers are used to manufacture a vast array of commercial goods including automotive tyres, gloves and footwear [12]. Natural rubber latexes are commonly used to form latex modified cementitious compounds [1, 2, 6, 8]. When a new cementitious finish

26

Common Polymers Used in the Formation of Concrete and Cementitious Products is applied on top of old concrete, the concrete is normally shot-blasted or scabbled in order to remove any dirt or latency. The resultant concrete has a textured finish, which aids the bonding of the new cementitious finish. Incorporating natural rubber into cementitious materials improves the flexibility and bond adhesion. This prevents debonding of the material from the substrate to which it is applied. Certain sub-floor compounds containing natural rubber can be applied directly over a number of substrate surfaces such as old tile adhesive, ceramic and terrazzo tiles without the necessity to prepare the substrate beforehand. It is not advisable to use natural rubber where a high degree of compressive strength is required, as a reduction in compressive strength is noted [6].

2.2.3 Polyvinyl Acetates (PVA)

2.2.3.1 Brief Description of the Manufacturing Process The active monomer (vinyl acetate) is initially synthesised by the reaction of the ethylene and acetic acid in the presence of a catalyst [23] (Figure 2.4). Klatte initially synthesised vinyl acetate in 1912, which was subsequently combined with other monomer components to produce a variety of co-polymers in 1917 [24]. The first large-scale commercial production of vinyl acetate was undertaken in 1937. PVA is produced by emulsion polymerisation of the vinyl acetate.

Figure 2.4 Structure of vinylacetate [25]

2.2.3.2 Physical Properties of PVA PVA is a thermoplastic and is used in a number of manufacturing procedures to produce a variety of materials including adhesives, bonding agents, as well as being incorporated into concrete and other cement-based products [6, 10]. PVA was initially used as a substitute for natural rubber when problems with the integrity of this polymer in a moisture rich environment were seen [6, 10, 26]. A reduction in the bond adhesion of the modified mortar has also been observed in the presence of water. Research has indicated that the PVA hydrolyses when exposed to moisture and subsequently converts to acetic acid and polyvinyl alcohol when exposed to these conditions [6].

27

Polymers in Cementitious Materials PVA was commonly used during the 1960-1970s although consumption has decreased as more technically advanced and more chemically and moisture stable synthetic polymers have emerged [10]. However, PVA is still recommended when the surrounding environment will remain constantly dry (i.e., interior use only), due to its economic price in relation to other commercially available polymers such as acrylics [7, 10].

2.2.4 Copolymers of Vinyl Acetate/Ethylene

2.2.4.1 Synthesis of Vinyl Acetate Ethylene Co-polymers Copolymers of vinyl acetate ethylene (VAE) are produced by the reaction of an acid with an alcohol [6] (Figure 2.4). Typically a polyvinyl alcohol is dispersed within an aqueous solution heated to the desired temperature [27]. The vinyl acetate monomer is then introduced into this hot solution along with the initiator. Ethylene is then incorporated into the mixture and the temperature/pressure required to promote polymerisation applied. High pressures are normally required during this polymerisation process which increases the stability of the resultant polymer [6, 10]. The presence of the ethylene group in the polymer reduces the tendency of the acetic group to undergo hydrolysis [7]. Hence this polymer is more stable when exposed to moisture and therefore is favoured if the modified cement-based material is an exterior product. The ethylene monomer is hydrophobic in nature and hence has a natural dislike of moisture [6, 10, 27]. The reaction conditions are tailored to produce a high molecular weight polymer, which subsequently has a performance that equates to PVA in a moisture free environment [10]. From the 1980s this type of polymer has been commonly used as a bonding agent within internal renders and plasters, whilst one of the more recent applications has been its incorporation into self-smoothing cementitious floor screeds [6, 10, 28]. The use of this flooring compound to resurface substrates such as concrete has increased in recent times as they are capable of withstanding foot traffic 24 hours after the application and typically forklift trucks after three days. The key advantage of such cementitious-based flooring systems is they do not emit volatile organic compounds (VOC) during their curing cycle and minimise the time the area being renovated is out of action due to their rapid cure rate.

2.2.4.2 Redispersible Polymer Powders VAE copolymers are commonly spray dried to produce redispersible polymer powders [27, 29-31]. Such systems have increased in popularity as one component blended polymer

28

Common Polymers Used in the Formation of Concrete and Cementitious Products modified cementitious systems can be more efficiently produced. A reduction in errors which could potentially occur by having to mix a separate polymer component into a cementitious mixture is removed. A reduction in manufacturing and packaging costs is also noted, as the necessity for a secondary liquid polymer component is removed which makes the inclusion of such polymer powders advantageous [6, 27, 28, 32]. As described later (in Section 4.5.3) the spray drying process is tailored to maintain the composition and particle size of the original latex products when redispersed in water [28]. The factor that particularly influences the performance of the polymer within the cementitious slurry is the addition of water. Commonly VAE emulsions are processed to produce redispersible powders although styrene acrylic and other co-polymers are also used [33]. A colloid, commonly, polyvinyl alcohol is introduced during the manufacturing process to prevent agglomeration of the particles and hence maintain a free flowing powder [28]. An anti-caking agent such as a clay is normally incorporated (5-13% by weight) to prevent the polymer clumping together when subjected to a load or comers into contact with moisture [1, 6, 10, 27, 28]. Other substances which may be present in the redispersible polymer powders include antifoamers, antioxidants, biocides, flow aids such as superplasticers and thixotropes. The final composition of the redispersible polymer powder will depend on the final applications it is intended for such as combination with cementitious materials [2, 27].

2.2.4.3 Effect of Incorporating Vinyl Acetate Ethylene Polymers into a Cementitious Material The effect of incorporating a VAE polymer in the form of either an emulsion or redispersible powder into a standard cement mortar was discovered by Walters [27]. A polymer to cement ratio of 0.10-0.15:1 was used whilst the sand to Portland cement ratio was 3:1 by weight. In this study a slight reduction in the degree of bond adhesion of the modified mortar was noted when the polymer was in a powdered form. In addition a decrease in the permeability of the structure to moisture was also witnessed. Although no absolute answers were given as to why this had occurred, uneven distribution of the polymer dispersion throughout the mortar may have caused the changes in performance witnessed. In turn the anti-caking agents and other chemicals used in the spray drying process may have influenced the physical performance of the final polymer modified cementitious material. However, this study does highlight the advantages of adding a redispersible powder rather than a latex into a cementitious material. Since the polymer is incorporated as a powder into the actual cementitious system the need to combine the liquid polymer (latex) with the cement-based compound is removed. The quantity of polymer/cement can therefore

29

Polymers in Cementitious Materials be more carefully controlled. Introducing too much water and polymer emulsion has been observed to retard the setting and results in poorer strength [34].

2.2.5 Acrylic Polymers

2.2.5.1 Introduction Methyl methacrylate (MMA) along with other acrylate monomers are the key ingredients in the manufacture of acrylic polymers, the first of which was synthesised in excess of 60 years ago [35]. MMA is one of the most commonly utilised monomers and is produced from methylacrylamide sulfate. This sulfate is produced by heating a mixture of acetone, cyanohydrin with sulfuric acid [36] (Figure 2.5). Acrylic emulsions are primarily produced on a commercial scale by emulsion polymerisation, involving the polymerisation of the acrylic ester monomer by a suitable initiator. The addition of this initiator under the ideal reaction conditions generates free radicals which partake in the polymerisation process resulting in the formation of polymer molecules. This process occurs in the presence of a surfactant in water, resulting in the desired emulsion. Acrylic polymers are typically manufactured by the reaction of an acrylic ester monomer (i.e., methyl acrylates, ethyl acrylates), with itself or another monomer species such as styrene, butadiene, acrylonitrile by free radical polymerisation. Free radical polymerisation of MMA results in the formation of polymethyl methacrylate [37]. This is a plastic, which has no natural colour and exhibits a good resistance to weathering and impact when compared other polymers such as polystyrene [36].

Figure 2.5 The structure of methylmethacrylate [25]

2.2.5.2 Physical Performance of Acrylic Polymers Acrylic polymers are thermoplastic in nature hence their flexibility increases when exposed to heat [10, 26, 35, 38]. They do not show any deterioration when exposed to UV radiation. The stability of this polymer to oxidation and discolouring makes its use in external coatings favourable [6, 7, 35]. Acrylics are also less susceptible to hydrolysis via acidic or alkaline means in relation to other commercially available polymers such as vinyl acetate based copolymers. Resistance to weak acids and solvents is generally

30

Common Polymers Used in the Formation of Concrete and Cementitious Products good, however, the structure of certain acrylics deteriorates when exposed to chemicals such as ethers, ketones, alcohols, aromatic and chlorinated hydrocarbons and strong acids. Research has shown that the degree of resistance to chemical attack by alcohols is directly associated to the molecular weight of the actual polymer [10]. The higher the molecular weight the greater the stability of the final acrylic polymer. The inclusion of styrene groups into the acrylic polymer chain enhances the degree of chemical stability along with an improvement in strength whilst maintaining a good degree of abrasion resistance [10, 35]. The overall cost of the polymer is reduced by including this type of co-monomer [7, 10].

2.2.5.3 Typical Uses of Acrylic Polymers Acrylic polymers are commonly used to modify cement-based systems which will be exposed to a variable environment along with the possibility of a high degree of moisture [6, 7, 39]. Unlike PVA homo-polymers, acrylics do not undergo further hydrolysis upon contact with water [10]. These polymers are utilised in flooring compounds and mortars where the highest level of physical performance of the modified compound is required [20]. Acrylic monomers have also been combined with aggregates to produce a resin-bound concrete suitable for use as repair mortar or as an overlay for bridge decks [8-10, 39, 40]. Acrylic polymers are used in paints suitable for both external and internal application due the degree of toughness and stability to UV radiation. These types of polymers are also used to manufacture medical equipment such as incubators, cuvettes and syringes to name but a few. One of the key advantages of using acrylic plastics is that they do not tend to react with a number of chemicals which could be encountered within the medical industry. Contact with isopropyl alcohol should however be avoided as this tends to induce ‘crazing’ on the surface. When exposed to gamma radiation, acrylics exhibit as tendency to discolour, i.e., a yellowish tinge is seen. In most cases this effect disappears and the original appearance is obtained once more when exposure ceases. This type of polymer is not recommended, however, when a material has to be capable of withstanding an exceptional degree of impact pressure.

2.2.6 Styrene-Butadiene Rubber

2.2.6.1 Introduction The polymerisation of styrene and butadiene to produce a synthetic rubber was undertaken in the early part of the 20th century [41]. This latex was mass produced during the

31

Polymers in Cementitious Materials Second World War throughout Europe and the USA as a synthetic alternative to natural rubber due to the shortage of this material during heightened periods of demand [42] (Figure 2.6). The manufacturing process principally involves the co-polymerisation of the styrene monomer in the presence of butadiene along with the appropriate surfactants, initiators and other ingredients [41]. The vast majority of styrene butadiene rubber (SBR) dispersions used as cement additives contain approximately 60% styrene [6]. In order to improve the stability and enhance the bonding capability, SBR emulsions are commonly carboxylated when used in conjunction with cementitious materials.

Figure 2.6 Styrene monomer [42]

2.2.6.2 Raw Materials The styrene monomer has been synthesised by the reaction of ethylene and benzene in the presence of a catalyst (Freidel-Crafts) [42, 43] (Figure 2.6). Polymerisation of radicals can occur by the introduction of an appropriate chemical initiator or by the application of heat (involving a Diels-Alder adduct) [42]. The butadiene utilised in the formation of this type of latex is commonly produced by the dissociation or ‘cracking’ of naphtha and other hydrocarbon fractions by the petrochemical industry using steam [44]. This process results in the formation of ethylene with butadiene as a secondary product [45]. Commercially, the most useful isomer is 1,3 butadiene, which is a gas at room temperature but becomes a liquid when stored under the correct temperature and pressure conditions (Figure 2.7) [46].

Figure 2.7 1,3 Butadiene [25]

2.2.6.3 Physical Characteristics and Uses of Styrene-Butadiene Rubbers SBR is one of the most significantly utilised styrene co-polymers within general industry. The rubber has a good stability to hydrolysis in the presence of water, once film formation has been completed, along with a high degree of abrasion resistance. This makes the

32

Common Polymers Used in the Formation of Concrete and Cementitious Products polymer ideally suited for use with cementitious materials such as flooring, which will be subjected to a significant amount of abrasion, wear and tear [6, 7, 10]. A notable improvement in the modified cementitious matrix to the penetration of water has been recorded [2]. The SBR incorporated into the cementitious material is capable of sealing the pores, that normally develop as the microstructure forms during the curing process and hence prevents water entering. Research has indicated that SBR do not possess a regular structure, this in turn means that this polymer does not have a tendency to crystallise [47]. Oxidation of this polymer actually hardens the structure, which subsequently becomes more brittle [6]. This can be controlled by incorporating antioxidants into the polymer to prevent this oxidation process occurring. One of the main disadvantages of such polymers is their tendency to be affected by UV radiation and develop a yellowish discoloration over a period of time [7]. Hence, their use is not advisable in materials which are used as decorative finishes. The final polymer becomes harder as the proportion of styrene present in the polymer is increased [2, 10, 43], whilst conversely, increasing the quantity of butadiene in the polymer softens the resultant matrix. Both of these initial constituents are hydrophobic, thus possess a resistance for water and the resultant polymer is not affected by variable environmental (i.e., external), exposure to moisture. SBR are elastomeric and thermoplastic in nature and are used in a number of applications including the manufacture of tyres, rubber adhesives and additives for modifying a variety of cementitious materials [6, 26, 47]. Indeed, SBR are commonly used in conjunction with concrete to enhance the abrasion resistance, flexural strength and workability. The resultant modified cementitious material is then used in applications such as the resurfacing of roads making the surface more hard-wearing and less likely to crack [6, 8, 9]. As with most latexes a notable improvement in the resistance of the modified cementitious matrix to the penetration of water has also been observed [2]. They are also commonly incorporated into or used in association with cementitious floor underlay as a substrate primer [48, 49]. The physical properties of bitumen are also modified with SBR resulting in a reduction in the tacky consistency of the modified bitumen. In addition the modified bitumen has an improved resistance to solvent exposure and toughness to name but a few [50].

2.2.7 Chloroprene

2.2.7.1 Introduction Polychloroprene is also commonly commercially known as Neoprene [6, 43] (Figure 2.8). The formulation of this product occurred in 1930 and polychloroprene was the first

33

Polymers in Cementitious Materials

Figure 2.8 Structure of chloroprene [23, 51]

commercial elastomeric polymer to be manufactured and marketed successfully [51]. Although the polymer can exist in several structural isomers the more common is the trans 1,4 structure [22].

2.2.7.2 Synthesis of Chloroprene This polymer is manufactured by the polymerisation of chloroprene (2-chloro 1,3butadiene) (CR) in the presence of a catalyst (Ziegler-Natta) [21, 23, 51]. The chloroprene monomer was originally manufactured from vinylacetylene, however, more recently it has been produced by the chlorination of isoprene or butadiene [47, 51]. Antioxidants and preservatives are added to the resultant emulsion to prevent oxidation and consequently the loss of physical properties [2, 6, 10]. Research has shown that if such additives are not present, hydrolysis occurs due to contact of the CR with moisture, and hydrochloric acid forms [6].

2.2.7.3 Physical Properties This polymer has a particularly good degree of bond adhesion and possesses a good resistance to oils [6]. Polychloroprene is superior to other rubbers such as SBR in this respect [47]. Analysis has determined that it is the inclusion of the chlorine ion in the hydrocarbon backbone that makes the polymer resistant to any adverse effect when immersed in hydrocarbon oils [43]. This structural design also improves the stability towards environmental weathering, ozone and imparts a degree of flame resistance [21, 47]. In turn such characteristics make the product expensive. The addition of metal oxides to this polymer also improves the heat resistance and helps prevent deterioration of the polymer structure upon storage [51]. Vulcanisation of this polymer occurs when metal oxides are added which increase the hardness of the polymer and thermal integrity. This process can also be achieved in Neoprene by exposing this polymer to heat.

34

Common Polymers Used in the Formation of Concrete and Cementitious Products

2.2.7.4 Common Uses within the Construction Industry Polychloroprene has been used as a primer for sealing concrete substrates before a selfsmoothing cementitious floor screed is applied. They are also incorporated into bitumen emulsions to improve the elasticity and workability - when this bitumen is combined with concrete, a material suitable for resurfacing car parks is produced [6]. Incorporating polychloroprene improves the resistance of this system to general environmental weathering. The addition of cationic surfactants during the emulsion polymerisation process causes the resultant latex to adopt this charge, it is this grade which is used in combination with cements [2, 6, 10]. However, modification of the cementitious material with polychloroprene is not recommended when the resulting product is likely to be in contact with steel reinforcing bars [10]. If any chloride ions remain in the material as a result of unreacted species from the polymerisation process they will corrode any steel present when they come into contact with it.

2.2.8 Polyvinylidene Chloride

2.2.8.1 Composition, Physical Properties and Common uses within the Construction Industry Polyvinylidene chloride (PVDC) is typically produced by free radical polymerisation of vinylidene chloride [52] (Figure 2.9). The monomer vinylidene chloride is synthesised from 1,1,2-trichloroethane. Commercially, this polymer is manufactured by a suspension polymerisation process where a dispersion of the polymer in a solid form is produced [53]. Vinylidene chloride in its purest form is a transparent liquid [52].

Figure 2.9 Repeating structure of polyvinylidene chloride

35

Polymers in Cementitious Materials This polymer exhibits a superior resistance to the penetration of gases (such as carbon dioxide), water and oils, as well as good strength characteristics. This is attributed to the steric hindrance associated with the presence of the chloride atom on every second carbon atom resulting in restricted structural mobility [43]. One of the primary advantages of using PVDC in conjunction with cementitious materials is the significant improvement in the physical and mechanical properties of the resultant products [6, 8]. Research has indicated that a general improvement in the strength characteristics, in particular the compressive strength along with durability, was achieved when the cementitious matrix was modified with polyvinylidene copolymer compared to when a styrene butadiene latex was used [6, 26]. The same mortar formulation and polymer to cement ratio was utilised in both these experiments. A greater resistance to wear (e.g., durability), either from mechanical means such as fork lift trucks or environmental factors has also been noted relative to that achieved when PVA are used. Once dry, the polymer film does not re-emulsify and this prevents moisture movement, hence polyvinylidene chloride has been used as a priming agent before the application of cementitious underlayments and self-smoothing floor screeds [6]. However, its use is not recommended as a cement additive in materials, which are going to have direct contact with steel structures due to the possibility of corrosion as described for polychloroprene.

2.2.9 Nitrile Butadiene Rubber (NBR)

2.2.9.1 Manufacture of Nitrile Rubbers Acrylonitrile butadiene copolymers commonly known as nitrile rubbers are manufactured in a similar manner to other synthetic rubber compounds (SBR) via free radical emulsion polymerisation of butadiene and acrylonitrile [47, 54]. Principally, these monomers are dispersed within water along with the required free radical initiator, which initiates polymerisation of the acrylonitrile and butadiene monomers. A surfactant is also present which forms the micelles, within which polymerisation and formation of the polymer molecules occurs. This rubber product was initially synthesised to fulfil the escalating demands for rubber during the First and Second World wars which could not be sustained by the retrieval of natural rubber alone [6, 10]. Acrylonitrile, which is a clear liquid, is manufactured from nitrogen-containing compounds such as amines, nitriles and ammonia [54]. Commercially it is now produced by the ‘ammoxidation of propylene’. The proportion of butadiene to acrylonitrile in the resultant co-polymer can be varied accordingly to produce the desired physical properties. The quantity of acrylonitrile used in the polymerisation process is observed to vary between

36

Common Polymers Used in the Formation of Concrete and Cementitious Products 20-50% by weight [43, 47]. Increasing the quantity of acrylonitrile present accelerates the polymerisation process.

2.2.9.2 Common Uses of Nitrile Rubbers Incorporating this compound into cementitious materials generally improves the overall resistance to abrasion and an enhanced tolerance to oils and certain solvents (aliphatic and aromatic) [43]. Concrete modified with nitrile rubber is therefore recommended in areas where a high degree of oil resistance is necessary, i.e., engineering facilities and garages [6]. The greater the acrylonitrile content in the polymer, the better the oil resistance [47]. O-rings and hoses are manufactured from this co-polymer due to its resistance to swelling when exposed to oils [54]. This resistance to swelling is noted to increase as the proportion of acrylonitrile present in this rubber constituent is raised. However, the flexibility of the system is lower when compared to other polymers [43]. Due to the high price of this rubber, its use tends to be limited to applications where resistance to oil and solvents is required [43].

2.2.10 Epoxy Resins

2.2.10.1 Introduction Epoxy resins have been used extensively in a variety of applications within the construction industry for numerous years to produce polymer concrete, adhesives and high performance coatings [1, 9-10, 39, 48, 55-59]. This is primarily due to the excellent adhesion, high chemical and solvent resistance exhibited by this type of polymer. Minimal shrinkage after curing along with a degree of flexibility are also properties which have made the use of epoxy resins favourable within the construction, automotive and aerospace industries [59]. Epoxy resins were first introduced to the commercial market in the form of adhesives in the 1940s and are capable of adhering a wide variety of materials such as glass, wood, metal and concrete to a produce a permanent bond [10, 57]. Analysis of the total consumption of resin composites predicted that the year 2000: 4.5 x 107 kg of resin would be utilised in commercial applications of which 80% would be epoxy in origin [60]. The Swiss chemist, Castin patented the synthesis of a resin in 1939 which, when hard, possessed exceptional mechanical properties and the curing process resulted in a minimal degree of shrinkage [61]. The epoxy pre-polymer is characterised by the presence of an epoxide (ethylene oxide) group at the end(s) of the molecule [57, 59, 61]. This group is highly reactive and will react readily with thiols, alcohols, acids, phenols, amines and amides which open the epoxide ring to form an hydroxyl group which in turn takes part

37

Polymers in Cementitious Materials in further chain propagation (polymerisation) [61, 62] (Figure 2.10). One of the main advantages of epoxy resins is that a vast array of substances can be combined with the pre-polymer to produce polymer compounds which are multifunctional [61, 63].

Figure 2.10 Epoxide ring [25] During the 1980s, epoxy resins were manufactured on an industrial scale by two main procedures, the polymerisation of compounds containing a double bond, via an oxidation process utilising peroxides and the glycidation of epichlorohydrin under alkaline conditions [64]. Oxidisation of an unsaturated compound has not found particular favour due to the economic cost involved in this procedure. The vast majority of commercially available epoxy resins used within the construction industry are two- or three-component systems comprising of a base (pre-polymer), hardener (curing agent) and aggregate [10, 39, 62, 65]. Single component system where curing is initiated by a certain temperature are used but to a lesser extent [62].

2.2.10.2 Chemical Composition Epoxy resin is the name assigned to describe a type of product synthesised by a crosslinking process encompassing a two-stage polymerisation process [66]. Initially a straight chain, low molecular weight pre-polymer is produced [65, 66]. The vast majority of epoxy pre-polymers are produced by the glycidation of bisphenol A with epichlorohydrin [57, 59, 64]. A three dimensional crosslinked network is formed by the addition of an appropriate curing agents, i.e., a polyamide [59]. Epoxy resins can basically be separated into three categories details of which are given next [59]: 1. Glycidated resins, 2. Epoxidised oils, 3. Cycloaliphatic epoxies.

38

Common Polymers Used in the Formation of Concrete and Cementitious Products By far the most predominant in terms of a commercial sense is glycidated resins [57, 59, 64, 67]. One of the initial epoxies produced involved the reaction of epichlorohydrin (2 mols) with bisphenol A, resulting in the formation of a diglycidyl ether of bisphenol A (DGEBA) [66]. Nowadays bisphenol F or combinations of both bisphenol A and F are used depending upon the physical properties of the epoxy required.

• Bisphenol A/Bisphenol F Epoxy prepolymers are produced by the glycidation of bisphenol A or F with epichlorohydrin. Bisphenol A is manufactured by reacting acetone with phenol [61, 67]. Whilst bisphenol F is manufactured by condensing phenol in the presence of formaldehyde [61, 64]. This process results in a number of reaction products, isomers and oligomers of bisglycidyloxyphenyl methane. In order to achieve the basic reaction at least 2 mols of epichlorohyrin are required to react with 1 mol of bisphenol A resulting in the formation of bisphenol A diglycidylether (Figure 2.11) [57]. Polymerisation will proceed until the hydroxyl groups of the aromatic rings of the bisphenol A have all reacted. The length of the resultant epoxy pre-polymer chain is related to the initial molar ratio of the epichlorohydrin and bisphenol A. In order to achieve a repeating polymer unit of n within the epoxy molecule an excess of epichlorohydrin (i.e., n + 2), must be added to the bisphenol A (n + 1). The value of n varies between 0-25 [57, 59, 61]. Varying the length of the polymer governs the consistency of the resin from a low viscosity to an epoxy possessing a high molecular weight. If a liquid consistency is required, n is normally below 1. Epoxy resins manufactured from bisphenol F shows an improved resistance to solvent attack along with a reduction in the viscosity compared to those epoxies produced from bisphenol A [61]. The resultant product is actually a conglomeration of ‘oligomers’ [61, 64]. Obviously these structural isomers will react with the available epichlorohydrin to produce a variety of polymerisation products.

Figure 2.11 Bisphenol A diglycidylether [57, 59]

39

Polymers in Cementitious Materials

• Epoxidised Novolaks Epoxidised novolaks are manufactured by initially reacting formaldehyde with phenol and then the reacting compound undergoes glycidation with epichlorohydrin [61]. The viscosity of such products is high: ranging from a very thixotropic liquid to a solid mass. These materials possess a substantial resistance to chemical attack especially organic solvents along with a high stability to thermal degradation. However, one of main disadvantages of epoxy novolaks is their lack of flexibility, which can be attributed to the high degree of crosslinking within the resin structure. This is due to the fact that they do not possess reactive hydroxyl groups.

• Alternative Methods of Forming Epoxies Lignin is one of the most abundant compounds on this planet and is one of the major constituents of wood. It is typically produced as a by-product during the manufacture of paper [12, 68]. Work has been undertaken to determine whether this substance could be utilised in the formation of epoxy resins, as lignin itself is a polymer, which possesses phenolic rings. In one particular exercise, lignin extracted from a particular wood, was initially combined with propylene and ethylene oxide resulting in the formation of a ‘chain extended’ hydroxylalkyl lignin. This compound contained hydroxyl groups which in turn could undergo epoxidation by reacting with epichlorohydrin to form lignin-epoxy prepolymers. These synthesised prepolymers reacted slowly with an aromatic amine, i.e., meta phenylene diamine. Adhesives and coatings could be produced from these lignin epoxy prepolymers. The authors concluded that varying the chain length of the prepolymers can result in the cured epoxy possessing a wide range of glass transition temperatures (Tg) and is hence suitable for use in the manufacture of a number of products such as adhesives.

2.2.10.3 Curing Agents The reaction of bisphenol A, bisphenol F or a combination of the two with epichlorohydrin produces an epoxy pre-polymer [2, 39, 61]. A significant reduction in the physical performance of epoxies is observed if crosslinking of the pre-polymer is not initiated, this process ultimately increases the molecular weight of the resultant epoxy [61, 65]. In order to achieve the desired physical characteristics such as strength, chemical resistance and durability a particular chemical type of curing agent is required [61]. These include amines, acid anhydrides or polyphenols. Curing of the epoxy can also be accelerated by the addition of a catalyst to the mixture [61, 69].

40

Common Polymers Used in the Formation of Concrete and Cementitious Products The combination of the base ‘pre-polymer’ and curing agent initiates further polymerisation, which in turn results in the release of heat, that is described as an exothermic process [39]. The greatest heat evolution is noted when the resin is still in its liquid form. Polymerisation of the epoxy generally proceeds via the epoxide ring, however, it can also occur through hydroxyl groups when present [59]. Crosslinking of the homopolymer can be catalysed or occur via the incorporation of ‘reactive intermediates’. The use of aliphatic amines as epoxy-curing agent is generally favoured at ambient temperature. Epoxy resins which can withstand heat have been manufactured and contain aromatic diamines such as m-phenylenediamine and 4,4´-diaminodiphenyl sulfone.

• Polyamines One of the most used classes of epoxy curing agents by far are polyamines which can be either aliphatic or aromatic in nature [57, 65]. Typical hardeners used to cure epoxies include aliphatic polyamines, cycloaliphatic polyamines and aromatic polyamines [59]. The reactive epoxide group present in the pre-polymer, reacts with a hydrogen atom on the amine, this in turn produces a hydroxyl group and subsequently a secondary amine group [61]. The latter is then able to undergo the same reaction with an available epoxide group: both these amino groups react at a similar rate. However, the actual rate of the reaction will depend upon the chemical nature of the amine utilised. The addition of nitromethane into the epoxy-polyamine systems has been observed to slightly increase the rate of the reaction [59]. Introducing water or 2-propanol into the mixture has been noted to have a similar effect. However, the rate of reaction depreciates when benzene or acetone is present. Aliphatic amines will react with the available epoxy pre-polymer and cure at ambient temperature in the absence of additional heat [61]. Heat is required to induce curing when epoxy pre-polymers are reacted with unmodified aromatic amines (i.e., not adducted with compounds, which will react at room temperature). Hence, their incorporation is advantageous in the production of coatings - a cured polymer coating is produced without the necessity for heat. Polyamines are used as the hardener when epoxy resins are required to completely cure in conditions where the temperature is low.

• Acid Anhydrides Acid anhydrides and polybasic acids are also used as curing agents [59, 61]. In the absence of a catalyst the epoxide group available on the epoxy pre-polymer reacts slowly with polybasic acids and acid anhydrides even at elevated temperatures such as 200 °C. The addition of a catalyst, either acidic or basic in nature, is necessary in order allow the reaction to occur at a reasonable pace. The anhydride group reacts with the available hydroxyl

41

Polymers in Cementitious Materials group on the epoxy to form an ester. An epoxide group is then capable of reacting with the carboxylic group present within the newly synthesised ester resulting in the production of a diester and an additional hydroxyl group. In all cases the crosslinking reaction is dependent upon the anhydride ring being opened first [59]. Whilst this reaction is occurring the epoxide group present on the pre-polymer is also capable of reacting with the hydroxyl group present on the acid anhydride (i.e., etherification) [61]. In the case of acids such as carboxylic acid, the carbonyl group reacts with the epoxide group resulting in the formation of an ester and hydroxyl group. Increases in the surrounding temperature encourage the hydroxyl and acid constituents to react. Consequently, water is eliminated from the structure and the epoxy hardens by itself. Acid anhydrides are regularly used as epoxy-curing agents due to the enhanced chemical and electrical properties achieved for the resultant thermosetting material [57]. Acid anhydrides have been typically used as the curing agent within epoxy powder hardeners [61].

• Phenol Derivatives Phenols are not readily used as epoxy curing agents when compared to amines and polybasic acids/anhydrides. In the absence of a catalyst, a reaction was only initiated at high temperatures, i.e., 200 °C. Analysis of the constituents, which had formed, indicated that the phenol had reacted with the epoxide group to produce an alcohol, which had subsequently reacted with the available epoxide group. However, it is addition of a catalyst such as a compound containing hydroxyl groups which is normally required in order to achieve a film or hardened composite within a suitable time period. Under these conditions the curing process will occur at lower temperatures, i.e., 100 °C, and involves the direct reaction of the phenol with the epoxide ring.

• Catalysts A variety of curing agents are used in the synthesis of epoxy resins and they can be acidic, basic or catalytic in nature [66]. The curing agent utilised determines the final structure and physical properties of the hardened resin. Acidic curing agents include anhydrides, phenols, carboxylic acids, dibasic acids, boron trihalides or Lewis acids. Basic curing agents include primary, secondary and tertiary amines. If the curing agent is catalytic in nature, further polymerisation of the pre-polymer is initiated, whilst the curing agent acts as a co-monomer when co-reactive. The latter interact by either a cationic/anionic reaction or by reacting with the epoxide or hydroxyl groups present within the prepolymer backbone.

42

Common Polymers Used in the Formation of Concrete and Cementitious Products

• Benefits of Certain Curing Agents The type of curing agent utilised will ultimately depend on the physical properties of the desired epoxy, i.e., heat/chemical resistance, glass transition temperature (Tg) and temperature range within which curing must occur [57]. Amine derivatives are commonly used rather than pure amines as they have certain handling issues associated with them. In order to perpetuate crosslinking of this type of epoxy it is imperative that the curing agent is poly-functional, hence mono-functional amines cannot be used [59]. If curing agents are aliphatic in nature, they are capable of initiating curing of the epoxy polymer without the necessity for elevated temperatures [61, 70, 71]. However, if the amine contains aromatic groups then heat is normally required to promote active polymerisation [59]. Using aromatic amines has the advantage that a longer pot life can be achieved at ambient temperature allowing the material to be worked and manipulated to achieve the desired shape or finish before curing is initiated. Even longer open times can be achieved if acid anhydrides are used. Good electrical properties have been observed for epoxies cured at elevated temperatures (150-170 °C) in the presence of a catalyst.

• Other Constituents used in the Manufacture of Epoxies Butylglycidylether and other glycidylethers of aliphatic polyols possessing a low viscosity are used as plasticers or diluents in (solvent-free) epoxy systems [57, 61]. Unlike cementbased products, a wide variety of pigments can be incorporated into epoxy resins without any chemical incompatibilities. Incompatibilities between pigments and cementitious materials may result in the formation of cracks within the microstructure [65, 72]. Liquid dispersion of pigments is generally preferred as this avoids the possibility of lumps of unmixed powder remaining in the resin and hence causing colour variations. A number of fillers are added into epoxies in order to increase the bulk volume, influence the rheology and reduce the potential shrinkage by reducing the exotherm experienced during the curing process. Talc, barytes and silicates are commonly added due to their morphology which consists of flakes, particulates or rods, in order to achieve the desired rheology [57].

2.2.10.4 Curing Conditions In epoxy systems, like cementitious systems, the rate of curing and strength development is inherently linked to the surrounding temperature [39]. A 50% reduction in the degree of polymerisation has been noted as a consequence of a drop in ambient temperature in the region of 10 °C. The application of a wide variety of epoxy resins is generally recommended when the ambient temperature is within the region of 5-25 °C. Below 5 °C the curing

43

Polymers in Cementitious Materials efficiency is affected and development of the microstructure is comprised. Depending upon the chemical nature of the epoxy resin component, a product can be produced which will efficiently cure at low temperatures. However, if curing is accelerated a degree of heat evolution during the polymerisation can result in problems with matrix expansion when used in warmer environments (i.e., where the temperature exceeds 30 °C). Above 20 °C, the polymerisation is accelerated and hence application of the material can be difficult depending upon the nature of use. In such cases where application may occur in an environment at 35 °C, the workability of the epoxy resin can be extended to overcome application problems. Generally, in order to enable all year round use most epoxy products are produced in two grades suitable for application in the summer or winter.

2.2.10.5 Heat Resistant Epoxies As research into the formulation of heat resistant polymers accelerated, the synthesis of new epoxy resins escalated. Results of research on the modification of the epoxy resin by integrating quinozolone and imide rings into the polymer structure was published during the 1970s [66]. It was shown that the resistance of the structure to thermal degradation upon exposure to high levels of heat improved as the concentration of aromatic compounds within the polymer backbone was increased. The reaction of epichlorohydrin and tetrabromobenzimidazolone utilising hexahydrophlathalic anhydride as the crosslinking agent produces fire resistant epoxies, i.e., bromine-substituted N-N´-diglycidyl-benzimadazolene. Bisquinazoline or phenol derivatives of bisquinazoline are reacted with epichlorohydrin to produce an epoxy resin incorporating quinazoline rings. This resin has an outstanding resistance to thermal decomposition when exposed to high temperatures registering a Tg of 200 °C. Similar heat resistant resins are produced when phenol and epichlorohydrin react with quaternary ammonic salts. A Tg of 153 °C has been measured for this type of epoxy system. Epoxy resins, which are resistant to high temperatures and possess a low degree of flammability, can be produced by incorporating fire retarders into the structure [64]. Previous research has examined the inclusion of N´,N-bis (2,3-epoxypropyl)-2,4,6tribromalin and in combination with N,N-bis(3-chloro-2-oxypropyl)-2,4,6,-tribromalin with a bisphenol A-epoxy derivative [73]. Good results were achieved, i.e., the material effectively extinguished the fire when the concentration of the halide present within the epoxy modifier exceeded 50%. Commercial flame retardants formulated for incorporation into epoxy resins can also contain a phosphorus-based compound [74]. Typically a mixture of stabilisers, bisphenol

44

Common Polymers Used in the Formation of Concrete and Cementitious Products A epoxy resin and a red amorphous phosphorus-based flame retardant are blended. This type of product is used in adhesives (conductive, dielectric), laminates and moulding. Alternatively, components such as aluminium trihydrate have also been used for this purpose.

2.2.10.6 Biodegradable Epoxy Resins Over the past few years with increasing environmental concerns, the biodegradability of and the ability to recycle epoxy resins materials has been brought into question particularly since a number of epoxy resins are formulated to posses a high resistance to structural degeneration when exposed to heat [58]. However, research has now been undertaken to incorporate chemical bonds, which will decompose when exposed to lower temperatures in relation to those normally required to cause deterioration of these types of epoxy resins. One important factor was the ability to degenerate the structure in such a manner as to allow the reuse of the epoxy in the design of new materials. Experimentation resulted in various cycloaliphatic diepoxides which would structurally deteriorate due to the breakdown of carbamate linkages. Both solid and liquid epoxy resins were produced. The diepoxides were cured in a similar manner to conventional epoxies by the use of cyclic anhydride curing agents. Analysis revealed that cured epoxies produced using diepoxides started to decompose at 200-300 °C. This is below the temperature required to decompose cured epoxies produced from cycloaliphatic epoxides.

2.2.10.7 Physical Properties A range of epoxy resins are available commercially which possess an excellent bond adhesion, act as electrical insulators and have a resistance to a wide range of chemicals [10, 69]. Epoxies absorb very little moisture, exhibit minimal shrinkage, are adaptable and possess excellent adhesion to a vast variety of substrates. They are commonly used in the manufacture of adhesives, a variety of coatings as well as in high performance mortars and grouts [10, 39]. Epoxies have been combined with cold tar typically 25-30% by weight to produce a coating system, which is more economical than a pure epoxy but performs in a similar manner to an epoxy system [75]. Epoxy resins manufactured from bisphenol F have a high chemical resistance to solvents along with a lower viscosity [61]. The actual epoxy content is also higher, however, they also have a tendency to crystallise. By utilising a combination of these bisphenol products (A and F) an epoxy system can be produced which has a viscosity in between that of the

45

Polymers in Cementitious Materials individual components whilst eliminating the tendency to crystallise. This type of epoxy is frequently used in the construction industry. Epoxy resins can also be produced which are thermoplastic in nature [61]. Epichlorohydrin/bisphenol A are combined with linear polycondensates and in turn are utilised as coatings which harden without the need for the introduction of a curing agent. One of the main disadvantages of epoxies, particularly those formulated to resist and function at high temperatures, is they are inherently brittle due to the nature of their structure [60, 76]. Research has therefore focused on producing epoxies, which possess a greater resistance to fracture upon impact. Strengthening the matrix has been achieved by incorporating a carboxyl terminated butadiene-acrylonitrile polymer [60]. However, the resistance of the structure to degeneration when exposed to high temperatures was reduced along with the overall modulus. The inclusion of siloxane-based rubbers into epoxies has however, removed such problems. Another method currently being favoured is the incorporation of polyethersulfonates, polyetherimides or polyesters, i.e., ‘ thermoplastic’ strengthening agents. These compounds are commonly incorporated into the base and phase separate during the curing process, which toughens the epoxy matrix. An enhanced degree of toughness is observed due to the deformation of the thermoplastic compounds present in the epoxy matrix.

2.2.10.8 Common Uses of Epoxy Resins Due to their superior bonding capabilities and high resistance to a wide variety of chemicals, epoxies are frequently used to coat walls and floors in industrial factories/warehouses where exposure to chemicals is commonplace or food preparation areas, hence hygiene is paramount [40, 57, 65]. Other applications include adhesives, laminates and tools to name but a few [56, 72, 77]. Another basic advantage of this type of coating is it is easy to clean, as the smooth impervious finish prevents dirt becoming trapped. However, such coatings are not advisable in areas, which will require regular steam cleaning, i.e., abattoirs, as the structure will disintegrate due to thermal shock. In this type of situation the installation of an epoxy screed (three-pack systems: base, hardener and aggregate) at an average thickness of 6-9 mm is recommended. Coatings and screeds are generally flexible and possess a substantial degree of abrasion and impact resistance along with toughness and durability. Epoxies are formulated to prevent shrinkage during curing [58]. It is important for every application to take account of the curing temperature and ideal section thickness [39]. If too large a volume/section is applied to the area being repaired, or refurbished at one time the heat evolution associated with the curing process and exotherm development can adversely effect the structural integrity. This causes cracking within the matrix, which must be avoided at all costs. If cracks occur they provide a route

46

Common Polymers Used in the Formation of Concrete and Cementitious Products for moisture, chemicals and other contaminates such as oils to enter into the coating, screeds and substrate underneath. This can result in the coating or screeds also debonding from the substrate.

2.2.11 Polyurethanes

2.2.11.1 Introduction Polyurethanes were initially produced by reacting a polyol with a diisocyanate and were introduced into the market place during the 1940s [78, 79]. The development of this type of resin advanced with the synthesis of diphenylmethane diisocyanate (MDI) which was subsequently used to synthesise a variety of adhesives, rubbers, foams and coatings. A high degree of flexibility along with a resistance to an array of solvents has been noted for polyurethanes as well as a superior abrasion resistance [80]. Initial research highlighted the superior adhesion of polyurethanes, these compounds were therefore readily used in the development of adhesives and coatings [79]. As development of this type of resin system intensified, polyurethane foams and materials suitable for moulding were launched in the commercial market place during the 1960s and 1980s. Thermosetting polyurethanes can be cast in either open or closed moulds by injection - certain systems can even be sprayed.

2.2.11.2 Brief Description of the Synthesis of Polyurethanes The synthesis of polyurethanes by the reaction of a polyol (compounds containing active hydroxyl groups) with a diisocyanate compound was developed over 60 years ago [81]. The isocyanate groups react with the hydroxyl groups present on the polyol [82]. These are either polyester- or polyether-based depending on the final properties required [79, 83]. In order to achieved the desired substituted polyurethane it is important there is an excess of diisocyanate present relative to the polyol concentration, i.e., 2 mols of diisocyanate for every 1 mol of polyol [82]. In order to achieve complete film formation it is important that one of the reactive species possesses an excess of three functional groups. The chemical nature of the polyol used and how they react with the available isocyanate groups determines the degree of crosslinking. Polyurethanes with a high degree of elasticity to rigid structures can be manufactured [79, 81]. The isocyanate group present in the polyurethane can also react with moisture present within the atmosphere and produces a diamine, which reacts with the available isocyanate forming the resin [80]. Such a reaction occurs within single pack moisture curing

47

Polymers in Cementitious Materials polyurethanes. As well as hydroxyl groups, the isocyanate can also react with carboxyl, amino or imino groups [80, 82, 84]. The addition of certain compounds also accelerates the formation of polyurethanes - these include tin salts and tertiary amines [84]. Thermosetting as well as thermoplastic polyurethanes can be manufactured [79].

2.2.11.3 Formation of Diisocyanates Diisocyanates are commonly used in the formation of polyurethanes, i.e., toluene diisocyanate, 4,4´ diphenyl methane diisocyanate (Figure 2.12), naphthalene diisocyanate and isophorone diisocyanate [79]. These are commonly produced by the reaction of the desired amine with phosgene, hence, the process is called ‘phosgeneation’. Initially a carbamic chloride and amine salt are produced by the rapid agitation of amines in the presence of phosgene and a solvent. The actual isocyanate is formed as a result of heating the carbamic chloride which initiates the loss of hydrogen chloride. Hydrogen chloride, solvent and excess phosgene are all then removed from the process leaving the synthesised isocyanate. Alternative routes to the synthesis of isocyanates without the need for phosgene due to its toxicity to humans have been assessed. These routes focus on the ability of a nitro-containing compound to react with carbon monoxide, thus removing the necessity for the amino group [79].

Figure 2.12 4,4´-Diphenylmethane diisocyanate [79]

2.2.11.4 Other Ingredients Used in the Manufacture of Polyurethanes As with epoxy systems, a number of additives are incorporated into polyurethane systems to improve the physical characteristics [79, 82]. These include stabilisers, antifoamers and rheology aids [82]. In order to prevent the pre-polymer becoming thixotropic over a period of time, low viscosity isocyanates were developed [79]. Principally, the unmodified isocyanate was combined with carbodimide, which alters the crystalline nature of the isocyanate and thus the viscosity. This compound can be used on its own or in conjunction with glycol modified MDI, to achieve materials with a variety of functions and degrees of viscosity.

48

Common Polymers Used in the Formation of Concrete and Cementitious Products Aromatic hydrocarbons, which do not contain any active hydroxyl groups, are used as solvents or thinners, therefore, xylene, toluene, esters and ketones are generally favoured [82]. There is no chance of solvent inadvertently reacting with the available isocyanate groups thus depleting the quantity capable of undergoing polymerisation. The chemical nature of the pigments and fillers also has to be scrutinised as they may react with the NCO groups available on the diisocyanate. Typical materials to avoid are certain types of organic pigments, carbon black and zinc oxide. Generally, as with cementitious compounds inert pigments and fillers are favoured in order to eliminate any undesirable reactions.

2.2.11.5 Volatile Organic Compounds A concerted effort has been made in the last few years to remove potentially harmful compounds (such as VOC) from the manufacturing process as well as the final product composition [79]. The development and improvements in the performance of water-based polyurethanes has intensified over this time period, giving emulsions containing anionic or cationic polyurethane ionomers (i.e., polymer molecules which possess ionic groups) [79, 85, 86]. Of these, polyurethane anionomers containing sulfonate and carboxylate groups are the most common [85]. The main advantage of these systems is they do not emit any chemical vapours during the curing process therefore do not harm the environment or workers utilising them [85, 87]. Due to the absence of solvents they are less flammable which is certainly an advantage [86].

2.2.11.6 Recycling of Polyurethanes The recyclability of such resins has also increasingly become an issue in recent years, as a consequence the development of renewable sources of polyurethanes has been investigated [88]. Naturally occurring polymers such as cellulose and sugar [88, 89] have been investigated as these materials contain hydroxyl groups and hence can be used to produce polyurethanes. Whilst the flame resistance is improved by the inclusion of polysaccharides, the incorporation of lignin enhances the structural stability of the resultant polyurethane [88]. Glycerol can also be used to initiate the formation of a high molecular weight ‘polyether polyol’. It can also act as a crosslinking agent. The advantage of using these types of materials has been investigated by Hinz and co-workers, including an assessment of the physical properties of the polyurethanes formed and the impact of utilising such raw materials upon the cost of the resultant resin. However, one disadvantage of using lignins is the dark colour of the resin formed, which restricts the final colour of the cured product.

49

Polymers in Cementitious Materials

2.2.11.7 Common Properties of Polyurethanes Polyurethanes are one of the most expensive resins currently available, their abrasion resistance and durability is exceptional, hence less of this material is required in comparison to more economical alternatives in order to achieve the necessary performance [79]. A high impact resistance has also been noted for the hardened systems along with a good chemical resistance to gamma radiation, oxygen, oils and grease. The physical properties of polyurethanes are linked to the ‘two phase morphology’, i.e., hard and soft section of the polymer due to the diisocyanate and polyol, respectively [86, 87, 90, 91]. Previous studies have shown that the chain length of the polyester side chain is governed by the extent of segregation of the hard and soft portions of the polyurethane. The effect of varying the length and structure of this chain upon the physical properties of the resultant resin has also been investigated [90]. Improvements in the resistance of polyurethanes to the deterioration of the structure and mechanical properties when exposed to heat can be achieved by manufacturing a resin with an organic and inorganic component [92]. These resins were manufactured by a solgel method. Two types of polyol were used in the formation of such polyurethanes along with titanium tetra-isopropoxide, triethoxysilane end capped poly(oxytetramethylene) glycol and triethoxysilane endcapped poly(oxytetramethylene) triol. The proportion of triethoxysilane compound was varied in relation to the titanium component between 46/54% and 70/30% by weight. The polyurethanes produced, as a consequence of these reactions possessed greater dynamic mechanical properties and heat resistance.

2.2.11.8 Typical Uses of Polyurethanes A number of polyurethanes are used as raw materials in the manufacture of a variety of materials, adhesives, coatings and foams [78, 81, 93]. The actual polyurethane constituent can be segregated into four main categories [79, 80]: •

Two-pack polyurethane systems, where the base (polyol) and the curing agent are combined in order to achieve a hard composite.

•

One component systems containing polyisocyanates which when applied directly to the substrate/surface, cure by reacting with atmospheric moisture.

•

One or two component polyisocyanate binder systems which are blocked with the addition of amines.

•

High molecular weight polyurethanes, which are principally non-reactive and cure as they dry.

50

Common Polymers Used in the Formation of Concrete and Cementitious Products The main advantage of high molecular weight polyurethanes is that they contain a significantly lower quantity of solvents than polyurethanes in other categories thus reducing their emission into the atmosphere and subsequent environmental consequences [82]. Polyurethane foams originated from the discovery that the introduction of moisture into the system caused the formation of carbon dioxide as a consequence of isocyanate groups reacting with the water, which was present [79]. A similar effect was achieved by introducing fluorocarbons during the manufacturing process. These foams can be manufactured possessing a flexible or rigid structure, which depends upon their cell structure either open (soft) and closed (hard). Due to the insulation properties and structure, this type of rigid foam has been used to prevent heat loss and for panelling suitable for use within walls and roofs. Flexible foams can also be synthesised and are utilised in applications where their chemical resistance to solvents and oxidation is beneficial in conjunction with their flexibility. Typical applications include bed mattresses, and cushions. As well as forming foams or fibres, polyurethanes are commonly used as high performance coatings, which can be applied to concrete floors and walls [79, 94]. One of the first coatings to be manufactured was produced by initially combining a diisocyanate with trimethylol propane and subsequent a polyol. The resultant coating dried in air and possessed physical properties on a par to those observed for wire coatings [80]. Coatings containing a high solids and low solvent content are used within the construction industry to produce hygienic, dust free chemically resistance surfaces [82].

References 1.

Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 186.

2.

D.G. Walters in Proceedings of ACI 1986 Fall Convention, American Concrete Institute, 1986, Baltimore, MD, USA, p.1-6.

3.

ACI Committee 548, State-Of-The-Art Report on Polymer-Modified Concrete, Report No.548.3R-3, American Concrete Institute, Detroit, MI, USA, 2003.

4.

Adhesives Age, 1988, 31, 9, 42.

5.

L.A. Kuhlmann, J.W. Young, Jr., and D. Moldovan, inventors; The Dow Chemical Company, assignee; US 5576378, 1996.

6.

R. Dennis in Construction Materials Reference Book, Ed., D.K. Doran, Butterworth Heinemann, UK, 1992, Chapter 39, p.1-12.

7.

D.G. Walters in Proceedings of the ACI Convention, 1997, Atlanta, GA, USA, p.1-5.

51

Polymers in Cementitious Materials 8.

J.A. Manson, Materials Science and Engineering, 1976, 25, 1-2, 41.

9.

E. Kirhikovali, Polymer Engineering and Science, 1981, 21, 8, 507.

10. J.D.N. Shaw, P.J. Brown, R. Cather, R. Dennis, P.C. Hewlett, R.A. Johnston, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Polymers In Concrete, 2nd Edition, Technical Polymers No. 39, Concrete Society Working Party, The Concrete Society, 1994. 11. O. Moy, Cellulose Polymers, 1999, http://www.dalton.org/students/moy 12. M.P. Stevens, Polymer Chemistry: An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p 389-396. 13. J. McMurry, Organic Chemistry, 2nd Edition, Brooks/Cole Publishing Company: Pacific Grove, CA, USA, 1988, p.886. 14. J. McMurry, Organic Chemistry, 2nd Edition, Brooks/Cole Publishing Company, Pacific Grove, CA, USA, 1988, p.117. 15. J. McMurry, Organic Chemistry. 2nd Edition, Brooks/Cole Publishing Company, Pacific Grove, CA, USA, 1988, p.106. 16. B. Staynes and B. Corbett, Civil Engineering (London), 1988, March, 28. 17. D. Feldman, Polymer News, 1993, 18, 9, 261. 18. R. Donges, Non-Ionic Cellulose Ethers, Hoechst AG, Wiesbaden, Germany, 1989. 19. R. Harbron in Proceedings of Calcium Aluminate Cements 2001, Edinburgh, UK, Eds., R.J. Mangabhai and F.P. Glasser, 2001, Cambridge University Press, Cambridge, UK, p.597-604. 20. I. Nelson, Multi Functional Crosslinking Acrylic Polymers For Concrete And Mortar, in Proceedings of the ConChem Conference, 1997, Dusseldorf, Germany, p.1-12. 21. J. McMurry in Organic Chemistry, 2nd Edition, Brooks/Cole Publishing Company, Pacific Grove, CA, USA, 1998, p.1122-1123. 22. M.P. Stevens in Polymer Chemistry An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p.104-109.

52

Common Polymers Used in the Formation of Concrete and Cementitious Products 23. D.A. Smith in Addition Polymers: Formation and Characterisation, Ed., D.A. Smith, 1968, Butterworths, London, UK, p.111-126. 24. D.E. Fahey, Concise History of Plastics, 1999, Australian National Plastics and Rubber Industry Training Council, Australia, p.1-22. 25. M.P. Stevens in Polymer Chemistry An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p.3-26. 26. V.R. Riley and I. Razl, Composites, 1974, 5, 1, 27. 27. D.G. Walters in Chemical Admixtures, American Concrete Industry (ACI), Farmington Hills, MI, USA, p.8-11. 28. A. Hoffman, Effect of Redispersible Powders On The Properties Of Self-levelling Compounds, Wacker Polymer Systems, Burghausen, Germany, 1999. 29. D.J. Schulze, The Use Of Redispersible Powders In Cement Mortars, Wacker Chemie GmbH, Münich, Germany, 1985. 30. D.J. Schulze and F. Jodlbauer, Vinnapas® Redispersible Powders For Building Renovation, Wacker-Chemie GmbH, Münich, Germany, 1997. 31. D.Z. Zurbriggen, Influence Of Redispersible Powders On Shrinkage, Hydration Behaviour And Microstructure Of Tile Adhesives, 1999, Elotex AG, Sempach Station, Switzerland. 32. Polymer Modified Factory-Made Dry-Mix Mortars As Modern Building Materials, Wacker Polymer Systems GmbH & Co. KG, Münich, Germany, 1999. 33. DLP Redispersible Polymer Powder in the Construction Industry, Dow Chemical Company Limited, 2000. 34. R.N. Swarmy, Cement and Concrete Composites, 1995, 17, 3, 175. 35. W.A., Whitaker III, Medical Plastics and Biomaterials Magazine, 1996, January, p.12. 36. F.W. Jr. Billmeyer in Textbook Of Polymer Science, 3rd Edition, 1984, John Wiley & Sons, Inc.: New York, NY, USA, p.383-391. 37. M. Annemieke, A.M Aerdts and B.K. van Herk in Materials Science and Technology, A Comprehensive Treatment, Synthesis of Polymers, Eds., R.W. Cahn, P. Haasen and E.J. Kramer, Wiley-VCH, New York, NY, USA, 1999, p.274283.

53

Polymers in Cementitious Materials 38. J. Daintith in A Concise Dictionary of Chemistry, Ed., J. Daintith, Oxford University Press, Oxford, UK, 1990, p.62. 39. T.P. O'Brien, W.B. Long, P.C. Hewlett, J.D.N. Shaw, L.J. Tabor and P.J. Winchcombe, Repair of Concrete Damaged by Reinforcement Corrosion, Technical Report No.26, The Concrete Society, London, UK, 1984. 40. C. Ellis, High Performance Plastics, 1989, 6, 6, 1. 41. D.P. Tate in Encyclopaedia of Polymer Science and Engineering, Volume 2, Eds., A. Klingberg, J. Muldoon and A. Salvatore, John Wiley & Sons, New York, NY, USA, 1985, p.553. 42. W.D. Watson and T.C. Wallace in Applied Polymer Science, 2nd Edition, Eds., R.W. Tess and G.W. Poehlein, American Chemical Society, Washington, DC, USA, 1985, p.367. 43. R.D. Deanin in Polymer Structure: Properties and Applications, Cahners Books, Boston, MA, USA, 1972, p.422-433. 44. PEP'97, Report 35D, Butadiene as a Chemical Raw Material, in Process Economics Program Report 35 Butadiene, Process Economics Program, SRI Consulting, Houston, TX, USA, 1998. http://process-economics.com/Reports/ peprpt035.htm 45. PEP'94, Report 35C, Butadiene, Process Economics Program, SRI Consulting, Houston, TX, USA, 1994. http://process-economics.com/Reports/peprpt035.htm 46. PEP'80, Report 35B: Butadiene, Process Economics Program, SRI Consulting, Houston, TX, USA, 1982. http://process-economics.com/Reports/peprpt035.htm 47. F.W. Billmeyer, Jr., in Textbook of Polymer Science, John Wiley & Sons, Inc., New York, NY, USA, 1984, p.372-379. 48. D.W. Fowler in the Proceedings of the 43rd Annual Conference and Focus '88, 1988, Cincinnati, OH, USA, Session 16-B1. 49. More Than Just Cement, Synthomer GmbH, Frankfurt am Main, Germany. 50. We Don’t Make Bitumen, We Make Bitumen Better, Synthomer GmbH, Frankfurt am Main, Germany.

54

Common Polymers Used in the Formation of Concrete and Cementitious Products 51. C.A. Stewart, Jr., T. Takeshita and M.L. Coleman in Encyclopedia of Polymer Science and Engineering, Eds., A. Klingsberg, J. Muldoon and A. Salvatore, John Wiley & Sons, New York, NY, USA, 1985, p.441-443. 52. R.A. Wessling, D.S. Gibbs and P.T. DeLassus in Encyclopedia of Polymer Science and Engineering, Eds., A. Klingsberg, J. Muldoon and A. Salvatore, John Wiley & Sons, New York, NY, USA, 1985, p.492-495. 53. F.W. Billmeyer, Jr., in Textbook of Polymer Science, John Wiley & Sons, Inc., New York, NY, USA, 1984, p.132. 54. F.M. Peng in Encyclopedia of Polymer Science and Engineering, Volume 13, Eds., H.F. Mark, Bikales, N.H. Overberger, C.G. Menges and J.I. Kroschwitz, John Wiley & Sons, New York, NY, USA, p.426-430. 55. R.N. Swarmy, Cement and Concrete Composites, 1992, 14, 3, 155. 56. P. Landau, Chemical Marketing Reporter, 1998, 253, 24, 8. 57. P. Ku, Advances in Polymer Technology, 1988, 8, 1, 81. 58. L.Wang and C.P. Wong, Journal of Polymer Science, Part A: Polymer Chemistry, 1999, 37, 15, 2991. 59. R.S. Bauer in Applied Polymer Science, 2nd Edition, Eds., R.W. Tess and G.W. Poehlein, ACS Symposium Series No.285, ACS, Washington, DC, USA, 1985, p.931. 60. J.N. Hay, B. Woodfine and M. Davies, High Performance Polymers, 1996, 8, 1, 35. 61. Pitture e Vernici, 1991, 67, 3, 47. 62. K.W. Harrison in Adhesion 12, Ed. K.W. Allen, Elsevier, London, UK, 1988, p.121. 63. I. Dobas, S. Lunak, S. Podzimek, K. Dusek and S. Pokorny, International Polymer Science and Technology, 1985, 12, 4, T/111. 64. Penczek and J. Rejoych, International Polymer Science and Technology, 1987, 14, 2, T/111. 65. P. Maslow in Organic Coatings & Plastics Chemistry Preprints, 1979, Volume 40, 348.

55

Polymers in Cementitious Materials 66. A.K. Srivastava and P.Mohan, Journal of Macromolecular Science, 1997, 37C, 4, 687. 67. S. Sugiyama and K. Sakashita, Hydrocarbon Processing, 1998, 77, 3, 100. 68. K. Hofmann and W.G. Glasser in Proceedings of the ACS National Meeting, Symposium on Chemical Modification of Biopolymers, Boston, MA, USA, 1990, 31, 1, p.657. 69. A. Blaga, Thermosetting Plastics, in Canadian Building Digest 159, 1974, Division of Building Research, National Research Council Canada: Ottawa. 70. Pacific Anchor Speciality Epoxy Resins. Ancarez 718NC Diluted Epoxy Resin, Pacific Anchor Chemical, Performance Chemicals Division, Air Products and Chemicals, Inc., Allentown, PA, USA, 1991. 71. I. Wiesner, International Polymer Science and Technology, 1986, 13, 12, T/70. 72. Araldite Epoxy Resins, Ciba Geigy (UK) Ltd., Plastics Division, Cambridge, UK, 1984. 73. S.N. Suleimanov, R.G. Agadzhanov and M.S. Slakhov, International Polymer Science and Technology, 1996, 23, 6, T/71. 74. Amgard CPC 700, Albright & Wilson, Oldbury, West Midlands, UK, 1999. 75. S.H. Alexander in Applied Polymer Science, 2nd Edition, Eds., R.W. Tess and G.W. Poehlein, ACS Symposium Series No.285, Washington, DC, USA, 1985, p.1229-45. 76. K-F. Lin and Y-D. Shieh, Journal Of Applied Polymer Science, 1998, 69, 10, 2069. 77. Chemical Industries Newsletter, 1988, March/April, 4. 78. M.S. Bhatnagar, Popular Plastics and Packaging, 1992, 37, 7, 43. 79. B.K. Howe in Proceedings of Rubberplas 84 Conference, Singapore, 1984, Volume 1, Paper No.2. 80. M. Brannen and J. Broeder in Proceedings of TAPPI Hot Melt Symposium Conference, Hilton Head Island, SC, USA, 1992, p.31. 81. J. Johnston, Shell Chemicals Europe Magazine, 1995, 2, 8.

56

Common Polymers Used in the Formation of Concrete and Cementitious Products 82. DD Coating Raw Materials Introduction: Chemistry, Products, Applications, Industrial Hygiene, Bayer AG, Leverkusen, Germany. 83. K. Tharanikkarasu and B.K. Kim, Journal of Applied Polymer Science, 1999, 73, 14, 2993. 84. H. Warson, Polymers Paint Colour Journal, 1999, 189, 4414, 26. 85. D. Dieterich in Proceedings of the 6th International Conference in Organic Coatings Science and Technology, Athens, Greece, Advances In Organic Coatings Science and Technology Series, Volume 4, 1982, p.51. 86. H.L. Manock, Resin and Pigment Technology, 2000, 29, 3, 145. 87. B-K. Kim and J.C. Lee, Journal of Polymer Science: Polymer Chemistry, 1996, 34, 6, 1095. 88. W. Hinz, P. Horn and H. Larbig in Proceedings of Polyurethanes '95 Conference 1995, Chicago, IL, USA, p.230. 89. C. Dajun and L. Yaojun, China Synthetic Rubber Industry, 1997. 20, 4, 244. 90. W. C. Chen, Y.S. Chen, T.L. Yu and Y.H. Tseng, Journal of Macromolecular Science A, 1997, 34, 8, 1369. 91. H. Janik and J. Foks, in Proceedings of Utech '92 Conference, Hague, The Netherlands, 1992, p.170. 92. M. Furukawa and K. Morimoto in Proceedings of IRC ‘96, International Rubber Conference, 1996, Manchester, UK, Paper No.68. 93. Urethane Plastics and Products. 1992, 22, 8 1. 94. R. Hussein and N.P. Cheremisinoff in Elastomer Technology Handbook, Ed., N.P. Cheremisinoff, CRC Press, Boca Raton, FL, USA, 1993, p.875-907.

57

Polymers in Cementitious Materials

58

Polymer Concrete

3

Polymer Concrete Author

3.1 Introduction Principally developed as a substitute for marble, the use of polymer concrete increased in popularity from its initial conception throughout the 20th century [1]. The use of polymer concrete first attracted a degree of interest within the construction industry during the early 1950s. As news of the superior properties achieved during the initial testing procedures spread, the number of research projects established to further assess the performance and uses of polymer concrete escalated. Subsequently during the early 1970s a vast array of field-testing was undertaken and the use of the material increased especially during the 1980s where it was frequently used to repair pre-cast Portland cement concrete structures, i.e., bridges, motorways and pavements [2, 3]. Polymer concrete has been utilised as a replacement for standard concrete and cementitious repair mortars due to its ability to overcome the primary disadvantages of the aforementioned materials. The physical properties such as durability and enhanced chemical resistance were quickly recognised [4, 5]. Structural stability when exposed to the freeze-thawing process was also advantageous as degeneration of the cementitious microstructure when exposed to sub-zero environmental conditions and subsequent thawing cycle is a common problem with Portland cement systems [6]. Polymer concrete/mortars are basically produced by combining thermosetting resin(s) with grading aggregates and has been used in one form or another since the 1950s [1, 4, 7-10]. Although not containing any actual ‘Portland cement’ this material is referred to as a ‘concrete’ due to its actual definition. The word ‘concrete’ describes an aggregate bound together by a substance to form a hardened mass [10]. The polymer binds the desired aggregate together to form a stable structure when hard. One of the principal advantages of polymer concrete systems is they can be formulated to fit a particular requirement such as viscosity, mechanical strength or curing temperature [11]. The thermosetting resin effectively replaces the actual cement component, i.e., Rapid Hardening/Ordinary Portland cement or High Alumina Cement [12]. The liquid

59

Polymers in Cementitious Materials component could contain monomers, pre-polymer, curing agent or catalyst as well as other ingredients such as pigments, which were subsequently mixed with the chosen aggregate [13]. Methyl methacrylates, unsaturated polyester and epoxy resins are commonly used to produce polymer concrete [1, 8, 11]. It is the exceptionally high physical performance achieved by most types of polymer concrete/mortars in relation to standard Portland cement concrete which makes its use advantageous [4, 7, 11, 13]. Due to the increased longevity, a workable life of in excess of sixty years has been quoted for the final material [3]. The vast majority of polymer concrete produced utilises a thermosetting resin binder, however, there have been occasions when a thermoplastic polymer has been used [13].

3.2 Chemical Composition of Polymer Concrete

3.2.1 Advantages of Thermosetting Resins In polymer concrete, thermosetting resins are used as the principal polymer component due to their high thermal stability and resistance to a wide variety of chemicals [14, 15]. During curing of thermosetting polymers by the application of a certain degree of heat/pressure a defined chemical reaction occurs. After this process has been completed, the physical shape of the thermosetting polymer cannot be altered because of the structure formed during the chemical reaction [14, 16]. Radiation and catalysts may also be used in the curing process resulting in an inflexible crosslinked polymer [16]. The combination of the linear chains in this manner imparts the enhanced physical properties commonly associated with these types of resin systems compared to those observed for latexes which ‘film form’ by coalescence of the polymer particles [15]. In turn the hardened polymer structure maintains a significant level of strength when exposed to high temperature [16]. Generally, it is not advisable to use thermosetting resins in conjunction with cement. The alkalinity of cement can adversely affect the polymerisation of certain monomers such as methyl methacrylates, hence their use with the latter should avoided [5]. Cement also requires a certain quantity of water in order to sufficiently hydrate and develop the cementitious microstructure that forms the hardened composite. The use of polymers dispersed in water is therefore generally favoured with cementitious materials [12]. As discussed in Chapter One, the cementitious microstructure which forms as a consequence of the hydration of Portland cement with water typically comprises various hydrate components such as calcium silicate hydrates, and calcium aluminate hydrates along

60

Polymer Concrete with calcium carbonate and residual silica [17-20]. The proportion and crystallinity of these hydrates determines the resultant physical properties of the cured cementitious material.

3.2.2 Thermosetting Resins Typically Used in the Manufacture of Polymer Concrete A variety of different thermosetting resins are utilised in the formation of polymer concrete [1, 2, 8]. Typical thermosetting monomer resins utilised in the formation of polymer concrete are epoxies, urea/melamine formaldehyde polyurethanes, furans, unsaturated polyesters, vinyl ester and methyl methacrylates [2, 12, 13, 16, 21]. The specific nature of the polymer used will depend on the desired rate of curing, application, desired setting time along with flexural, tensile and compressive strength required of the polymer concrete and environment which the resultant composite will be subjected to [2]. Table 3.1 details the main monomer(s)/resins commonly used in the manufacture of polymer concrete. Such materials are also fast curing, achieving an initial set within 30-90 minutes on average and the curing process is much less susceptible to the effects of temperature [2]. Certain types of polymer concrete can be applied from –18 °C up to 40 °C [11]. In order to achieve the maximum performance it is important that the correct proportion of resin and aggregates along with the desired fillers/admixture is weighed accurately and mixed sufficiently to produce a homogeneous mixture [7, 13]. The earliest polymer concrete formulations used a combination of methyl methacrylate (MMA) with polyester/styrene [2] due to the cost effectiveness of such resin binders. Methyl methacrylates were also used in conjunction with trimethylolpropane trimethacrylate (TMPTM) where benzoyl peroxide was added to initiate polymerisation of these monomers [8].

Table 3.1 Common monomers used in the manufacture of polymer concrete (adapted from [1, 2, 8]) Type of monomer MMA MMA/TMPTM Polyester/styrene Urethane Furan Epoxies

61

Polymers in Cementitious Materials

3.2.2.1 Methyl Methacrylates Polymethyl methacrylates (PMMA) (acrylics) are commonly used in the manufacture of polymer concrete as they exhibit a low degree of structural shrinkage during curing (0.01-0.1% variation in length) and are highly durable, exhibiting excellent strength properties (see Table 3.2) [13]. These polymers effectively cure in the same way as polyesters yet they do not exhibit the high degree of shrinkage associated with that type of resin [4]. This low level of shrinkage has been attributed to the greater structural flexibility of PMMA in comparison to polyester. The resin matrix is less susceptible to structural movement due to expansion and contraction during the curing process. Polymer concrete manufactured using acrylics is also not susceptible to structural deterioration as a consequence of freezing and then thawing, due to an exceptional resistance to the absorption of moisture [13]. Polymer concrete was commonly produced using this type of resin binder to repair bridge decks due to its high strength and minimal shrinkage capabilities. The primary disadvantages of using MMA were the obnoxious smell and flammability due to its low flash point [2, 13]. Hence a number of precautions had to be taken such as excluding machinery which could cause a spark or the presence of naked flames during the mixing and application of polymer concrete produced from MMA. Such factors have made the use of MMA less favourable and hence a general reduction in their use subsequently followed. This has become especially prevalent since there has been a concerted effort to reduce the annual consumption of volatile organic compounds within the general industry due to environmental and health issues [22-24].

3.2.2.2 Polyesters Polyester resins are commonly supplied in the form of an unsaturated pre-polymer. The resin base nominally contains 60-80% polyester and other monomers such as styrene [13, 16]. Polyester resins cure by the reaction of the monomer with a catalyst, which is generally a peroxide and is normally added to the liquid component as a powder [4].

Table 3.2 General physical properties of polymer concrete regularly utilised (adapted from [13]) Resin identity

Compressive strength (MPa)

Tensile strength Flexural (MPa) strength (MPa)

Modulus of elasticity (GPa)

Epoxy

50-150

8-25

15-50

20-40

Polyester

50-150

8-25

15-45

20-40

PMMA

70-210

9-11

30-35

35-40

62

Polymer Concrete The pre-polymer undergoes the crosslinking process via the breakage of its double bond [13]. The resultant polyester polymer concrete is capable of achieving a strong bond to a number of materials and registers a good mechanical strength. Due to the physical properties and the fact that they are low cost, polyester styrene resins are favoured for use in specific manufactured polymer concrete products [8]. By combining approximately 52% sand with 27% hydrated alumina and 21% by weight of a polyester resin a compound has been produced that was suitable to use in underwater applications and to encase waste, possibly radioactive in nature. This type of resin was commonly used to produce polymer concrete overlays [2]. One of the main advantages of using this type of resin over epoxies is the price - polyesters are certainly more economical [13]. However, the curing process is exothermic in nature resulting in the maximum release of heat from the matrix once the resin has set [4]. This can result in the actual volume of the cured structure being lower than that of the original resin material. The structural shrinkage of this type of resin concrete tends to be high in comparison to unmodified concrete, values of between 0.3-0.5% in length have been quoted [25]. This obviously restricted the use of such a product over a significant area due to the degree of shrinkage, which will be incurred [13]. However, research has shown that the shrinkage can be reduced to minimum of 0.15% in length by adding thermoplastic additives to certain types of isophthalic polyester resins [25].

3.2.2.3 Epoxy Resins Epoxies have a particularly good chemical resistance and bond adhesion thus they will adhere to a variety of substrates [26]. These resins also exhibit a low degree of shrinkage. The epoxy pre-polymer is combined with a suitable curing agent in order to produce a hard crosslinked resin. A variety of curing agents are used in the formation of epoxy resin concrete, the most popular being polyamines followed by polyamides and polysulfides [13, 26-28]. The compounds react with the epoxy pre-polymer produced typically by the reaction of bisphenol A with epichlorohydrin. The chemical nature of the curing agent principally influences the physical properties of the resultant polymer concrete [13]. A high degree of chemical resistance is achieved when a polyamine is utilised. Whilst an improvement in the structural flexibility is observed when polyamides and polysulfides are incorporated in the epoxy pre-polymer. Epoxy resin systems tend to develop the vast majority of their mechanical strength (approximately 80%) within the first 1-2 days when cured at a temperature of 20 °C [4]. In the case of epoxy resin concrete, compressive strengths in the region of 55-110 N/mm2 are

63

Polymers in Cementitious Materials commonly achieved [4, 7, 29]. This is significantly higher than that nominally achieved by standard Portland cement concrete, i.e., 20-70 N/mm2. The resultant resin composite can withstand an operating temperature of 40-80 °C [4, 7]. As with the majority of these types of resin compound a reduction in mechanical strength would be expected as the temperature tends towards the higher end of this temperature range [7]. However, the resin system will generally maintain its physical integrity over this temperature range. A flexural strength up to ten times that associated with ordinary concrete has also been observed [4, 29]. Although epoxy resins are expensive, the polymer concrete produced displays a minimal degree of shrinkage during curing [27, 30]. A superior chemical resistance particularly in relation to organic liquids, i.e., aromatic ketones, exceptional structural durability and the ability to bond to the vast majority of substrates to which it is applied are also characteristics of this material [13, 27, 30, 31]. The application of polymer concrete manufactured using epoxy resins is normally favoured when a high mechanical strength and chemical resistance are required [13].

3.2.2.4 Other Resin Constituents Furan, polyester amide, vinyl esters and urethanes have also been used, but generally to a lesser extent, in the manufacture of polymer concrete [2]. Furan polymer mortars and grouts were used due to their exceptional chemical resistance particularly to organic liquids such as aromatics and ketones [13]. In this type of polymer concrete the furan pre-polymer was polymerised by the addition of formaldehyde and furfuryl alcohol. The resultant polymer concrete was capable of withstanding significant changes in temperature in addition to an excellent resistance to chemicals.

3.2.2.5 Introducing Thermosetting Resin into Ordinary Portland Cement The effect of combining thermosetting resins such as epoxies and furans with wet concrete was studied during the 1970s [10]. Generally, variable results were achieved, depending upon the chemical nature of the monomer and curing conditions used. In certain cases an improvement in the physical parameters was observed particularly the compressive strength, which increased by up to 50%. Such results were normally achieved at substantial polymer concentrations, i.e., 30% by weight. The differences in the results achieved were attributed to the high alkalinity of the concrete, which subsequently was preventing adequate polymerisation of certain monomer/pre-polymers. Conversely, the effect of introducing the monomer/pre-polymers upon the hydration of the cement also has to be considered [8]. If the introduction of monomer disturbs the hydration of the cement component the development of the cementitious microstructure will inadvertently be

64

Polymer Concrete hindered. Cracks may also form throughout the cementitious structure as a consequence of introducing the monomer into the cementitious system.

3.3 Brief Introduction to Mixing and Application of Polymer Concrete 3.3.1 Summary The monomer or ‘pre-polymer’ used in the manufacture of polymer concrete will depend upon the type of application and working characteristics required, i.e., abrasion/impact resistance, increased ability to withstand chemical attack, structural stability and durability [13, 30]. The effect of temperature upon the curing time of monomer, pre-polymer, strength, workability, etc., also has to be taken into consideration [4]. Polymer concrete is basically produced by blending the chosen monomer or pre-polymer with the appropriate hardener and catalyst in order to promote crosslinking and the final polymer formation [13]. Once thoroughly mixed this compound is combined with the desired aggregate. The curing time can be adjusted to fit a certain requirement by either increasing the proportion of catalyst added or the surrounding temperature. This is turn accelerates the crosslinking process and consequently reduces the period over which curing occurs [4]. Alternatively reducing the catalyst level and temperature has the opposite effect. It is important that the polymer binder used in the formation of the hardened ‘polymer concrete’ matrix contains an effective antifoamer. The entrapment of air within the mixture, which can occur during the mixing of the polymer with the required aggregate, can lead to a high porosity and consequently a poorer compressive strength [32].

3.3.2 Mixing A simplistic method of producing polymer concretes is detailed as follows [1]. A quantity of the chosen monomer (50%), or pre-polymer is mixed with the designated initiator, whilst the remaining proportion is combined with the promoter. Both of these components are then mixed with the specific type of aggregate required. The resultant mixture is placed onto the substrate and trowelled to produce the desired finish. The surface should then be covered to prevent loss of the resin due to evaporation.

3.3.3 Aggregates Generally the most economic and widely sourced aggregates are utilised in the formation of polymer concrete, i.e., granite, sandstone or slate [13]. However, silica fume along with

65

Polymers in Cementitious Materials metallic aggregates have also been utilised as alternative fillers to the standard aggregates commonly used in the manufacture of polymer concrete. In simple terms, the aggregate component has to possess a solid form, negligible moisture content and be non-absorbent. Vermiculite and expanded shale along with other lightweight aggregates can be used in conjunction with a monomer, pre-polymer to form a material, which possesses a similar strength to concrete but is lighter in density [10]. Waste products such as glass from nonreturnable bottles have also been processed to provide a suitably sized aggregate, which was consequently mixed with a monomer, to produce a polymer concrete. Since the resin was by far the most expensive component used in the manufacture of polymer concrete, it was sensible to limit the quantity of this material used [30]. By using aggregates with a precise particle size distribution a polymer concrete could be produced which possessed sufficient flow whilst allowing adequate introduction of monomer into the voids formed between the aggregate particles, yet using a minimal quantity of resin. Research showed that the actual monomer content required to completely fill all pores and theoretically combine the aggregate could be as low as 7-8% by weight. As the particle size of the aggregate decreases, the quantity of monomer pre-polymer required to produce sufficient resin to bind all of the aggregate together increases, in some cases up to 30% by weight was required [13].

3.3.4 Workability and Shrinkage Characteristics Increasing the quantity of resin used in the manufacture of polymer concrete improves the workability and flow of the concrete [7]. The polymer binder was, however, the most expensive constituent hence increasing the proportion used made the resultant polymer concrete more expensive. Generally enough resin is used to produce the minimum degree of workability which is acceptable. Limiting the proportion of the binder in this way also prevents any noticeable shrinkage during curing along with reducing the tendency of the hardened composite to expand and contract when subjected to varying temperatures. It also enables a modulus of elasticity similar to that registered for standard concrete to be achieved. The ability to efficiently mix polymer concrete in summer and winter is also desired. The polymer component should therefore possess sufficient workability at temperatures commonly experienced during the summer, i.e., 30 °C in order to allow mixing and placement of the concrete within a suitable period of time. Conversely, in winter the polymer concrete should set within a reasonable time period, at the colder winter temperatures.

66

Polymer Concrete

3.3.5 Voids One of the primary problems with conventional concrete is the formation of voids within the cementitious matrix [10]. Analysis of a typical cementitious microstructure reveals the formation of discrete hydrate phases, which are inter-linked with each other and any aggregate present, rather than a continuous phase throughout the matrix resulting in the formation of pores within the hardened matrix. In turn these pores or cracks introduce points of weakness and subsequently fracture when the composite is subjected to a load. They also provide an opening within the structure through which acid rain, chlorides and carbon dioxide can enter and react with the cementitious medium resulting in deterioration of the internal/external matrix. In polymer concrete the hardened polymer should ideally fill all the space present between the aggregate particles thus removing the ability of contaminants to enter the internal structure. The resin provides a continuous phase throughout the cured concrete, which encapsulates the combined aggregate [8, 10]. Hence this is the reason this type of concrete is less susceptible to corrosion and consequently possesses a greater structural stability [10, 13]. However, it should be noted that the removal of voids within the structure would also depend on how efficiently the resultant polymer concrete has been compacted during application [7, 10]. The monomers used are also commonly hydrophobic in nature hence naturally repel the intrusion of water [33].

3.3.6 Priming of the Substrate It is important that the surface onto which the polymer concrete, grout or repair mortar is applied to is free from dust, latency and other contaminants such as oil prior to the application [2, 4, 7]. Any areas of the substrate, which are damaged or deteriorating, should also be removed. The bond adhesion and durability of the polymer concrete or grout could be adversely affected by the presence of such contaminates or areas where the structural integrity of the substrate is poor. One of the most popular methods used to clean the surface of the substrate is shot blasting [2, 7]. If for example the repair mortar is being applied directly onto the substrate it is important that it is done within a few hours of the initial substrate preparation, i.e., four hours. This was to prevent a build up of dirt upon the prepared surface, which would adversely effect the adhesion. In the majority of cases the substrate requires sealing with an appropriate primer. The first primer coat is allowed to dry before the second is applied, up to 16 hours later [4]. This in turn seals any minor imperfections in the initial coat, i.e., pinholes which are caused by air bubbles within the primer coat bursting and leaving a hole. If the primer coat

67

Polymers in Cementitious Materials is to act as a bonding aid between the substrate and the applied polymer compound (i.e., repair mortar), it is important that this coat remains tacky at the point of application. If this is not the case the correct bonding capabilities between the repair mortar and bonding coat will not be achieved.

3.3.7 Mixing In order to achieve maximum physical performance from the concrete, grout or mortar it is imperative that the components are mixed together sufficiently in order to achieve a homogeneous mix [4, 7]. The majority of failures in the past within epoxy resin mortars have been attributed to poor mixing of the individual components, i.e., base, hardener, and aggregates [4, 7]. A common way of initially mixing such materials is to distribute the aggregate onto a flat surface onto which the various liquid components are poured [7]. These are then crudely mixed together using a shovel, hence a number of the individual components did not receive any mixing at all. The mortar possessed an incorrect ratio of base and hardener, which consequently affected the curing efficiency. In many cases a hard surface was achieved but the strength characteristics were extremely poor thus the mortar failed when subjected to only a slight load. If only small quantities require mixing an appropriate trowel can be used to blend the various components [4]. However, if the size of the application dictates that large quantities of resin material require mixing, it is more appropriate to use a forced action mixer. Again care has to be taken to ensure that all material is sufficiently mixed and does not collect around the sides of the mixer.

3.4 Application Techniques Polymer concrete can be applied in the same manner as conventional cementitious mortar [1]. Once the material has been sufficiently mixed it should be placed onto the appropriate substrate, compacted and distributed to the necessary thickness [4]. A trowel can be used if the volume is sufficiently small. Polymer concrete has also been applied to bridge decks using paving equipment and then screeded to the produce the required finish [2]. Voids within the mortar must be avoided. If these remain within the microstructure, the porosity increases and the mechanical strength is reduced as these act as points of fracture when a load is applied to the hardened mortar. One of the advantages of polymer concrete over a polymer-impregnated system is that a heat source is generally not required to cure the product [8]. If the resin-based mortar has to be applied in layers the previous section should be textured to improve the bond adhesion between this and the newly applied layer [4]. Too long a period between the application of successive layers should be avoided as the bond adhesion

68

Polymer Concrete is reduced. If the latter occurs then a bonding/tack coat should be applied before additional layers are laid in order to improve the cohesive strength between the layers. Cracks and damaged sections of concrete can be repaired using epoxy resin mortars when the actual thickness of the repair required is below 12 mm [7].

3.5 Decision Model When undertaking any construction project it is important to assess the time scale over which the project has to be completed, raw materials required and any foreseen difficulties which may occur. If the project involves restricting the public access to a particular service, (i.e., motorway bridge), minimising the time period where the bridge is out of action is an essential consideration. Other factors to be considered may involve providing an alternative route or temporary structure in order to minimise distribution. The validity, time scale, suitability of raw materials and building constraints, which would or could be experienced on a construction project have been determined. From the research undertaken a decision model has been formulated [34]. The model enables a particular project to be assessed and the best course of action and most suitable materials to rectify the problem to be determined. One advantage of the model is the fact that it is not overtly complicated and does not require the user to be extensively computer literature.

3.6 Repairing Polymer Composites Repairing a polymer composite can be a difficult task, generally areas which visually appear damaged are rectified, however, the problem may occur further within the internal structure [35]. Such faults can result in structural failure or in the loss of mechanical integrity. A number of repair methods are currently used including infrared welding, injecting a resin into the damaged area in order to seal the cavity, or bonding the damaged sections together with an adhesive. However, these types of repairs ultimately depend upon the nature of the problem and the ability to clearly access the damaged areas. Cracking within the polymer matrix is not visible to eye and hence difficult to repair. Damage of this nature can result in a loss in the physical properties of the polymer composite. Recently, research has been undertaken to design a polymer-modified composite, which is capable of repairing itself at a macro, and microscopic level. The idea principally involved the storage and release of chemical components within the matrix of the composite, which can fill and hence effectively repair the internal damage. Metal and borosilicate glass hollow fibres containing polymer were incorporated into a polymer composite. Inserting hollow glass fibres into the matrix produced composites, which showed the principal of self-repair.

69

Polymers in Cementitious Materials

3.7 Physical Properties of Polymer Concrete The resultant physical properties of the polymer concrete will ultimately depend upon the chemical nature of the monomer used, degree of polymerisation, the particle size of the aggregate, along with the degree of hardness it possesses [1, 4, 11, 13, 30]. Initiators, pigments and thixotropic agents were also added to improve the workability, setting characteristics and aesthetics of this material [1, 8]. The same application characteristics as with standard Portland cement concrete can be used for polymer concrete [8]. Another advantage of polymer concrete is it can be made virtually any colour without experiencing any chemical incompatibility with pigment and resin [36]. Good adhesion to the vast majority of substrates and a high degree of chemical resistance depending on the type of monomer pre-polymer used, is normally achieved [11]. However in order to achieve the maximum bonding capabilities it is important that the substrate is prepared as described earlier and hence is free from dust and contaminants. One advantage of using polymer concrete is its resistance to movement once cured, therefore it is very difficult to change its physical form. This is principally due to the fact that the initial shaping process can require heat and pressure initiating specific chemical reactions during the curing process [16].

3.7.1 Typical Strength Characteristics Observed for Polymer Concrete By comparing the physical properties of traditional Portland cement concrete/mortars with polymer concrete/mortars it is clear that performance of the latter is superior (Table 3.3). This means that in certain applications only half the volume/weight of polymer concrete was required relative to that necessary if ordinary Portland cement concrete was be used in order to achieve the required performance and specification [13]. Concrete and mortars containing thermosetting polymers are favoured within the construction industry due to the good compressive strengths which are observed, i.e., 55-110 N/mm2 for 24-48 hours (see Table 3.3). Their resistance to structural movement once cured is also an advantage [12, 29]. Polymer concrete with a compressive strength exceeding 80 N/mm2 has been produced containing as little as 8% polymer binder by weight [7]. Typically, a polymer content between 5-15% has been used in the manufacture of polymer concrete and mortars [13]. The quantity of polymer added will be influenced by the grading and nature of the aggregate to be used. The finer the aggregate the greater the quantity of resin required. The end use and required physical properties of the polymer concrete or mortar will also govern the type and quantity of polymer/aggregate used.

70

Polymer Concrete

Table 3.3 Physical properties associated with epoxy resin concrete and Portland cement concrete (adapted from [29]) Physical properties

Epoxy resin concrete/ mortars

Portland cement concrete/mortars

55-110

20-70

25-50

2-5

Tensile strength (N/mm )

9-20

1.5-3.5

Water absorption at 7 days, 25 °C

0-1

5-15

6-48 h

1-4 weeks

Compressive strength (N/mm2) 2

Flexural strength (N/mm ) 2

Strength development at 20 °C

Polymer concrete is generally capable of withstanding significantly higher loads in relation to standard Portland cement systems due to the enhanced deformability and crack resistance of the matrix [5]. When comparing the typical physical properties of polymer concrete with those of ordinary Portland cement based concrete, the tensile strength is commonly six times greater. A substantial increase in flexural strength has also been recorded [30]. However this parameter will obviously be influenced by the interaction of the cured binder and the aggregate contained within. The ability to withstand a high degree of impact without substantial structural deterioration is also a significant advantage particularly if the structure is to be exposed to significant loads or a high degree of wear and tear [8].

3.7.2 Bond Adhesion of Polymer Concrete to a Substrate and the Effect of Expansion and Contraction The bond adhesion of polymer concrete systems to ordinary Portland cement is superb [2]. In order to ensure a good bond, all contamination and latency on the substrate surface must be removed [37]. It is important the interaction between the polymer concrete and subsequent substrate (conventional concrete, metal, etc.), is sufficient in order to maintain stability at the interface and thus an adequate bond. Polymer concrete also has to be capable of withstanding a degree of structural movement due to expansion and contraction of the substrate it is adhered to and high resistance to crack formation [2, 7, 37]. The potential stress/strain, which could be experienced between the two composites, should therefore be evenly distributed [37]. Deterioration of the polymer overlay due to the strain of these processes will reduce the bond between the latter and the substrate [2]. There is also a possibility of opening a route for chloride ions to penetrate into the concrete resulting in deterioration of the microstructure.

71

Polymers in Cementitious Materials

3.7.3 Permeability of the Polymer-Aggregate Matrix and Performance when Subjected to Varying Atmospheric and Environmental Conditions The permeability of the resultant polymer concrete/mortar is considerably lower than that typically observed in standard Portland cement concrete and mortar systems [7, 13]. The impervious nature of these polymer-aggregate matrices mean they are less susceptible to corrosion by chlorides, acids and carbonation as a result of exposure to carbon dioxide [11]. This is particularly advantageous if the ordinary Portland concrete contains reinforcing bars, corrosion of the latter can be directly linked to exposure to these types of chemical compounds and replacing them is an expensive process [4]. The chemical resistance of polymer concrete is typically higher than standard Portland cement concrete, the actual extent ultimately depends upon the chemical nature of the resin used as the binder [13]. Polymer concrete and other resin-based systems are able to withstand a variety of atmospheric and environmental conditions such as seawater without showing any significant signs of degradation [5]. This is one of the reasons why such materials are used in areas where a high structural performance and durability is required, yet the material has to be lightweight.

3.8 Common Uses of Polymer Concrete Although initially formulated during the 1950s within Europe, polymer concrete was not used in any significant volume throughout countries like the USA until the 1980s [2, 3]. With its enhanced physical performance in relation to ordinary concrete, the use of such systems was advantageous due to the increased life span expected of the material (estimates in excess of 60 years have been quoted) [3]. In addition to this it is mixed and applied in the same manner as ordinary cementitious materials hence utilising common skills and equipment [1, 2]. The main disadvantage of such systems is the cost of the actual material, as the polymer is significantly more expensive in relation to ordinary Portland cement, hence the proportion utilised is normally kept to a minimum [11, 13]. Generally the use of polymer concrete is only specified when the application requires the superior properties of the latter [13]. The volume of actual material required to accomplish the performance specification of certain specialised applications is reduced, by up to 50% in some cases. In these situations, the use of polymer concrete becomes more attractive. As well as being directly applied to the substrate surface, polymer concrete is commonly cast into blocks in a vast array of shapes and sizes [7]. This process and the resultant

72

Polymer Concrete product is commonly utilised in Europe. Polymer concrete can be used for a wide variety of applications, i.e., pipe linings, on and offshore concrete structures, for securing anchor bolts in railroads, motorways and even sports arenas [1, 8].

3.8.1 General Road and Bridge Repair Initial research assessed the feasibility of using polymer concrete as a repair system for bridges, motorways and other road surfaces. The majority of repair systems initially used MMA and/or polyester-styrene as the resin binder [2]. Rapid curing of this type of repair system, normally 30-90 minutes, meant the downtime of a particular structure, (i.e., bridges), was kept to a minimum. The superb bond adhesion between the applied polymer concrete and the substrate made this a popular repair compound [2, 8]. The versatility of this polymer concrete originates from the exceptional physical properties achieved. These include a greater compressive and tensile strength, the impervious nature of the hardened structure to water, significant acid and chemical resistance. Polymer concrete can also be applied over a wide temperature range (–18 °C to 40 °C) [7, 11, 13]. In one case, sections of a bridge which in certain areas showed signs of significant deterioration of the concrete were removed. These sections were replaced with new precast concrete sections which were embedded onto a polymer concrete mortar [2]. Due to the rapid curing characteristics associated with most polymer concrete, the repair work could be undertaken during the night allowing the bridge to be opened and suitable to accept traffic the following morning. Polymer concrete has also been designed for the repair of concrete and asphalt surfaces in adverse weather conditions [2]. Typically the polymer concrete consisted of a furfuryl alcohol monomer which was combined with a promoter, catalyst and a retarder as well as the graded aggregate. This combination of compounds was designed to give the necessary workability and then subsequently promote rapid polymerisation of the monomer. The one key advantage of this type of polymer concrete was that it could be applied even when the surface was wet or it was raining.

3.8.2 Overlays and Coatings Polymer concrete is also commonly used as a bridge decking overlay due to its high chemical and corrosion resistance [2, 11]. It is imperative that a sufficient bond between the overlay and original concrete surface is achieved otherwise the performance and working life of the overlay is reduced. One of the advantages of using polymer concrete, as a bridge deck overlay is the superior bond that is achieved between the latter and the normal Portland

73

Polymers in Cementitious Materials concrete. The increased modulus of elasticity and flexibility that polymer concrete possesses over normal concrete has the added advantage that a much thinner layer of material could be placed in order to obtain the same load, strength and wear resistance [2, 13, 36]. The polymer concrete overlay utilised has to be capable of responding to the expansion and contraction, which will occur with the bridge structure due to variations in temperature. If the elongation of the resin is not sufficient to accommodate this movement then a reduction in the adhesion of the overlay to the concrete surface can occur. Analysis of the bridges which have been resurfaced with polymer concrete overlays throughout United States of America and Canada indicated that the vast majority were polyester-based followed by epoxy resin with a small number using MMA [2]. Polymer concrete produced from unsaturated polyester resins was common due to its relative low cost in relation to other suitable binders [13]. The superior resistance of furan resins to chemical and acid attack, means it is commonly favoured to produce overlays for use in chemical factories and plants [2, 13]. Acid and chemical tanks are also coated with a polymer overlay containing furan resins due to their exceptional chemical resistance [2]. The concrete must be sealed with a suitable primer before the application of this material can take place. Polymer concrete coatings have also been formulated which will adhere to a variety of metal structures utilising an epoxy resin in conjunction with a number of fillers such as kaolin, silica fume and talc along with a catalyst [38]. The viscosity of these coatings is such that they are commonly sprayed onto the surface.

3.8.3 Resin Grouts and Mortars Since the 1960s epoxy resin grouts and repair mortars, have been utilised in a vast array of applications. Epoxy resin grouts are generally used to repair 6-12 mm thick splits in concrete structures [7, 29]. Whilst epoxy grouts can be used to secure starter bars and dowels in cementitious materials due to their good bond adhesion and negligible shrinkage [7]. They are generally used when the diameter between the fixing and the outer region of the hole surpasses 6 mm. The ‘pull-out’ strength achieved will depend upon the size of the hole filled along with the depth to which the fixing is submerged and the chemical nature of the resin utilised. The greater the depth the higher the ‘pull out’ strength. The superior compressive, flexural and tensile strength obtained by epoxy resin mortars along with their high stability towards chemical attack makes the use of such materials for precision grouting advantageous [7, 13]. It is important that air entrapment during the mixing process is minimised in order to prevent a reduction in the physical properties of the resultant mortar due to an increase in the porosity of the matrix. Nowadays, epoxy

74

Polymer Concrete grouts are formulated to contain antifoamers, which stop the aeration of the mix. However before chemical antifoamers were commercially available a vacuum mixer was used to ensure the grout was air free [7]. These types of mortars were favoured for securing, and embedding heavy-duty machinery. The purpose of the concrete structure under repair, ambient temperature and the period of time in which the repair had to be completed all have to be taken into consideration when undertaking this process. Basically, the epoxy used must be capable of adequately filling and bonding to the concrete surface, cure when exposed to moisture and develop an acceptable degree of mechanical strength in the required time and at the surrounding temperature. Unless the concrete area to be repaired is small, the use of polyester-based mortars should be avoided [4]. This is primarily due to their susceptibility to shrinkage whilst curing. The repair therefore no longer fills the void completely which subsequently renders the repair ineffective [13]. Grouts and mortars produced using these resin components are however used to fix dowels and starter bars in place due to the rapid rate they gain strength. Compressive strengths in the region of 50 N/mm2 have been registered in as little as three hours when the surrounding temperature was 20 °C [4]. Before the application of a resin-based repair mortar/grout proceeds, the substrate surface normally has to be primed with two coats of primer. It is imperative that the resin-based mortar is able to adhere to the primer, thus the latter are also generally epoxy-based to prevent any bonding incompatibilities. The primer also has to be capable of bonding to materials such as steel. Primers that contain a high concentration of zinc are not recommended for use when the repair is going to contribute to the overall structural performance as a general reduction in cohesive strength is observed.

3.8.4 Sealants If cracks have arisen within a concrete structure due to corrosion of the reinforcing bars contained within, sealing these by injecting resin into the cracks will not prevent further corrosion of the reinforcing bars contained within the concrete structure [4]. This is primarily due to the fact that this method will not stop the further corrosion or in turn further cracks within the repair due to the latter. In such cases it is advisable that the reinforcing bars are sealed first with a protective membrane and the crack then filled with a suitable epoxy repair mortar. Again, it is imperative that the concrete surface and crack interface show no signs of deterioration and are thoroughly cleaned before application proceeds. Sealing cracks with an epoxy resin is however, a viable method of repair in order to improve the structural integrity if there are no visible signs of corrosion or structural movement.

75

Polymers in Cementitious Materials In order to penetrate the surface and fill the voids contained within the microstructure a low viscosity resin is required [28]. If the crack undergoing repair is in a vertical position the area will have to be sealed or shuttered to prevent loss of the material whilst it cures [4]. If access to the external surface of the crack is difficult and hence cannot be efficiently sealed, a thixotropic epoxy can be used instead [4, 28] Movement of the latter stops when the external force is removed. The resin may be injected into the void using either a special pressurised injection system or simply via resin cartridges compressed in a suitable gun [4]. In order to prevent further cracking during or as a consequence of the injection process due to tensile stress it is important that the pressure is carefully controlled. If the crack is damp or contains water it is important that a moisture tolerant epoxy is utilised. Methyl methacrylates (high molecular weight) are ideally suited for filling cracks within concrete bridge decks, or where access is difficult, due to their superior adhesion [39].

3.8.5 Castable Systems Polymer concrete can also be cast and moulded into a variety of shapes to form a multitude of components used in a wide variety of applications [40] detailed earlier in the text, and as a result precast polymer concrete has increased in popularity over the last decade. From building blocks and panels to decorative surfaces and finishes such as kitchen worktops, a vast array of products have been produced using this material [10]. The high strength and durability of such materials makes their use ideal in harsh corrosive conditions (i.e., high temperatures or continual exposure to chemicals, seawater, etc). Combining the resin component such as epoxies and vinyl esters with perlite has also produced lightweight polymer concrete. Ultimately the physical performance of the cast polymer concrete will be governed by the actual method to which it is manufactured and mixed [41]. A variety of methods were assessed from hand and batch mixing to continual processing. When mixing by hand or as a specific batch there was a greater potential to introduce error into the process and hence reduce the quality and properties of the resultant material. Whilst a continuous casting process was determined to produce the best and most consistent results allowing the addition of compounds which are more difficult to handle when using other methods. Polymer concrete is also used as an interior pipe lining to improve the durability when exposed to liquids which can corrode or erode the unprotected pipe surface [42]. When styrene is used in conjunction with acrylonitrile along with other ingredients such as reaction initiators, promoter, silica sand and Portland cement, a polymer concrete is produced which has a significant resistance to high temperatures up to 260 °C [2]. This polymer concrete has been used as a lining for pipe work, which can be subjected to a significant degree of heat as a cheaper alternative to alloy steels. The polymer concrete

76

Polymer Concrete was placed inside the pipe, which was capped at each end and subsequently rotated at high speed in order to distribute the concrete evenly around the internal circumference. Research within this area has also resulted in the development of an economical polymer concrete which does not require the use of harmful substances such as acrylamide or acrylonitrile or high temperatures to initiate curing of the concrete [42]. The resultant polymer concrete enhances the durability and chemical resistance of the pipe interior against corrosion or erosion. Testing revealed the resultant polymer concrete lining is capable of operating in a harsh geothermal environment without any noted deterioration in the chemical resistance. With the developments in nuclear power, the problem of safely storing the radioactive waste produced from the fission process has arisen [10]. Hence, research has been undertaken to determine the feasibility of long-term storage of radioactive waste encased in polymer concrete and polymer impregnated concrete. The resistance of these concretes to corrosion and chemicals makes the use of such materials favourable for this type of application. However, the resistance of these concretes to significant variations in temperature, stability to radioactivity and possibility of the radioactive material leaching through the microstructure has to be assessed. It is important to bear in mind the time period, (i.e., thousands of years), over which the polymer concrete and polymer impregnated concrete must maintain its maximum performance due to the radioactive substances enclosed within.

3.9 Polymer Impregnated Concrete Another common area where polymers have been used in conjunction with cured Portland cement concrete structures has been in the formation of polymer impregnated concrete [11]. A chosen monomer/monomers are impregnated into the prepared concrete surface and subsequently polymerised, typically by the application of heat and the activation of chemical initiators within the monomer constituent [8, 10]. An improvement in the physical properties such as the mechanical strength (compressive, flexural and tensile) and modulus of elasticity have generally been observed when this impregnation process had been completed [9, 11]. Theoretically any voids within the cementitious matrix could be sealed by this impregnation process, thus preventing the penetration of water and carbon dioxide from entering the structure [43]. Analysis also suggested that the corrosion of reinforcing bars within the concrete was subsequently prevented by using this process if all the voids are sealed [44]. An increase in the durability and resistance to the damage that can result in concrete as a consequence of water freezing within the pores and expanding, (i.e., freeze-thawing cycle), has also been identified [43]. A variety of monomers can be used to produce polymers with either a glassy or a rubbery consistency [45]. Typically

77

Polymers in Cementitious Materials monomers impregnated into concrete structures included MMA, styrene, acrylonitrile and vinyl chloride [1, 10].

3.9.1 Concrete Preparation and Impregnation Process Most polymer impregnation applications (such as concrete bridges, floors and walls) involved the partial impregnation of the monomer(s) species into the cementitious substrate to depths of up to 0.04 m [8]. Full impregnation of the monomer species involved initially removing the air contained within the cementitious microstructure by applying a vacuum. The monomer was then introduced into the microstructure under pressure to aid the impregnation process. The techniques involved in this process cannot be applied practically to large surface areas such as concrete floors or bridges and are generally used to fully impregnate precast concrete structures such as beams. The process of impregnation principally involves three stages, removal of moisture from the concrete, introduction of the monomer, polymerisation followed by curing [8]. Before impregnation of the monomer could begin the cementitious material had to be prepared. Initially the concrete surface was carefully cleaned to remove any contaminants and loose materials, which hinder the absorption of the monomer into the substrate. It is generally accepted that the concrete will contain a certain degree of moisture and its removal is generally achieved by heating the surface [43]. The disadvantage of this process is that the heat was normally applied to one surface only of the concrete structure, (i.e., the surface undergoing the impregnation process). Hence the distribution of moisture could vary throughout the concrete matrix. Heat was generated from fuel burners or infrared heaters and directed towards the surface of the concrete [8, 43]. This process is carried out for several hours until the substrate surface has achieved a suitable temperature and the vast majority of moisture held within the microstructure is removed [1, 43]. A moisture content of less than 2% is desired within the cementitious matrix before the impregnation of the resin component proceeds [1]. One of the key disadvantages of utilising polymer impregnated systems in relation to polymer concrete is a heat source is required to initiate curing and produce a hardened composite [12]. A layer of dry sand is normally distributed over the surface onto which the monomer is poured [8]. The temperature of the concrete is generally not allowed to exceed 100 °C in order to avoid damage such as cracking [43]. The purpose of the sand is to act as a barrier in order to prevent the monomer dissipating to undesirable areas. Indeed the quantity and period over which saturation occurred would be governed by the type of monomer and degree of impregnation desired which in the vast majority of cases was partial rather than complete.

78

Polymer Concrete Once the impregnation of the monomer had been completed the substrate is generally covered with polyethylene to prevent a loss in the monomer concentration due to evaporation [43]. Typically the monomer is allowed to penetrate into the concrete matrix for a period of four to twelve hours in order to ensure penetration occurred to a suitable depth, i.e., 0.04 m. Once this period had expired, polymerisation of the monomer species was initiated. This was normally achieved by exposing the monomer to a suitable temperature in order to activate the initiator (typically benzoyl peroxide) contained within monomer. Free radicals are subsequently generated which initiate polymerisation of the monomer. The heat source used to initiate polymerisation was normally steam in order to avoid any potential hazards associated with the monomer, (i.e., flammability). This polymerisation process normally occurred over three to six hours [1, 8]. Other chemicals, (i.e., TMPTA), were incorporated into the monomer system in order to promote crosslinking and hence the formation of a crosslinked polymer matrix within the concrete microstructure [8, 43].

3.9.2 Physical Properties Certain researchers have stated that the impregnation of resin into concrete was the best way of combining such compounds. Indeed using data collected from studies at the time (1974) it was proposed that the most significant improvement in the physical characteristics of the concrete was observed by adopting this process [10]. Negligible creep was also observed unlike with traditional concrete systems. Whilst significant improvements in compressive strength of polymer impregnated concrete were also recorded. Research tended to suggest that the depth of penetration did inherently govern the stability of a concrete slab to freeze thawing and hence the overall durability. A slight improvement in the abrasion resistance of the impregnated concrete surface in relation to that observed without any polymer modification was also noted (Table 3.4). An assessment of the benefits of impregnating the cementitious microstructure with a resin determined a general improvement in the region of 30% to the coefficient of thermal

Table 3.4 Typical physical properties recorded for Portland cement concrete impregnated with MMA polymerised by cobalt radiation (adapted from [30]) Physical properties

Value registered

Compressive strength (MPa)

139.7

Tensile strength (MPa)

11.2

Modulus of elasticity (MPa) Water absorption (°C)

43436 0.29

79

Polymers in Cementitious Materials expansion. Whilst a dramatic reduction in the ability of water and other contaminants to penetrate the surface was observed, in some cases a reduction of up to 99% was recorded. The inclusion of the resin effectively reinforced the internal structure of the concrete substrate which was capable of withstanding structural changes to a greater extent during loading [30]. Certain investigations did indicate that long-term prevention of contaminants penetration into the microstructure was linked to the type of monomer which had been utilised. The ability of the monomer to penetrate into the concrete substrate will be governed by its viscosity and surface tension [43]. Penetration of the monomer obviously increased the porosity of the concrete structure. Of course this parameter will be influenced by a number of factors, water to cement ratio, chemical composition of the cement and temperature of curing. All these factors along with the degree of polymerisation will govern the overall depth of polymer impregnation. The potential of a reaction between the cementitious matrix, (i.e., calcium silicate hydrates), and the monomer was also a possibility which could affect the physical performance of the polymer impregnated concrete [30].

3.9.3 Common Uses of Polymer Impregnated Concrete Research undertaken throughout the 1970s indicated that a significant improvement in strength and durability of the concrete surface occurred when impregnated with an appropriate monomer, pre-polymer combination [2]. This type of process was initially favoured on concrete bridge decks due to the enhanced physical performance, durability and chemical resistance of the impregnated cementitious material. Impregnation with a resin also had the advantage of potentially sealing the pores present within the concrete matrix and hence making the surface more impervious to water [43]. However, it should be noted that the overall level of improvements would obviously depend upon the performance of the original Portland cement concrete structure and chemical nature of the monomer introduced. The effectiveness of this process to improve the lifetime of floors within heavy industry has been recognised [2, 45]. Resin monomer(s) have been impregnated into concrete floors within industrial situations in order to rectify cracking to the surface and enhance the long-term performance [45]. The absence of solvent within these resin constituents means the process is kinder on the environment and less hazardous to human health. This process allows the repair of the concrete surface rather than resorting to alternative methods such as repairing the concrete completely as recasting it is an expensive process. Such a process has the advantage of hardening the surface and improving the durability and resistance

80

Polymer Concrete to chemical attack. The impregnation of monomer, pre-polymers into stonework to seal and protect the exposed surface has also been undertaken. Alternative uses for this type of technique involved the complete impregnation of post tension beams with the desired monomer [2, 8]. These beams were ideally suited in areas where adverse environmental conditions would be experienced such as a high degree of corrosion which are normally observed with exposure to seawater (coastal areas). These beams exhibit excellent strength characteristics and durability along with a minimal creep which enable them to withstand higher loads than those capable of being supported by ordinary precast beams [2].

3.9.4 Disadvantages Associated with Polymer Impregnated Concrete Although an impressive increase in the strength and durability of partially impregnated concrete was observed in some cases, this material did tend to be more brittle than traditional Portland cement structures [2]. The use of this process to enhance the physical performance of concrete and prevent deterioration of the matrix was used throughout the 1970s. However, a link was established with this process and an elevated chloride content within the impregnated concrete structure. This would seem unusual as this procedure essentially was supposed to seal the voids within the microstructure and prevent the absorption of damaging elements. Analysis however, attributed this phenomenon to the drying, polymerisation process inducing cracking within the structure thus allowing compounds to enter the matrix once more. One of the principle areas which also caused concern was where areas of impregnated and ordinary concrete overlapped or were in close proximity to each other [43]. The sometimes significant differing in physical characteristics, i.e., strength, coefficient of thermal expansion could potentially produce a point of weakness with the structure resulting in failure of the concrete within these regions.

References 1.

E. Kirhikovali, Polymer Engineering and Science, 1981, 21, 8, 507.

2.

D.W. Fowler in Proceedings of the 43rd Annual Conference and Focus ’88, Cincinnati, OH, USA, 1988, Session 16-B.

3.

J.W. Prane, Adhesives Age, 1985, 28, 9, 48.

4.

T.P. O'Brien, W.B. Long, P.C. Hewlett, J.D.N. Shaw, L.J. Tabor and P.J.

81

Polymers in Cementitious Materials Winchcombe, Repair of Concrete Damaged by Reinforcement Corrosion, TR26, The Concrete Society, Crowthorne, UK, 1984. 5.

Plastics in Building Construction, 1988, 12, 7, 2.

6.

A. Blaga and J.J. Beaudoin, Polymer Modified Concrete, Canadian Building Digest No.241, National Research Council Canada, Division of Building Research, Ottawa, Canada, 1985.

7.

J.D.N. Shaw, P.J. Brown, R. Cather, R. Dennis, P.C. Hewlett, R.A. Johnston, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Polymers in Concrete, 2nd Edition, TR39, The Concrete Society, Crowthorne, UK, 1994.

8.

I. Berkovitch, Civil Engineering (London), 1984, August, 45.

9.

D. Feldman, Polymer News, 1993, 18, 9, 261.

10. M. Steinberg, Polymer-Plastics Technology and Engineering, 1974, 3, 2, 199. 11. C. Ellis, High Performance Plastics, 1989, 6, 6, 1. 12. I. Nelson, Multi Functional Crosslinking Acrylic Polymers for Concrete and Mortar in Proceedings of the ConChem Conference, Dusseldorf, Germany, 1997, p.1-8. 13. A. Blaga and J.J. Beaudoin, Polymer Concrete, Canadian Building Digest 242, Division of Building Research, National Research Council Canada, Ottawa, Canada, 1985. 14. A Concise Dictionary of Chemistry, Ed., J. Daintith, 1990, Oxford University Press, Oxford, UK, 230. 15. V.R. Riley and I. Razl, Composites, 1974, 5, 1, 27. 16. A. Blaga, Thermosetting Plastics, Canadian Building Digest No.159, 1974, Division of Building Research, National Research Council Canada, Ottawa, Canada. 17. J. Bensted in Advances in Cement Technology, Ed., S.N. Gnosh, Pergamon Press, Oxford, UK, 1983, 307. 18. S. Diamond in Hydraulic Cement Pastes Conference: Their Structure and Properties, Cement and Concrete Association, Slough, UK, 1976, p.2-30.

82

Polymer Concrete 19. I. Jawed, J. Skalny and J.F. Young in Structure and Performance of Cements, Ed., P. Barnes, Applied Science, London, UK, 1983, p.237. 20. H.F.W. Taylor, The Chemistry of Cements, 1966 Lecture Series, Series Number 2, The Royal Institute of Chemistry, London, UK, 1966, p.1-2. 21. L. Czarnecki, A. Garbacz, P. Lukowski and J.R. Clifton, Optimization of Polymer Concrete Composites, Final Report, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, 1999. 22. H.L. Manock, New Developments In Polyurethane and PU/Acrylic Dispersions, Industrial Copolymers Ltd, Preston, UK. 23. Pigment and Resin Technology, 1996, 25, 1, 30. 24. M. Cook, Pigment and Resin Technology, 1996, 25, 1, 12. 25. S. Chandra, Cement and Concrete Composites, 1992, 14, 4, 289. 26. Pitture e Vernici, 1991, 67, 3, 47. 27. P.I. Ku, Advances in Polymer Technology, 1988, 8, 1, 81. 28. P. Maslow, Proceedings of the ACS Division of Organic Coatings and Plastics Chemistry, Honolulu, Hawaii, USA, Volume 40, p.348. 29. J.D.N. Shaw, Civil Engineering (London), 1983, July, 24. 30. J.A. Manson, Materials Science and Engineering, 1976, 25, 41. 31. Araldite Epoxy Resins, Ciba Geigy (UK) Ltd., Plastics Division, Cambridge, UK, 1984. 32. Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 186. 33. D.H. Lutz, Modification of Mineral Plasters with Hydrophobic Polymeric Binders, Wacker Polymer Systems GmbH & Co. KG, 1 Burghausen Germany, 999. 34. M.A. El-Mikawi and A.S. Mosallam in Proceedings of SPI Composite Institute 51st Annual Conference, Cincinnati, OH, USA, 1996, Paper 3-D. 35. M. Motuku, U.K. Vaidya and G.M. Janowski, Smart Materials and Structures, 1999, 8, 5, 623.

83

Polymers in Cementitious Materials 36. Civil Engineering, 1987, April, 39. 37. W.O. Nutt in Symposium on New Polymer Applications in the Building and Construction Industry, Surrey University, Guildford, UK, 1973, p.11. 38. B.T. Tran, T.J. Toerner and P.E. Titus, inventors; Power Lone Star Inc., assignee; US Patent 5573855, 1996. 39. S. Chandra, Cement and Concrete Composites, 1991, 13, 4, 277. 40. J.T. Dikeou in International ICPIC Workshop on Polymers in Concrete for Central Europe, Bled, Slovenia, 1996, PC Session. 41. R. Kreis in International ICIPC Workshop on Polymers in Concrete for Central Europe, Bled, Slovenia, 1996, Industrial Applications Session. 42. W.C. Allen, inventor; Union Oil Company of California, assignee; US Patent 5384355, 1995. 43. D.R. Paul and D.W. Fowler, Journal of Applied Polymer Science, 1975, 19, 1, 281. 44. K.W. Harrison in Adhesion 12, Ed., K.W. Allen, Elsevier Applied Science Publishers Ltd., London, 1987, p.121. 45. P. Seidler in International ICPIC Workshop on Polymers in Concrete for Central Europe, Bled, Slovenia, 1996, Industrial Applications Session.

84

Polymer Portland Cement Concrete

4

Polymer Portland Cement Concrete Author

4.1 Introduction From the mid to late 20th century, the manufacture and subsequent uses of polymers within the construction industry has grown substantially [1, 2]. Actual interest in using polymers to modify the physical properties of cementitious materials escalated from the 1950s [36]. In 1956, the first polymer modified bridge overlay was produced by incorporating a styrene butadiene rubber (SBR) into the concrete mortar, which was subsequently applied to the Cheboygan bridge in Michigan, USA [7]. From this point the use of such products for this type of application along with concrete repairs intensified. Common problems associated with the initial polymers used in the modification of cementitious systems included a susceptibility to moisture and deterioration due to weathering [8]. A reduction in composite density due to an increase of air trapped during the mixing process and hence heightened porosity was also noted [9]. Whilst the ability of the polymer to effectively film form to its fullest capacity decreased as the thickness of the applied section increased. Developments in polymer technology have been tailored to address these issues and subsequently minimise such problems whilst maintaining the benefits such polymers impart when used as a cement additive [10]. The end use of the cementitious materials and the environment into which it to be placed will determine the quantity and type of polymer incorporated [11]. Today with the development of polymer modified repair mortars, self-smoothing cementitious floor screeds, paving, tile adhesives as well as the actual latex being utilised as a primer system, the use of such polymers is enormous [8, 11, 12]. In the vast majority of polymer modified applications SBR and acrylics have been utilised due to their high performance, stability, bond adhesion and flexibility [3, 6, 13].

85

Polymers in Cementitious Materials

4.2 Brief Summary of the Advantages of Incorporating Polymer into Cementitious Materials An increase in the flexural and tensile strength of the resultant cementitious system has been noted when a latex emulsion has been incorporated into the latter [2, 4, 9, 12-17]. This in certain products such as two component latex modified cementitious underlayments, enables such materials to be applied over substrates which require minimal surface preparation such as old tile adhesive, asphalt and quarry tiles. However, from the mid part of the 20th century the increasing demands for high performance materials within the construction industry perpetuated a demand for far more technologically advanced and adaptable products such as cementitious flooring, repair mortars and grouts [2, 4, 9, 14-16, 18]. Such products were required to possess the desired workability in combination with a minimum drying time, and to rapidly gain strength within the first 24 hours of room temperature cure [12, 19]. A heightened degree of abrasion resistance along with a general reduction in the ability of water to be absorbed into the modified composite and enhanced chemical and frost resistance are generally achieved [13, 14, 17, 18]. Indeed the structural and technical demands of most modern buildings necessitate the use of polymers in one form or another in combination with cementitious materials [13].

4.3 Physical Requirements of Latexes in Order to be Suitable as Cement Additives In order for a latex (polymer dispersion) to be compatible for use as a cement additive it has to meet certain requirements [9, 20, 21]. It must be chemically stable towards ionic species such as Ca2+, Al3+ which are produced during the hydration of the cement phase [9]. These ionic species are involved in the formation of the calcium silicate hydrates (C-S-H) which primarily constitute the hardened cementitious microstructure [1, 22-26]. Hence, if the active species react with the added polymer, the development of the microstructure would be hindered and thus the resultant physical properties, such as strength, would be seriously affected. One of the primary problems of adding a polymer constituent, such as emulsions, monomer(s), or pre-polymers, into fresh concrete was the increase in stickiness which could potentially cause problems during the application and finishing of the final product [27]. In order to resolve this issue, polymers have been formulated which do not impart this ‘tacky’ effect due to their ‘high glass transition temperature’ (Tg). No skinning upon the surface of the modified mortar, concrete was noted which had been commonly witnessed previously. The addition of the latex should not adversely entrap any air within the slurry during the mixing process and it should have a high degree of stability towards mechanical sheer [1,

86

Polymer Portland Cement Concrete 9, 20]. A polymer film should assemble throughout the cementitious structure even at low temperatures (i.e., 5 °C), and bond to the aggregates present. The polymer film should bond to any hydrate phases, i.e., calcium silicate hydrates precipitated within the resultant matrix [9, 11, 28]. The particle size and distribution of the polymer particles also influence the properties of the dispersion and how the polymer subsequently ‘film forms’ [8, 29]. An ability to withstand a wide variety of temperatures and still perform is also required [9]. A continuous film forms once the water present has evaporated and the remaining polymer particles have agglomerated [1, 8, 14, 28]. As water is removed by evaporation or consumed chemically during cement hydration, the polymer particles initially come into closer contact with each other. These polymer molecules subsequently deform and coalesce to produce a series of polyhedral particles, which make up the resultant film [14]. When dispersed throughout a cementitious mortar, the polymer must maintain a degree of flexibility and mobility as the microstructure forms [1, 14]. In order to enable the formation of the film within a cementitious material it is imperative that the temperature at which particles adjoin to one another is below that required to promote the hydration and development of the host cement based matrix, (i.e., room temperature). If this were not the case, the cementitious matrix could form and harden before film formation of the polymer was initiated and hence the full benefits of incorporating this polymer would not be observed [17]. For every polymer there is a minimum temperature at which film formation can effectively occur. This is termed the ‘minimum film formation temperature’ [1, 9, 14]. Below such conditions a powdery residue is formed rather that a continuous film [1].

4.4 Polymer Emulsion A polymer emulsion is principally ‘a dispersion of organic polymer particles in water’ [8, 20, 30]. Indeed a latex normally comprises a dispersion of fine particles in an aqueous solution, i.e., water. Latexes (polymer emulsions) are commonly formed by an ‘emulsion polymerisation’ process [1, 2, 6, 9, 20, 31, 32]. This procedure involves the addition and reaction of a designated ‘monomer’ (i.e., vinylacetate for vinylacetate latexes), a co-monomer where necessary (vinylester, versatic acid), a surfactant, initiator and water. Latexes can possess a rubbery consistency and hence be elastomeric in nature like SBR, natural rubber, polyacrylonitrile and polychloroprene [8, 10]. Alternatively they can be thermoplastic, hence the structure becomes more elastic and pliable when exposed to chemicals or heat, i.e., polystyrenes, polyvinylacetates, copolymers of styrene and vinylacetates. Depending on the surfactants utilised during the polymerisation reactions the latex polymers can be positively (cationic), negatively (anionic) charged or possess no charge

87

Polymers in Cementitious Materials whatsoever (non-ionic) [8, 9, 20]. Non-ionic polymer particles are generally the most suitable for use with Portland cements [8, 20].

4.4.1 Manufacture of Latex Emulsion

4.4.1.1 General Preparation Generally a number of ‘emulsion polymerisation’ methods can be used to manufacture these dispersions [8, 33, 34]: 1. Semi-continuous polymerisation, 2. Batch polymerisation, 3. Continuous polymerisation. The monomer is added in certain proportions over a specific timescale in the semicontinuous method. Whilst, the entire monomer concentration is added in a single addition during the batch manufacturing process. Typically 20 tons of polymer emulsion will be produced by the batch or semi-continuous process [8]. After the final additions of the monomer and the initiators, the temperature of the reaction mixture is maintained at a particular setting to allow the latter to polymerise, resulting in the minimum concentration of monomers remaining. This quantity will obviously be determined by the acceptable limits for a particular monomer. The emulsion is then allowed to cool and commonly sieved to remove any agglomerates [20, 31]. Simply, the procedure for polymerisation involves the addition of a proportion of monomers, water, stabilisers and any additional components where necessary, which are subsequently combined within an appropriate reactor under the necessary conditions (i.e., heat, agitation) [1, 2, 5, 8, 20, 32]. The monomer is initially distributed throughout the aqueous medium in the presence of an emulsifier. Subsequently the appropriate initiator is added and reaction conditions tailored to initiate the formation of free-radical species and hence the polymerisation of the monomer present [35]. The initiator basically produces a free radical, which then initiates the monomer to polymerise and form a polymer by chain addition [1, 5, 20, 31]. Typical initiators are persulfates, which decompose at relatively high temperatures to produce the free radicals necessary to promote monomer polymerisation [9, 20, 31]. Alternatively, peroxides and hydrogen peroxides can produce radical species by the transfer of an electron or dissociation at lower temperatures [9, 31, 36]. This method is therefore the most economical in terms of energy requirements.

88

Polymer Portland Cement Concrete The other components incorporated during the emulsion polymerisation process are added to obtain the required particle size of the resultant polymer and desired molecular weight and pH [1, 20]. By the addition of specific chemicals (i.e., surfactants) or alterations in temperature the polymerisation process progresses to a point where an acceptable concentration of monomer remains [2, 8, 20, 31]. This concentration will be dependent on the type of monomer used. The pH, total solids content and viscosity of the resultant polymer emulsion is normally checked to determine whether there were any problems during the synthesis and to make sure that the desired polymer material was produced [20]. In certain circumstances, the polymer particles undergo further polymerisation with secondary monomer constituents [37]. This process is called ‘graft copolymerisation’ The chosen secondary monomer reacts with active groups situated in appropriate sites on the polymer. The addition of a suitable initiator and subsequent formation of free radical species again initiates copolymerisation between the previously formed polymer and secondary monomer.

4.4.1.2 Monomer Composition The monomer constituents are commonly organic in nature, contain a double bond within the carbon chain and a functional group such as COOH or Cl in an adjacent position to the latter [31]. The decomposition of the initiator results in the formation of a free radical species, which subsequently causes the double bond in the monomer to break and forms a new bond with the radical species, thus forming a new reactive compound [20]. The latter then reacts further with other monomer units and forms a polymer by chain addition [1, 31, 38]. The type of monomer utilised in the polymerisation process is determined by the physical properties required of the resultant polymers, such as flexibility, hardness and polymer formation temperature [31]. Table 4.1 shows the physical properties of typical monomers used in the formation of emulsion/redispersible polymer powders.

Table 4.1 Typical rigidity of common monomers used to manufacture latex emulsions. Adapted from [1, 31, 39] Monomer composition

Physical properties

Vinyl acetate

Hard

Styrene

Hard

Butadiene

Soft

Acrylic acid

Hard

89

Polymers in Cementitious Materials

4.4.1.3 The Effect of Surfactant Addition Surfactants are added during the initial stages of the polymerisation process and also during delayed nucleation (i.e., grafting) [1, 8, 9, 20, 31]. They are commonly incorporated to stabilise the latex dispersion [1]. Part of the surfactant molecule is hydrophilic in nature and hence has an affinity with water along with an hydrophobic segment [31, 40-42]. The chemical nature of the hydrophilic and hydrophobic segments determines whether the surfactant is cationic, anionic or non-ionic [8, 9, 20]. The surfactant molecules agglomerate at a certain concentration determined by their nature to form micelles which form the hydrophilic segment giving water solubility [31, 40, 41]. The hydrophobic part of the surfactant molecules is housed within the micelle [41]. Typical surfactants used in emulsion polymerisation include quaternary ammonium salts (cationic), sodium lauryl sulfates (anionic) sodium dodecyl sulfate (anionic) and nonyl phenol polyoxyethylene (non-ionic) [31, 40, 41] The surfactant carries out a number of functions and its addition influences the nature of the final latex emulsion produced [1, 2]. Principally, the rate at which polymerisation occurs, particle sizes of the nucleated polymer as well as the stability of the polymer particles are influenced by the surfactant [40, 41]. Research into the nucleation of polymer particles and the role of surfactants in the emulsion polymerisation process is complex and is outside the scope of this publication. The addition of a surfactant aids the dispersion of cement particles throughout the mortar as it acts as a plasticer and hence the workability of the mortar improves [1, 20].

4.4.1.4 Formation of Free Radical Species Free radical species can be formed by an oxidation and reduction process, by the heat of the reaction or light [36]. A common oxidising agent is hydrogen peroxide [9, 20, 31, 36]. A chemical reaction is initiated whereby the latter loses an electron and thus is effectively oxidised during this process which results in the formation of the free radical species [36]. The advantage of producing the reactive species in this way is the temperature requirements for promoting the emulsion polymerisation of the monomer constituents are lower. The higher the quantity of surfactant used during manufacture, the greater the stability against the agglomeration of the polymer particles [8]. On average the quantity of surfactant remaining within the synthesised dispersion is within the region of 2-4% by weight [20]. The reaction rate of the polymerisation process is governed by the actual quantity of the initiator added, whilst the proportion of the monomer made available controls the overall molecular weight of the polymer molecule formed by chain addition [31].

90

Polymer Portland Cement Concrete

4.4.1.5 Other Ingredients Added into Latexes Antioxidants and ultraviolet protectors are also commonly incorporated into the resultant latex emulsion to prevent the molecular structure decomposing and maintain a good shelf life (normally 6-12 months from actual date of manufacture) [1, 8, 32]. Additives such as biocides are also added to prevent bacterial growth inside the medium and hence extend the active use of the resultant product. The performance should not be impaired when subjected to cold or hot environments [9]. Whilst a reduction in the ability of water to enter the hardened structure, and a high resistance to alkalis and acids is also favoured. Antifoamers are normally added when latexes are used to polymerise modified cement based materials as the addition of the latex encourages the entrapment of air within the resultant mortar or slurry [1, 8, 9, 20]. This adversely increases the proportion of voids present within the hardened matrix [9]. These pores in turn become a point of fracture when the material structure is subjected to a load [43, 44]. In the early days, antifoamers such as silicone oil were added directly to the slurry or mortar during the mixing process [10]. Increased aeration of the mix has also been linked with the presence of surfactants within the latex. However, the latexes commercially available today either as powders or emulsions normally contain effective antifoamers in order to maintain the degree of air entrapment to an absolute minimum [1, 9, 20, 32, 45].

4.5 Redispersible Polymer Powders

4.5.1 Introduction As well as emulsions, latexes are also available in a solid form termed ‘redispersible polymer powders’ [1, 8, 9, 14-16, 29, 32]. The redispersible polymer powder incorporated into the cementitious material subsequently polymerises upon contact with water [12, 14, 15, 29, 32, 46]. Research into this area has shown that the incorporation of such polymer powders into cement-based materials inherently improves the flexural strength but has a much greater impact on the tensile strength of the resultant hardened composite [1, 12, 14-17]. However, their effect upon the compressive strength is much less pronounced and analysis has shown a slight reduction in this parameter to be common. It should be noted that the water to cement ratio, and porosity of the hardened cementitious structure principally governs the compressive strength [1, 5, 9, 47]. A heightened degree of workability is achieved by adding polymers to the mix, thus less water is effectively required to obtain the desired consistency [9, 14, 47].

91

Polymers in Cementitious Materials Today, redispersible polymer powders are commonly used in the manufacture of onepack polymer modified cementitious floor screeds, underlayments and repair mortars [8, 18, 29, 48]. These systems basically comprise a blended cementitious material (cements, aggregates, superplasticers, antifoamers etc) which generally contain 1-3% redispersible polymer powder to improve the flexural/tensile strength and bond adhesion [48, 49]. Certain techniques are utilised during the manufacturing process of redispersible polymer powders [9, 14, 15, 29, 32, 46, 50]. Initially, emulsion polymerisation of the latex occurs which can be either a pressurised or non-pressurised process. This is subsequently followed by formation of the spray mix formulation, which is a wet blending procedure before the final stage encompassing the actual spray drying process. Since these polymer powders are produced from a water-based dispersion no solvents are used during the manufacturing process hence they are free of volatile organic compounds and thus environmentally friendly [14, 51]. All redispersible polymer powders are thermoplastic in nature and possess a Tg below 30 °C [15, 17]. A number of companies are based in the UK, which manufacture and supply redispersible polymer powders to the market place for use in the formulation of high performance polymer modified construction products [15, 18, 46, 52, 53].

4.5.2 Conception and Development As described earlier, the first significant advances in synthetic polymer technology occurred during the 1930-1940s. Initially the redispersible polymer powders manufactured throughout the 1950s were based on polyvinyl acetate (PVA) [15, 29, 53, 54]. Use of this type of polymer system escalated as the advantage of single pack cementitious systems was increasingly noted [46, 53]. However, a major development in redispersible powder technology occurred in the 1960s when pressure co-polymerisation was first successfully completed resulting in the synthesis of co-polymers of vinyl acetate [53]. This procedure removes the necessity for the addition of a plasticiser and the co-polymers of vinyl acetate formed were technologically more advanced than the previous formulated PVA. The manufacture and uses of redispersible polymer powders escalated during the 1970s and today homopolymers and copolymers of vinyl acetate combined with ethylene, styrene and vinyl ester of versatic acid and vinyl laurate are typically produced [1, 15, 16, 18, 50, 53, 55]. Their inclusion into cementitious mortars enables the resultant materials to achieve the structural requirements demanded by modern day construction.

4.5.3 Spray Drying Procedure Redispersible polymer powders are principally manufactured by a spray drying procedure [1, 14, 15, 29, 32, 46, 50]. The desired polymer dispersion is initially produced via the

92

Polymer Portland Cement Concrete emulsion polymerisation process previously discussed (Figure 4.1). To which a watersoluble protective colloid such as polyvinyl alcohol (PVOH) is added in order to prevent the solid particles agglomerating resulting in a lumpy consistency instead of a free flowing dry powder [1, 8, 14, 15, 29, 32, 46]. A secondary colloid such as a high molecular weight polymer or a melamine sulfonate condensate may also be used depending upon the chemical nature of the polymer being manufactured [15]. Droplets of polymer dispersion are produced via an atomising device during the spray drying procedure where the water insoluble polymer particles are contained within the protective colloidal matrix [15, 29, 32]. This procedure effectively forces the polymer particles to coalesce into agglomerates [1, 8, 14]. The aqueous phase is subsequently removed by evaporation as the chamber is flooded with hot air, (i.e., 170-200 °C), and a dry polymer powder remains [32]. This powder subsequently passes through a cyclone which removes any contaminates present. An anticaking agent is then added to the polymer powder to hinder any coalescing of the particles during storage or the application of a load. Normally this substance is calcium carbonate, silica or clay in origin and approximately 10-13% is added [1, 2, 8, 14, 15, 29, 32]. This free flowing powder then passes to the bagging plant [32]. Redispersible polymers commonly used in conjunction with cement tend to have 25% of the bulk density of the cementitious phase. In fact in reality the polymer particles which constitute the resultant dry powder are actually in a ‘quasi-solid state’ [14]. Since the polymer is primarily thermoplastic in

Figure 4.1 Formation of redispersible powder and redispersion in water Provided courtesy of and reprinted with permission of Elotex

93

Polymers in Cementitious Materials nature it will only become a true solid when the local environment is below the Tg. Above this value the material keeps a degree of solidity due to its molecular constituents being interconnected. The presence of a free emulsifier within the redispersible polymer also improves the workability and hence reduces the volume of water required, whilst an antifoamer prevents the entrapment of excess air during the mixing process which ultimately increases the porosity and reduces the resultant strength [13, 16]. In order to maintain the physical characteristics, i.e., flow and chemical composition of the resultant redispersible polymer powder, the material should be stored away from direct sunlight at a temperature below 30 °C [32]. This is to prevent the polymer particles agglomerating together. It is also generally advised that the pallets should be stored individually and not stacked on top of each other.

4.5.4 Particle Size of Redispersible Polymer Powders The majority of modern day redispersible polymer powders are vinyl acetate co-polymers where the secondary monomer is ethylene, styrene, vinyl ester of a versatic acid or vinyl laurate [1, 15, 16, 32, 46, 51, 53]. Analysis of the particle size distribution of a typical redispersible vinyl acetate ethylene (VAE) powder indicates an average diameter of 80 microns [32]. However, this size is misleading, as it is an actual measurement of the average dimensions of an agglomerate of latex particles. When in contact with water the latter disintegrates and a much more accurate assessment of the size grading is obtained. Generally, the particle size varies between 0.5-5 microns [9, 32]. The particle size of the powder is controlled during the polymerisation process to maintain the same characteristics of the original polymer emulsion. The particle size of the redispersible polymer powder also governs its ability to film form efficiently and the binding power of the polymer within the cementitious matrix [8, 29]. The properties of this type of polymer vary in accordance with the proportion of vinyl acetate to ethylene.

4.5.5 Advantages of Adding a Redispersible Polymer Powder There are a number of essential factors, which make the use of redispersible polymer powders more favourable over the addition of a standard latex emulsion [18, 29, 32, 51]. Principally the manufacturing and packaging costs are lower. The need for a protective biocide and isolation from frost are also removed, as a powder is not affected in the same manner as an emulsion [29, 46, 51, 52]. Inclusion of this type of polymer system is also more environmentally friendly as there is less packaging which requires disposal at the end

94

Polymer Portland Cement Concrete of a particular operation [18, 29, 32, 46, 51]. The risk of adversely affecting the desired cementitious product by incorrectly adding the polymer constituent to the cementitious component is also removed [8, 16, 29, 32]. The manufacturers have investigated the advantages of adding these types of solid polymer powders into cementitious materials [14-17, 29, 46, 51, 53]. These have been summarised as improved workability and an extension of initial setting time along with heightened strength characteristics (flexural and tensile), a reduction in drying shrinkage along with an enhanced degree of impact/abrasion resistance [16, 29, 55]. The synthesis of redispersible polymer powders has therefore enabled the development of single pack polymer modified products [46]. A number of which have been designed to be fast setting and rapidly gain strength over the first 24 hours, thus the curing time required to achieve a useable surface was dramatically reduced [12, 19]. Thus the time the actual area undergoing renovation is out of action can be restricted to a minimum [56]. Such systems have the obvious advantage of reducing the actual requirement for on-site mixing of the polymer and cement-based product [29, 57]. A single pack system is much easier to mix efficiently on site, reducing the need for more specialised equipment [29]. A saving in terms of manpower costs was also achieved as the necessity to hire a highly skilled work force was no longer paramount due to the ease of use associated with these products [18, 29].

4.5.6 Rehydrating Redispersible Polymer Powders Essentially upon contact with water the polymer particles contained within the colloid are released as the colloid is water-soluble and hence dissolves. The polymer particles then disperse in the water and effectively form an emulsion of the same chemical composition to that initially spray dried. In order to achieve the polymer dispersion it is essential that the protective covering encasing the polymer particles dissipate easily [1, 29]. In fact in order to achieve complete dispersion of the polymer powder the chemical nature, molecular weight and proportion of PVOH colloid incorporated during the spray drying procedure has to be carefully assessed and controlled during the manufacturing process [1]. The PVOH colloid is normally consumed within the aqueous phase or dispersed within the cements, aggregate or filler components present within the cementitious mixture being polymer modified [15, 29, 32]. Thus once in contact with the aqueous phase, the polymer particles polymerise and form a polymer network throughout the hardened cementitious microstructure [9, 32]. In the event of the protective covering not being removed the enclosed polymer particles cannot partake in the polymerisation and film forming process [29].

95

Polymers in Cementitious Materials

4.5.7 A Summary of the Various Redispersible Polymer Powders Commercially Available For several years the producers of redispersible polymer powders have manufactured numerous grades suitable for use in a wide variety of construction products [46, 51, 53]. These types of redispersible polymer powders vary in colour from white to a pale yellow and generally possess a particle size between 50-120 μm [46]. However, once dispersed throughout an appropriate aqueous phase the particle size of the individual polymer molecule has been registered between 0.5-5 μm. The degree of moisture present within the resultant powder is commonly less than 1% by weight of the total weight of the powder. The pH of the resultant dispersion is normally inbetween 4-12 depending upon the chemical nature of the polymer such as presence of acrylics or VAE. The minimum temperature at which the polymer particles will coalesce to form a film can vary between 0-18 °C depending upon the chemical nature of the resultant polymer. Below this temperature the ability of the polymer particles to coalesce and film form is restricted. Hence the full benefits of incorporating a polymer into the cementitious matrix will not be achieved. A Tg on average within the region of –5 to 25 °C has been noted for these types of polymers.

4.5.8 Typical Chemical Composition of Modern Day Redispersible Polymer Powders Vinyl acetate homopolymers are recommended for use in adhesives such as wallpaper adhesives [51]. A series of co-polymers are also produced by the incorporation of secondary monomer (i.e., ethylene, vinyl ester of versatic acid (VeoVa)) and acrylic. Research has shown that increasing the concentration of ethylene utilised improves the flexibility whilst incorporating vinyl versatate increases the hydrophobic nature of the resultant polymer [46]. When this polymer is used to in conjunction with a cementitious material the resultant cured cement based material should possess a higher resistance to the intrusion of water into the cementitious microstructure. VAE co-polymers powders redisperse within an aqueous phase very efficiently and depending upon the concentration of the monomer components and additional additives a highly flexible or a hard polymer film is formed possessing a degree of elasticity [15, 32]. The Tg of the polymer increases in unison with the degree of hardness [51]. A minimum film forming temperature for this type of polymer is in the region of 0-5 °C [15, 29]. The workability and flow of the modified cementitious material will also be influenced by the particle size of the polymer particles added. A number of VAE grades are suitable for use in cement, lime-based mixtures, i.e., plasters, tile grouts and thermal insulation systems [32].

96

Polymer Portland Cement Concrete The minimum film forming temperature for PVA is typically 18 °C [29]. The ability to redisperse in an aqueous phase and the binding power of this type of polymer is slightly poorer than that for VAE. A harder polymer film is also produced, hence the degree of flexibility compared to the former is lower. Other secondary monomers commonly used in conjunction with vinyl acetate are VeoVa and vinyl laurate. The Tg of these copolymers is generally higher than that noted for VAE polymers. The minimum film formation temperature (MFFT) for vinyl acetate/VeoVa are lower than that noted for VAE polymers commonly between 4-6 °C [15, 51]. A good degree of flexibility/toughness is noted for these types of co-polymers as well as them being hydrophobic in nature [15]. Subsequently these types of polymers are generally suitable for use within self-smoothing floor screeds, tile adhesives and repair mortars due to the heightened resistance to moisture intrusion into the polymer modified cementitious microstructure.

4.6 The Formation of the Polymer Modified Cementitious Matrix The polymer modification of Portland cement concrete and other cementitious products generally involves the addition of a polymer in the form of either a liquid dispersion or a solid powder [1, 8-10, 20, 21, 45, 46]. The polymer component cures in harmony with the developing cementitious matrix and subsequently forms a continuous film throughout the microstructure [9, 14]. Ultimately, the properties of the modified cement-based material will depend upon the chemical nature, quantity added and degree of polymerisation within the polymer along with the composition and proportion of the cement utilised [1, 5, 20]. Indeed, the molecular weight of the polymer, ability to crosslink, chemical composition of the dispersion and nature of the surfactant utilised in the manufacture of the redispersible powder will also determine the final physical properties of the resultant cementitious structure [1]. However, it is important to note that the environment in which the cement-based material is cured, i.e., temperature, humidity and so on, also have to be considered as such factors will influence microstructural development and hence the final properties achieved. If a two-pack system (separate liquid and powder component) is being used then the latex is mixed with the concrete or required cementitious material on site [1, 45]. Experimentation has shown that it is desirable to add the cementitious material (i.e., blended cement, aggregates), to the latex rather than the other way around as the workability of the resultant mortar is reduced which makes the mixing process and placement more difficult [10]. This effect has been attributed to the fact that the latex is absorbed rapidly into the dry material and hence the slurry becomes thixotropic very quickly.

97

Polymers in Cementitious Materials Upon mixing the latex (whether in form of a liquid of dehydrated powder) particles are dispersed throughout the cementitious medium [9]. In order to achieve a uniform matrix and the desired physical properties (i.e., strength, flexibility and durability), it is imperative that formation of the incorporated polymer film and development of the cementitious microstructure proceeds to its maximum level [9, 14]. Hence the interaction of the polymer and cement constituents must not adversely hinder the curing of each of the individual components [9]. Initially, hydration of the cement phase occurs via the dissolution of the ionic species from the surface of the cement grains and subsequent precipitation of the C-S-H, ettringite (Aft phase), calcium mono-aluminosulfate (Afm phase) and other minor phases/compounds which make up the hardened cementitious structure [1, 22-26]. During this process the polymer particles merge together to form ‘thread like membranes’ between any aggregate present and the discrete hydrate phases nucleated within the microstructure [10, 14]. Analysis has shown the addition of polymers such as polyacrylic esters into a cementitious matrix results in a interaction between the polymer film and the surfaces of the various aggregates present. This interaction also occurs with any calcium hydroxide crystals and hydrate phases such as calcium silicate hydrates which form within the cementitious microstructure during the hydration and curing process [9]. The polymer film forms within the cementitious microstructure at the same time as the microstructure forms. The development of these polymer films increases the bonding of the discrete hydrate phases such as calcium silicate hydrates and reduces the tendency of the microstructure to incur microcracking when exposed to particular degrees of strain, especially within the early stages of hydration [1, 10, 14, 53]. Indeed, an improvement of the bond adhesion of the polymer modified resultant cementitious composite when applied to a substrate such as old concrete in relation to an unmodified cementitious compound has been observed. The degree of microcracking within the structure reduces as the concentration of polymer incorporated into the cement based compound is increased [7, 9, 10, 14, 53]. A degree of chemical resistance and an increase in the durability is also observed for polymer modified cementitious material although this will obviously vary depending upon the nature of the resultant polymer incorporated [1, 4, 10]. Evidence suggests that the most favourable degree of polymerisation throughout the cementitious matrix will be achieved when a dry polymer solid weight of 10-20% is utilised [7]. Below this level the permeability of the structure to water, chemical stability along with the degree of flexibility of the cementitious microstructure will obviously reduce. However, increasing the proportion of the polymer to any significantly level above the quoted values will not greatly improve the previously physical properties stated. As the actual polymer is by far the most expensive component when incorporated into a cementitious mixture the cost implications adversely outweigh the benefits. An increase in the degree of air trapped

98

Polymer Portland Cement Concrete within the slurry has also been observed when the concentration of polymer surpasses this optimum level. This will obviously affect the porosity of the cementitious microstructure resulting in the formation of an excess of large voids within. Subsequently these voids are more susceptible to fracture when exposed to a load [9, 43, 44]. Research has also indicated that stabilisers commonly used in the manufacture of certain types of polymer dispersion, (i.e., polyvinyl styrene), can adversely effect the workability of the mortar [10]. Hence the use of polymer dispersions containing this compound should be avoided or the reduction in workability compensated for by the addition of super plasticisers. In the past furans and polyester styrene have been used to modify Portland cement concrete [58]. However, problems were encountered due to the high alkalinity of the cement phase, which is on average pH 13 and subsequently prevented polymerisation [21]. Efficiently mixing immiscible organic compounds with an aqueous phase also has caused problems.

4.7 Introduction to the Benefits of Modifying Cementitious Materials with Polymer Dispersions Latexes are utilised in the formation of a multitude of cementitious materials [1, 4, 810, 20, 32, 45]. The advantages of incorporating polymers into the mix are a general improvement in the flexibility of the resultant cementitious material and a reduction in the modulus of elasticity. An improvement in the bond adhesion of the polymer modified cementitious material to the chosen substrate, (i.e., brick, concrete), along with an enhanced chemical/carbonation resistance is also observed. Non-ionic polymer dispersions are generally used in combination with hydraulic cements as those possessing a charge do not have the desired chemical stability and can adversely react with the ionic species, (i.e., Ca2+ Al3+), which are involved in the formation of the C-S-H [8, 20]. Hence non-ionic surfactants are generally used in the manufacture of most polymer dispersions as the nature of the surfactant governs the resultant charge of the polymer formed during the emulsion polymerisation process. The polymer (either in a liquid or powder form) utilised as a cement additive will also depend on the cost and the material in which it is likely to be incorporated into [9, 14]. Acrylics are the most expensive polymer dispersion due to their exceptional physical properties followed by SA and SBR, then VAE and subsequent copolymers of vinyl acetate. PVA homopolymers are generally the most economical as their performance tends to be limited in relation to the polymers mentioned previously. The following section provides

99

Polymers in Cementitious Materials a brief description of the physical properties of the various types of polymer dispersions typically used in conjunction with cementitious materials.

4.7.1 Acrylics Acrylic polymers are generally used when the performance of the resultant modified material has to be high, especially in the case of repair mortars and industrial flooring [13]. This type of polymer is the most expensive to use [7]. Acrylics have a good degree of resistance to a wide variety of chemicals, oils, mineral and organic acids diluted in an aqueous phase as well as most alkalis either in a diluted or concentrated form [10, 39]. However, this polymer is not stable in the presence of alcohols, ethers, esters, aromatic and chlorinated hydrocarbons or strong acids such as nitric acid or hydrochloric acid. Although the chemical resistances of the majority of the latex modified systems are improved in relation to the base formulae there are still a number of chemicals, which can attack such polymer modified cementitious structures. These include organic solvents, organic and inorganic acids and sulfated compounds. The application of cementitious materials containing acrylics should be avoided in areas where exposure to such chemicals is likely. Today, acrylic-based polymers are available which incorporate a unique monomer within the backbone [59, 60]. The presence of cement encourages this special combination of molecular constituents to crosslink to form a polymer network throughout the cementitious matrix [60]. The standard benefits discussed previously (flexural/tensile strength and workability) are achieved when using this type of polymer as a cement additive for the modification of the cementitious materials such as floor screeds, mortars and grouts. However, encouraging the polymer to crosslink reduces the inherent problem of softening the matrix. This is especially true if a high proportion of polymer is used, such as 20% by weight, which in turn lowers the compressive strength achieved. A good resistance to a variety of organic and inorganic chemicals such as hydrochloric acid, sulfuric acid, sodium hydroxide and diesel fuel has also been observed when incorporating this type of acrylic into cementitious materials. Antifoamers have been developed which prevent aeration of the slurry when the desired cementitious material is mixed with water [9, 10, 60]. Generally, the incorporation of such antifoamers does not adversely affect the workability and consistency of the cementitious material.

4.7.2 Styrene Acrylics Copolymer dispersions of styrene acrylic (SA) esters where the secondary monomer is butyl acrylate are also commonly used as a cement additive [13, 61]. These types of

100

Polymer Portland Cement Concrete polymer dispersion are commonly added to improve the flexibility, heighten the degree of chemical resistance or restrict moisture access into resultant modified composite. A total solids content of 45-60% and a pH between 7.5-8.5 is normal. Non ionic dispersions are ideally favoured for use with cement-based materials as no adverse reaction between the polymer and cementitious matrices occurs such as consumption of reactive species, (i.e., Ca2+, OH-), which normally react to form the microstructure of the hardened composite. SA are noted for improving the all over performance of the modified system into which they are incorporated [13]. Indeed, this type of polymer is favoured when a high degree of flexibility is required within the modified cementitious material such as that desired in floor screeds and patch repairs. SA available in the market place today adhere extremely well to old concrete surfaces along with the advantage of stability towards freeze thawing and heightened impact and abrasion resistance [62].

4.7.3 Vinyl Acetate Ethylene Co-Polymers VAE co-polymer dispersions are commonly used as a cement additive or to seal the substrate surface such as concrete and brick before the application of a plaster, render or a screed [1, 2, 7-9, 20]. Such materials include self-smoothing floor screeds, cementitious underlayments, medium to light wearing cementitious surfaces and light weight repair mortars which can be applied onto vertical and overhead situations [8, 15, 48, 49, 50]. Today, VAE co-polymers can be produced without the inclusion of solvents, hence the copolymers are primarily water-based and do not possess any remnants of vinyl chloride [59]. Such properties are certainly beneficial in modern society, as there is a definite movement towards utilising non-volatile, environmentally friendly, protein free polymers throughout Europe. Removing the use of volatile organic compounds commonly used 20-30 years ago in the manufacture of certain polymers and epoxy resins is now advantageous. Research has shown the incorporation of this type of polymer within cementitious materials results in an improvement in bond adhesion of this material to the substrate. An increase in the abrasion/impact resistance and workability of the modified cementitious composite is also observed. This is especially true if the modified material is applied in thin sections, (i.e., 5-10 mm) [13]. A reduction in the quantity of water required to achieve the desired consistency (i.e., fluid), has also been noted which is advantageous in terms of the resultant strength development.

101

Polymers in Cementitious Materials

4.7.4 Styrene Butadiene Rubber Co-Polymer The incorporation of SBR into cement-based systems has been noted as being beneficial in terms of an improved resistance to the absorption of water, longevity, bond adhesion along with flexural and tensile strength [4, 7, 10, 63]. It is subsequently utilised in a variety of products such as pressure sensitive adhesives, waterproofing compounds, plasters, renders, mortars and self-smoothing underlayments/overlayments [64, 63]. By incorporating SBR into cementitious materials, low odour products such as cementitious underlayments and tile adhesives (two pack polymer emulsion/blended cement based material) have been synthesised. Cementitious underlayments are typically used to level uneven concrete and other substrates in order to provide a flat level surface before the application of vinyl tiles, carpet and wood [64]. Natural rubber has been used as the polymer constituent in cementitious underlayments and when mixed with the cementitious component an ammonia odour tends to be released. SBR also tend to possess less of an odour compared to many acrylics [65]. Ultimately the release of no strong odours during application of a particular polymer-based product or curing process is beneficial to the applicators and members of the public close to the area where the material is being applied. SBR are also used to manufacture a wide range of products such as primers, low odour adhesives for flooring applications such as carpet, tapes, carbonless copy paper and magazine paper [4, 63, 64]. Depending upon the type of polymer emulsion being utilised the tape can bond permanently to the required surface or be removed and replaced on several occasions

4.7.5 Epoxies Epoxy-based polymers have also been used to modify cementitious mortars and are principally synthesised by the condensation polymerisation of bisphenol A, bisphenol F or a combination of the two with epichlorohydrin [4, 9, 20]. An epoxy resin is subsequently formed by dehydrohalogenating the chlorohydrin component, which results from the reaction process [20]. Epoxy dispersions can also be produced by diluting the epoxy resin in water in the presence of minute quantities of non-ionic surfactants. These resins can also be used to modify concrete mortars although the base and hardener are added separately [9]. The resultant polymer modified cementitious mortar can be utilised and finished in a similar manner to ordinary concrete. Incorporating the epoxy resin imparts a degree of chemical resistance and resists the passage of water into the modified infrastructure. This type of product is generally used as a concrete repair mortar and as an overlay for bridge decks [3].

102

Polymer Portland Cement Concrete

4.8 A Summary of the Effect of Polymer Modifying Cementitious Materials Principally, the overall performance of the polymer modified cementitious material depends upon the nature of the polymer utilised, the ratio of polymer to cement content by weight, temperature at which curing occurs and environment (i.e., external or internal), in which curing takes place [13, 20, 46]. Any air entrapment during the process and subsequent formation of voids within the microstructure also has to be taken into consideration [9, 20]. The presence of antifoamers in the cementitious and polymer components prevents aeration of the slurry during mixing. As the hydration of the cement component proceeds, the polymer particles coalesce to form a network of ‘thread-like’ polymer throughout the hardened structure [10, 14]. Thus, as explained previously improving the tensile and flexural strength along with a greater degree of adhesion and abrasion resistance [3, 4, 8, 9, 13-16, 21, 28, 55]. A reduction in the modulus of elasticity and hence an increased ability to withstand deformation of the cementitious structure is also achieved. If the polymer is hydrophobic, then a greater resistance to water absorption within the cementitious material occurs [42, 52]. Modification of cementitious materials with redispersible polymer powders is principally the same as with polymer dispersions except it involves the incorporation of a dry powder into the cementitious product before mixing rather than the addition of a liquid component during the mixing process [14, 15, 32, 54]. Once the polymer has redispersed within the aqueous phase it will perform in the same manner as the original dispersion it was manufactured from [9]. The primary advantage of using such polymer powders such as the ease of use, less demanding storage conditions and longer shelf life [32, 50, 51].

4.8.1 Water Demand The addition of polymers to concrete has been observed to effectively reduce the overall water demand and extend the setting characteristics [1, 9, 13, 46, 47]. The severity of these effects are governed by the specific type of polymer utilised and the actual quantity incorporated into the cementitious material [47]. Although, it should be noted that retardation of the setting of the resultant mortar can be controlled by adequately adjusting the water to cement ratio, (i.e., the water contained within the polymer emulsion). The polymer cures in unison with the formation of the cementitious microstructure, forming thread like membranes throughout the matrix. The polymer is therefore able to bond with aggregates and hydrates formed within the cementitious matrix. This in turn enhances the overall flexibility of the hardened composite along with the tensile and flexural strength [3, 7, 14, 28]. Thus the polymer acts as an additional binder within the cementitious matrix [4, 9, 14, 28, 45, 66].

103

Polymers in Cementitious Materials

4.8.2 Bleed Water An additional advantage of modifying the cementitious medium with a polymer latex is any bleeding or segregation of the aggregate or cement phase is reduced to an absolute minimum. This has been associated with binding properties of polymer dispersions. This property is particularly useful in the formation of underwater cements as the aggregate has to remain bound with the cement component when placed underwater [67].

4.8.3 Workability Generally, the inclusion of a latex dispersion when added in the correct manner, i.e., the dry component into the liquid, improves the workability [1, 9, 14, 28, 46]. This is linked to increased shear achieved during the mixing process due to the actual spherical shape of the polymer particles. An analogy to this scenario is the addition of glass beads into lightweight repair mortars in order to enable a workable but stiff mix to be achieved which is not susceptible to slumping. Hence, the quantity of water required to obtain the desired working characteristics is lower, which in turn reduces the degree of drying shrinkage of the cementitious material during drying [14, 28].

4.8.4 Stability The physical properties of the resultant polymer modified cementitious material can vary considerably depending upon the type of monomer constituents used in the formation of the latex polymer [5, 13, 20]. SBR, SA and vinyl acetate co-polymers are commonly used to modify cementitious materials. This is due to the general improvement in the flexural and tensile properties of the modified cementitious material they impart along with the actual stability of the polymer dispersion [7, 13]. Initial experimentation with the early PVA caused concern due to their instability when exposed to moisture and more details will be given later in the text [1, 3, 7, 45]. Hence the incorporation of this type of polymer was only favoured when the environment would remain constantly dry. Since the exposure of construction materials to moisture is commonplace, styrene butadiene and SA are generally used to modify mortars and concrete, floor screeds as well as being used in manufacture of adhesives and bonding agents [3, 44]. This is primarily due to the fact that once formed the polymer film does not breakdown and re-emulsify when exposed to moisture unlike PVA. Incorporation of a polymer into the cementitious microstructure produces a barrier against the absorption of moisture from an external environment once film formation is complete [3, 10]. Research has also shown that the inclusion of a polymer such as an acrylic provides better resistance against chloride absorption and

104

Polymer Portland Cement Concrete subsequently the possibility of deterioration of the cementitious structure and corrosion of any reinforcing steel present within the structure [13, 68, 69].

4.8.5 Voids Since the C-S-H phases are precipitated onto the surfaces of the cement grains present and in water containing areas, their development is not uniform throughout the cementitious matrix and voids exist. These vary in size from capillary pores 0.0005 to 0.5 μm, i.e., macro pores [10, 43]. The tiny capillary pores are far too small for polymer particles to enter and subsequently form a film within [7, 9, 10]. However, the larger the void the easier it is for the polymer molecules to enter the pore and coalesce to form a film and hence a seal. A reduction in the porosity of the resultant cementitious microstructure has therefore been noted due to the inclusion of a polymeric material [1, 15, 53]. Analysis of polymer modified and an unmodified cementitious material has indicated that the shape, size and distribution of the pores throughout the resultant microstructure changes when a latex is included into the mix design [47]. Generally the concentration of voids is less, the larger ones have been sealed with polymer and those that are still present tend to be more spherical in shape. In fact analysis of the microstructure has determined there to be a significant decrease in the large pores, i.e., > 0.02 μm due to the fact that the polymer molecules are able to enter and seal the voids [1].

4.8.6 Drying Characteristics It would appear that the inclusion of a polymer dispersion can either influence the drying shrinkage in a positive or negative manner [9, 14]. Certain research suggests that the inclusion of a polymer prevents rapid water loss from the developing cementitious microstructure and hence reduces the degree of initial drying shrinkage that the cementitious material experiences [9, 10, 21]. The influence of the polymer incorporated in the cementitious material to inhibit evaporation from the moisture is however dependent upon the chemical nature of the polymer added. Excessive shrinkage has also been witnessed when the quantity of the latex exceeds 15% by weight. It has been surmised that at this concentration the polymer does not form a continuous network throughout the cementitious microstructure but rather a three-dimensional network. In turn, any shrinkage experienced by the polymer is imposed on the resultant bulk matrix [9, 14, 70]. However, it should also be noted that the addition of a polymer would not affect the drying shrinkage after 28 days curing. A slight retardation of the hydration and initial setting characteristics of cement-based materials has been observed when 5% of a VAE co-polymer was introduced [14]. In turn this effect is reduced as the proportion of polymer added decreases.

105

Polymers in Cementitious Materials

4.8.7 Strength Characteristics It should be noted that in the majority of circumstances, the overall influence of incorporating a polymer emulsion into a cementitious material upon the resultant compressive strength is a lot less [9, 15, 16]. Indeed, research has shown that incorporating a polymer in the form of a liquid or powder into a cementitious material does not necessarily enhance the compressive strength of a particular product. In fact a reduction in the compressive strength is normally observed [13-16]. However, it is important to remember that the overall strength characteristics will be determined by a number of factors, cement composition, water/cement ratio, proportion of polymer incorporated in relation to the cement content, degree of air entrapment and porosity of the resultant composite [9, 13, 15]. In order to achieve the maximum degree of strength it is imperative that the hydration of the cement component and subsequent development of the microstructure proceeds to its absolute limit [17, 21, 70]. This is especially important during the early stages of curing, (i.e., within the first 24 hours) [19]. It is also imperative that the polymer film forms to its maximum degree throughout the cementitious matrix during the curing process and hence development of the microstructure. One of the principal requirements when utilising a redispersible polymer powder or dispersion as a cement additive is an ability to film form at temperatures below those of the surrounding environments [14, 17]. The minimum application temperature for cementitious products is generally quoted as between 5-25 oC. Once film formation has been completed the latter must not be able to re-emulsify when exposed to water once more [15, 29]. It should also be noted that the compressive strength registered would also be influenced by the hardness of the polymer incorporated [15, 16, 71]. This factor affects the Tg – the polymer is often analysed to determine this parameter. The particle size of the polymer has also been found to influence the resultant compressive strength. An improvement in the compressive strength of the polymer modified cementitious material has been observed as the polymer hardness increases. It should, however be noted that generally a slight reduction in compressive strength is noted for these modified cementitious products in relation to that registered with no polymer present. A reduction in the compressive strength has also been linked to an increase in the particle size of the polymer incorporated in the cement-based material. An example of this was noted by Walters and co-workers [71]. He carried out a series of experiments using a number of different polymer dispersions and determined the effect of varying their chemical nature on the physical characteristics of standard cementitious mortar (a mixture of graded sand and Portland cement combined at a 3:1 ratio). The polymer dispersions utilised were a copolymer of vinylacetate and ethylene, PVA, a carboxylated styrene-butyl acrylate and a styrene-butadiene copolymer along with a carboxylated butyl acrylate-methyl methacrylate copolymer. Although, the polyvinyl

106

Polymer Portland Cement Concrete acetate homopolymer possessed the highest degree of polymer hardness, the cubes of this polymer modified cementitious mortar did not register the greatest compressive strength. This was in fact achieved by the carboxylated styrene-butadiene copolymer. The author has indicated that the reason for these results may be a consequence of the substantially larger particle size of the PVA polymer in comparison to those of the other polymers utilised. This may, however, have also been related to the formation of larger voids within the resultant cementitious microstructure modified PVA resulting in a porous structure. A common method of increasing the flexibility of a cementitious mortar was to add steel or asbestos fibres into the mix [17]. Indeed this method has been used since the 19th century. However, due to health hazards associated with the use of asbestos, (i.e., the development of asbestosis), this material is no longer used. Alternatively, polypropylene fibres can be used, however, such methods are costly and sufficient improvements can be achieved using an emulsion or redispersible polymer powders. The primary advantage of adding a latex to a cementitious mortar is the improvement in flexural and tensile strength [1, 8, 9, 14, 16, 17, 46, 55]. Research has shown that it is not only the polymer to cement ratio, which influences the flexural and tensile strength, but the actual monomer concentration within the polymer dispersion and whether it is a powder or dispersion [5, 32]. An increase in flexural strength of the polymer modified cementitious materials has been directly linked to the proportion of and the chemical nature of the polymer introduced [14, 15]. The heightened degree of tensile and flexural strength observed for polymer modified cement-based materials is principally due to the asymmetric stretching of the resultant latex film formed throughout the microstructure [14]. In fact a series of tests were undertaken on a number of cementitious mortar samples modified with varying quantities of VAE co-polymers [15]. The actual samples were cured in water for a period of 14 days before being transferred to an environment maintained at 20 °C, with 75% relative humidity until 28 days curing was achieved. An improvement in the parameter of 50% was noted when 20% (w/w) of an ethylene vinyl acetate (EVA) co-polymer was utilised. Improvements in flexural and tensile strength have also been linked to the hardness of the actual polymer [7]. Investigation, however, of this theory by Walters did not conclusively establish a link between the two parameters. In accordance with this relationship the inclusion of PVA homo polymer into a cementitious mortar should have resulted in the maximum increase in such strength properties. However, this turned out not to be the case. Again it was believed that this was a consequence of the larger particle size of this particular polymer relative to the other polymers analysed during this experiment. Research suggests that the incorporation of redispersible polymer powder into a cementitious material improves the adhesive strength due to drying time being extended, enabling the cement present to hydrate to a greater degree [14]. Experimentation has shown that the 107

Polymers in Cementitious Materials addition of 3% by weight of a redispersible polymer powder into a standard tile adhesive effectively improves the adhesive strength by a significant degree. If fact an increase in the adhesive strength and degree of deformation of the resultant polymer modified material has been noted as the proportion of polymer was varied between 0-6% by weight [18].

4.8.8 Influence of Application Method upon the Strength Characteristics of Polymer Modified Cementitious Materials Research has also been undertaken to determine whether the actual application method used to apply the polymer modified (incorporating a redispersible polymer powder) cementitious material significantly influences the final physical characteristics [16]. In these particular experiments the chosen Portland cement mortar with a water/cement ratio of 0.5 by weight was mixed in a conventional manner using a cement mixer and also in the nozzle of a spray dryer (Aliva 246 machine). The objective of this exercise was to determine whether adopting different mixing methods, and hence a different shearing action during the mixing process influenced the dispersion of the polymer powder contained within the dry cementitious powder. EVA and SA co-polymers were used during this experiment along with a control formulation, which contained no polymer. In terms of flexural strength the incorporation of 2% redispersible powder into the mortar enhanced the degree of strength obtained. The inclusion of VAE and SA polymer powders resulted in a slight improvement in the tensile strength of 1-1.5 N/mm2 and 1.5-2 N/mm2, respectively, whether the modified material was sprayed or applied manually. A marginal reduction in the compressive strength for both types of polymer and application method was noted compared to the control.

4.8.9 Abrasion and Impact Resistance The advantage of using polymer modified Portland cement cementitious materials is the noted improvement in the abrasion resistance [1, 3, 29]. The addition of these types of polymers (i.e., vinyl acetate, vinyl acetate-vinyl versatate and VAE), increases the binding between the discrete crystalline/amorphous phases, i.e., C-S-H and aggregates present within the cementitious composite [14, 17]. Research has shown that the incorporation of a latex at a polymer/cement ratio of 0.20 improves the wear resistance of the concrete by 20 times [10]. A significant reduction in abrasion resistance has also been noted with the addition of 4% of polymer by weight of a redispersible polymer powder [18]. The weight loss measured from the test surface using the chosen abrasion method fell from 0.2 kg with zero polymer addition to 0.01 kg when 4% was introduced into the cementitious material analysed.

108

Polymer Portland Cement Concrete Analysis has also indicated that a polymer modified cementitious material possesses a greater degree of impact resistance and can withstand a higher degree of structural distortion before cracking is induced [18, 52, 53]. An example being movement due to expansion or contraction of the substrate and surrounding building onto which the polymer modified cementitious material is applied. Hence thinner sections can be applied whilst achieving the desired physical characteristics due to the benefits of adding the polymer. During the construction of the Shanghai Bank in Hong Kong vast quantities of polymer modified Portland cement were used to encase the internal steelwork structure [65]. The primary advantage of using this type of cement was that only a 12 mm layer was required to provided the necessary physical properties and support rather than the 40-50 mm of ordinary Portland concrete which would normally be required.

4.8.10 Durability and Modulus of Elasticity The durability of the final polymer modified cement-based material will also be influenced by the chemical composition of the polymer used as an additive. A definite improvement has, however, been observed by introducing a polymer into the cementitious material [7, 28]. A reduction in the modulus of elasticity of the resultant modified structure has been observed when latexes are used [9, 41]. The degree of deformation, which the structure can endure before fracturing occurs increases [10]. This quality is linked to the proportion/chemical nature of the polymer added and the quantity of cement present.

4.8.11 Permeability of Polymer Modified Cementitious Materials One of the main advantages of using a latex in combination with a cement-based system is the noted reduction in the permeability of the modified cementitious structure, the matrix is therefore less susceptible to the intrusion of chloride ions, acids and carbon dioxide [9]. Chloride ions and carbon dioxide have been observed to react with reinforcing bars and consequently cause them to rust thus their presence is not desired within the surrounding cementitious structure. Research has shown that a relationship exists between the nature of the polymer used and the permeability of the resultant modified cementitious structure. Analysis has indicated that the permeability of the cementitious structure tends to decrease relative to the proportion of polymer addition [4, 14, 21, 45]. In one of the experiments undertaken by Walters and co-workers a good resistance to water intrusion was observed for all polymers (VAE, SB, SA, PAE) except for the material containing a PVA homopolymer [71]. The polymers analysed were VAE, SA, SBR and PVA. This has been linked to the instability of the acetate group to hydrolysis in the presence of water. Thus, it is important that the

109

Polymers in Cementitious Materials polymer used is not susceptible to deterioration or re-emulsification when subjected to water once the film has dried [1, 10]. The inclusion of a polymer material within the cementitious matrix also limits the degree of moisture movement within the structure. This in turn also restricts the penetration of water from the surrounding environment into the resultant cement-based material and hence increases the resistance to frost damage and carbonation [7, 10]. However, it should be noted that any polymer present within a cementitious microstructure is capable of filling any large pores present, i.e., 0.05 μm. The introduction of the polymer into the cement-based product therefore lowers the porosity and reduces the vacant voids, which are available to take in water within the matrix. The intake of water has been observed to affect the strength characteristics of polymermodified concretes and to a certain degree cementitious materials such as mortars, in particular the flexural strength [9]. This factor is however, still nominally higher than that achieved when no polymer is present. SBR, VAE have good chemical resistance and their physical properties are not compromised when cured upon contact with moisture unlike PVA [1, 7, 8]. The inclusion of secondary monomers such as ethylene and VeoVA in conjunction with vinyl acetate also lowers the tendency for the polymer to re-emulsify on contact with water [7]. Research has also shown the inclusion of additional side group into this polymer reduces the tendency to undergo hydrolysis when exposed to a degree of moisture. The only disadvantage associated when using of SBR systems is their tendency to change colour when exposed to ultraviolet light [7, 8]. When the consistency of colour is an important factor such as in paints, decorative finishes and plasters for external applications, acrylics are normally used.

4.8.12 Carbonation Evidence also suggests that the inclusion of a polymer can reduce the degree of carbonation within the cementitious matrix [9, 71]. This has been attributed to the polymer forming thread like membranes throughout the structure and filling any available voids which it can enter [9, 10, 14]. Hence the actual volume of the microstructure which can subsequently undergo carbonation is reduced. Resistance to carbonation also increases relative to the quantity and actual chemical nature of the polymer present [71]. This is of particular importance when the concrete substrate contains reinforcing bars. Carbonation has been linked to the corrosion of the steel reinforcing bars [9, 71]. Analysis has shown the incorporation of an acrylic polymer generally results in a poor carbonation resistance relative to that achieved when other polymer dispersions are used. A link between the particle size of the polymer and an increase in the degree of the carbonation experienced has been postulated.

110

Polymer Portland Cement Concrete

4.8.13 Chemical and Acid Resistance The chemical and acid resistance of a polymer-modified structure will principally depend on the type of polymer compound used and the quantity present in relation to the concentration of cement. The effect of incorporating a series of polymer dispersions in cementitious material and their subsequent effect on the chemical/acid resistance has been assessed. The polymer dispersions analysed during this particular experiment included SA, SBR, VAE and PVA. Analysis of the stability of a number of latex modified systems has revealed the highest degree of structural durability to be achieved when a SBR was added [71]. A slightly lower resistance was observed when a copolymer of vinyl acetate (VAc) and ethylene, SA and an acrylic copolymer were incorporated into a standard cementitious mortar. However, the addition of PVA resulted in the least structural stability. All samples were exposed over a 14 day period to an aqueous solution of 1% nitric acid and 2% sulfuric acid by weight.

4.9 Common Polymer Modified Construction Materials Polymers have been used as cement additives for modification of a variety of cementitious materials on a regular basis since the 1950s [1, 44]. With regard to mortars and renders, polymers (powder or dispersion) were primarily incorporated to alleviate the main problems noted with non-modified materials such as durability, workability and flexibility [28, 45]. However, it was important that the polymer utilised was not adversely affected by the alkaline nature of the cementitious environment [9, 10]. Equally the polymer should not influence the alkalinity of cement [45]. A few examples of the types of polymers used as cement additives in various construction materials is listed in Table 4.2.

4.9.1 Bridge Decking Polymer modified Portland cement concrete is suitable for use as a bridge decking overlay or as a general concrete repair mortar [4, 30, 45, 65]. By incorporating SBR, polyvinyl chloride (PVC) and polyvinylidene dichloride into cement mortars an excellent bond adhesion between the polymer-modified material and the substrate is achieved. However, except for SBR, use of the other polymers detailed above is not recommended when the modified cementitious material will come to contact with reinforcing bars. If the chloride component has not been completely consumed during polymerisation there is the possibility that this substance could leach out of the host material and come into contact with steel. Subsequent interaction with chlorides has been observed to corrode the reinforcing bars [10, 45]. Acrylic latexes are commonly used to give good colour uniformity and overall increased durability of the cementitious composite [7].

111

Polymers in Cementitious Materials

Table 4.2 General composition of polymers used to modify a variety of construction products. Adapted from [8, 10, 15, 46, 51, 59-61, 63, 64] Chemical composition of polymer Material description Acrylic

Self-smoothing floor screeds, repair mortars, primers, sealing compounds

SA

Self-smoothing floor screeds, primers, ceramic tile adhesives, sealing compounds

SBR

Repair mortars, primers, ceramic tile adhesives, grouts, adhesives

VAE

Plaster, repair mortars, self-smoothing floor screeds, ceramic tile adhesives, tile grouts

Chloroprene

Primers

VeoVa

Ceramic tile adhesives, tile grouts, joining compounds

Vinylidene chloride

Repair mortars

4.9.2 Mortars and Renders Polymer modified cement-based mortars and renders are utilised for a vast variety of applications such as motorway bridge repairs, waterproofing, lining manholes, repairing sea defences and offshore facilities to name but a few [1, 4, 15, 69]. The increased resistance to chemical attack by the addition of the polymer depends upon the composition of the polymer used. This makes these systems attractive for use in food factories or manufacturing facilities where chemical or oil spillages are likely to occur [1, 9]. The enhanced resistance to the intrusion of carbon dioxide into the cementitious microstructure also makes the use of polymer modified cement systems favourable. One of the main problems associated with the deterioration of concrete is carbonation. Research has shown that a resistance to the absorption of carbon dioxide equivalent to a 50 mm thickness of dense concrete can be achieved when modified mortar is applied at a thickness of 5-10 mm. Polymer modified mortars are commonly recommended for use in conjunction with engineering bricks and coping stones due to the improved bond adhesion and durability. These types of polymer modified cementitious systems are far cheaper than epoxy resin mortars and hence are commonly used instead of the latter unless the physical parameters of the repair are such that they require the performance of an epoxy-based material [1]. Polymer modified mortars are principally applied in the same manner as ordinary mortars containing no polymer [4]. The appropriate thickness should be placed in order to prevent slumping. If the application of several layers is required the previous layer should be textured, (i.e., scratched), to improve the adhesion between the previous layer and newly applied layer.

112

Polymer Portland Cement Concrete Application of these types of materials is recommended between 5-25 °C. Below 0 °C, any water present will turn to ice and hence is no longer available to take part in the hydration of the cement component and thus microstructural formation. Thus the development of the physical matrix, strength properties, and so forth, are adversely affected [25]. As the application temperature increases, the rate of hydration and cure is accelerated.

4.9.3 Flooring The demand for a more economical fast track alternative to the traditional resin flooring resulted in the formulation of self-smoothing cementitious screeds [4, 12, 19, 28, 56, 72]. Utilising a combination of cements, graded aggregates, fillers and additives, a material which possesses the desired workability to enable efficient placing whilst rapidly gaining strength within the first 24 hours of application can be produced. In the majority of cases the surface is sufficiently hard enough to withstand foot traffic after three to four hours of curing and suitable for forklift trucks within three days [19]. The principal advantage of these systems, which are used for the renovation or refurbishment of old concrete floors, is the surface preparation, screed application and the application of a coating can be undertaken and completed in a weekend. Thus the period over which the area being renovated is out of action is kept to an absolute minimum, which is of great importance for most business/manufacturing operations today.

4.9.4 Primers Self-smoothing floor screeds are commonly used to refurbish and renovate old concrete floors [4, 19, 56]. After the initial surface preparation of the substrate (i.e., shot blasting), to remove any loose material or contaminants it is imperative that this surface is sealed. This is to prevent the transfer of air contained in the old concrete into the newly applied screed [1]. If this process is not undertaken, any air from the substrate will result in numerous pinholes within the screed which are not aesthetically pleasing. There is also an increased risk of the screed debonding from the substrate surface. Polymer dispersions are commonly used as primers, which are normally diluted down with water (1:1 to 1:5 ratios by weight are commonly used) before being applied to the prepared concrete. Diluting the polymer down with water aids its absorption into the substrate [56, 61, 64]. In the vast majority of cases the primer is allowed to dry before the application of the desired cementitious or plaster-based material occurs and hence acts as a sealer. However, there are situations where the final product such as a self-levelling floor screed is placed onto a wet coat hence the latter is acting as a bonding coat between the required product and the substrate.

113

Polymers in Cementitious Materials By crosslinking acrylic polymer molecules the matrix is strong enough to withstand expansion/contraction due to changes in surrounding temperature or deterioration of the structure when exposed to moisture [59, 60]. Hence re-emulsification of the polymer film when in contact with water is eliminated. One of the principal advantages of using SBR as a primer is it does not breakdown when used in a moisture rich environment [8, 64]. This initial polymer coating aids the adhesion of the floor screed subsequently applied onto the surface of the old concrete substrate [1].

4.9.5 Ancillary Construction Products As well as modifying Portland cement systems, SBR latexes can be used to adapt the properties of bitumen [73]. Noted improvements in the flexibility of the system in cold environments (i.e., towards 0 °C), continuous suspension of the aggregate, abrasion/impact resistance and an increased longevity have been observed. The flow can also be altered at high temperatures in order to maintain the desired workability and reduce the stickiness. SBR can be used in hot bitumen with no degeneration in performance of the polymer. They are either positively or negatively charged (i.e., cationic or anionic), depending upon the type of surfactant used during the manufacturing process of this polymer. Cationic styrene butadiene latexes are also used to produce slurry seals and tacky coats, whilst anionic types are used in the formation of several roofing materials and sealing membranes [74]. These tend to be either non-carboxylated or carboxylated SBR that may contain acrylonitrile in the backbone. The pH can vary between 2-10, whilst the total solid content is within the region of 30-98% by weight with a butadiene concentration of 15-70% by weight. By introducing a polymer into the cementitious medium, tile adhesives can be produced whose performance does not deteriorate when exposed to moisture [46, 53]. Whilst, grouts with a greater stability to chemicals (i.e., chloride attack), and less of a tendency to crack under adverse conditions can be achieved by incorporating SBR [64]. However, it should be noted that the porosity of the composite also determines to what extent it will be affected by chemical attack, i.e., the easy way they can enter the structure. Other applications include the raw material component in the manufacture of latex gloves, corrosion resistant paints, and so on [63]. Coatings which can prevent the absorption of certain types of chemicals (i.e., mild acids, chlorides), as well as providing protection against carbonation have also been formulated utilising this type of polymer.

114

Polymer Portland Cement Concrete

References 1.

J.D.N. Shaw, P.J. Brown, R. Cather, R. Dennis, P.C. Hewlett, R.A. Johnston, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Polymers In Concrete, 2nd Edition, Technical Polymers No. 39, Concrete Society Working Party, The Concrete Society, 1994.

2.

ACI Committee 548, State-Of-The-Art Report on Polymer-Modified Concrete, Report No.548.3R-3, American Concrete Institute, Detroit, MI, USA, 2003.

3.

C. Ellis, High Performance Plastics, 1989, 6, 6, 1.

4.

D.W. Fowler in the Proceedings of the 43rd Annual Conference and Focus '88, 1988, Cincinnati, OH, USA, Session 16-B1.

5.

E. Kirhikovali, Polymer Engineering and Science, 1981, 21, 8, 507.

6.

R. Hussein and N.P. Cheremisinoff in Elastomer Technology Handbook, Ed., N.P. Cheremisinoff, CRC Press, Boca Raton, FL, USA, 1993, p.875-907.

7.

D.G. Walters in Proceedings of the ACI Convention, 1997, Altanta, GA, USA, p.1-5.

8.

R. Dennis in Construction Materials Reference Book, Ed., DK Doran, Butterworth Heinemann, UK, 1992, Chapter 39, p.1-12.

9.

Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 189.

10. V.R. Riley, and I. Razl, Composites, 1974, 5, 1, 27. 11. H. Guo, A.E. Hamielec and S. Zhu, Journal of Applied Polymer Science, 1997, 66, 5, 935. 12. R. Harbron in Calcium Aluminate Cements 2001, Eds., R.J. Mangabhai and F.P. Glasser, IOM Communications, London, UK, 2001, UK, p.597-604. 13. I. Nelson in Proceedings of ConChem Conference, Dusseldorf, Germany, 1997, p.1-12. 14. R. Zurbriggen, Influence of Redispersible Powders on Shrinkage, Hydration Behaviour and Microstructure of Tile Adhesives, 1999, Elotex AG, Sempach Station, Switzerland. 15. D.J. Schulze and F. Jodlbauer, Vinnapas® Redispersible Powders For Building Renovation, Wacker-Chemie GmbH, Munich, Germany, 1997.

115

Polymers in Cementitious Materials 16. D.J. Schulze and F. Jodlbauer, Vinnapas® Redispersible Powders For Building Renovation, Wacker-Chemie GmbH, Munich, Germany, 1997. 17. D.K. Adler, Thermoplastic Polymers In Cement-Based Materials - An Ideal Partnership, Wacker-Chemie GmbH, Munich, Germany. 18. Polymer Modified Factory-Made Dry-Mix Mortars As Modern Building Materials, Wacker Polymer Systems GmbH & Co. KG, Munich, Germany, 1999. 19. M. Roberts in Calcium Aluminate Cements 2001, Eds., R.J. Mangabhai and F.P. Glasser, IOM Communications, London, UK, 2001, p.605-614. 20. D.G. Walters, Presented at the ACI 1986 Fall Convention, Baltimore, MD, USA, 1986. 21. R.N. Swarmy, Cement and Concrete Composites, 1992, 14, 3, 155. 22. H.F.W. Taylor, The Chemistry Of Cement, Lecture Series Number 2, The Royal Institute of Chemistry, London, UK, 1966, p.1-2. 23. L.D. Mitchell, M. Prica and J.D. Birchall, Journal of Materials Science, 1996, 31, 16, 4207. 24. H.M. Jennings, B.J. Dalgleish and P.L. Pratt, Journal of the American Ceramic Society, 1981, 64, 10, 567 25. A.M. Neville, Properties of Concrete, 3rd edition, 1981, Pitman Publishing Inc., Marshfield, MA, USA, p.13-19. 26. I. Jawed, J. Skalny and J.F. Young in Structure and Performance of Cements, Ed., P. Barnes, Applied Science, London, UK, 1983, p.237-312. 27. L.A. Kuhlmann, J.W. Young, Jr., and D. Moldovan, inventors; The Dow Chemical Company, assignee; US 5576378, 1996. 28. J.A. Manson, Materials Science and Engineering, 1976, 25, 1-2, 41. 29. A. Hoffman, Effect of Redispersible Powders on the Properties of Self-levelling Compounds, Wacker Polymer Systems in Annual Contract Flooring Association Conference, Coventry, 1999. 30. I. Berkovitch, Civil Engineering (London), 1984, August, 45. 31. Technical Communication, Harlow Chemical Company Ltd, Harlow, Essex, 24th Jan 2000.

116

Polymer Portland Cement Concrete 32. D.G. Walters, Concrete International, 1992, 14, 4, 30. 33. C.L. Zhoa, J. Roser, W. Heckmann, A. Zosel and E. Wistuba, Progress in Organic Coatings, 1999, 35, 1-4, 265. 34. C.J. Samer and F.J. Schork, Industrial & Engineering Chemistry Research, 1999, 38, 5, 1792. 35. M.P. Stevens, Polymer Chemistry: An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p.130-131. 36. M.P. Stevens, Polymer Chemistry: An Introduction, Addison-Wesley Publishing Company, Inc., Reading, MA, USA, 1975, p.104-109. 37. F.W. Billmeyer, Jr., in Textbook of Polymer Science, 3rd Edition, Wiley, New York, NY, USA, 1984, p.121-122. 38. J. Daintith in A Concise Dictionary of Chemistry, Ed., J. Daintith, Oxford University Press, Oxford, UK, 1990, p.234. 39. W.A. Whitaker III in Medical Plastics and Biomaterials Magazine, 1996, January, p.12. 40. M. Annemieke, A.M. Aerdts and B.K. van Herk in Materials Science and Technology: A Comprehensive Treatment, Synthesis of Polymers, Eds., R.W. Cahn, P. Haasen and E.J Kramer, Wiley-VCH, New York, NY, USA, 1999, p.274283. 41. W. Cooper in Reactivity, Mechanism and Structure in Polymer Chemistry, Eds., A.D. Jenkins, and A. Ledwith, 1974, Wiley-Interscience, New York, NY, USA, p.176-178. 42. D.P. Tate in Encyclopaedia of Polymer Science and Engineering, Volume 2, Eds., A. Klingsberg, J. Muldoon and A. Salvatore, John Wiley & Sons, New York, NY, USA, 1988, p.553. 43. S. Mindess, Journal of The American Ceramic Society, 1970, 53, 11, 621. 44. R.F. Feldman and J.J. Beaudoin, Cement and Concrete Research, 1976, 6, 3, 389. 45. T.P. O'Brien, W.B. Long, P.C. Hewlett, J.D.N. Shaw, L.J. Tabor and P.J. Winchcombe, Repair of Concrete Damaged by Reinforcement Corrosion, Technical Report No.26, 1984, The Concrete Society, London, UK.

117

Polymers in Cementitious Materials 46. Your Partner For Redispersible Powders, Elotex AG (Division of National Starch and Chemical Company), Sempach Station, Switzerland, Company Presentation, 1998. 47. R.N. Swarmy, Cement and Concrete Composites, 1995, 17, 3, 175. 48. L. Holmberg, and J. Engstrand, Peramin Cementitious Screed Handbook, Perstorp Construction Chemicals, Perstorp, Sweden, 1999. 49. T.A. Bier and L. Amathieu in Proceedings of ConChem Conference, Dusseldorf, Germany, 1997, Paper No.F97. 50. U. Tribelhorn and D. Zweifel, Concrete, 2000, May, 52. 51. DLP Redispersible Polymer Powder In The Construction Industry, Dow Chemical Company Limited, Midland, MI, USA, 2000, p.1-4. 52. Ceramic Tile Adhesives Modified With Vinnapas® Redispersible Powders, Wacker Polymer Systems GmbH & Co. KG, Burghausen, Germany, 2000. 53. D.H. Lutz, Modification Of Mineral Plasters With Hydrophobic Polymeric Binders, Wacker Polymer Systems GmbH & Co. KG, Burghausen, Germany, Edition 12/1999. 54. Vinnapas® Redispersible Powders, Nr. 3828 0297, Wacker Polymer Systems GmbH & Co. KG, Burghausen, Germany. 55. J. Schulze, Cement and Concrete Research, 1999, 29, 6, 909. 56. M. Roberts, Industrial Contract Floors, 2000, p.11-12. 57. Vinnapas® Binders For Renovating Concrete, Wacker-Chemie GmbH, Munich, Germany, 1992. 58. M. Steinberg, Polymer Plastics Technology and Engineering, 1974, 3, 2, 199. 59. Cembond 006, Cembond 349, Cembond 401 Vinamul Polymers As Cement Additives, Vinamul Limited, Surrey, UK. 60. Cembond 006 Crosslinking Acrylic Cement Admixture, Vinamul Limited, Surrey, UK. 61. Harco Dispersions: A Guide to their Properties and Applications, Harlow Chemical Company Ltd., Harlow, UK.

118

Polymer Portland Cement Concrete 62. Revacryl 387 For Cement Modification, Harlow Chemical Company Ltd, Harlow, UK, 1996. 63. Synthomer The Synthetic Latex Specialists, Synthomer GmbH, Frankfurt, Germany. 64. More Than Just Cement, Synthomer GmbH, Frankfurt am Main, Germany. 65. Main applications, market areas and advantages of Synthomer 29Y* Cement Admixture, Technical Information, Synthomer Limited, Essex, UK, 1999. 66. Civil Engineering (London), 1987, April, 39. 67. B. Staynes and B. Corbett, Civil Engineering (London), 1988, March, 28. 68. T.K. Chatterjee in Advances in Cement Technology: Crtical Reviews and Case Studies on Manufacturing, Quality Control, Optimisation and Use, Ed., S.N. Ghosh, Pergamon Press, Oxford, UK, 1983. 69. S. Chandra, Cement and Concrete Composites, 1992, 14, 4, 289. 70. A. Blaga and J.J. Beaudoin, Polymer Modified Concrete, Canadian Building Digest No.241, 1985, Division of Building Research, National Research Council Canada, Ottawa, Canada. 71. D.G. Walters, ACI Materials Journal, 1990, 87, 4, 371. 72. T. Newton, Concrete, 1997, April. 73. We Don’t Make Bitumen. We Make Bitumen Better, Synthomer GmbH, Frankfurt, Germany. 74. Synthomer Technical Information Bitumen Systems, Synthomer Limited, Harlow, UK.

119

Polymers in Cementitious Materials

120

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials

5

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials Author

5.1 Introduction Developments in polymer technology over the last fifty years have resulted in ever more advanced polymer materials with superior physical properties [1]. As research proceeded in this field and the requirements for polymers to satisfy the demands of a constantly expanding market place increased, the manufacture of these products on a large scale was required. This resulted in the engineering of immense production plants capable of producing vast quantities of resin. In order to meet the ever increasing requirements for more technologically advanced composites and high strength construction materials, machinery was designed to process and manipulate resins into a vast array of shapes. Fibres have been manufactured by a variety of different methods including drawing the resin from a dilute solution, extruding in a semi-liquid state or solid state by hydrostatic extrusion. The resin can be extruded into a variety of different shapes, i.e., coils, tubes and rods [2]. Fibres in turn could be incorporated within a suitable binder whether it be resin or cementitious in origin to produce a reinforced composite [1]. The benefits of incorporating fibres with cementitious and resin binders has been well documented over the years [3]. These fibres can be carbon, steel, cellulose, glass, polypropylene (PP), polyethylene (PE) or polyester in origin. A variety of research projects have been undertaken during this time to assess the effect of introducing natural and synthetic fibres into these materials. Although the use of fibres in combination with concrete and other cementitious systems has increased over the latter part of the 20th century, the total quantity is still relatively small (< 5%) in relation to the overall volume of resin used throughout the construction industry.

5.2 Natural Fibres Natural organic polymers such as cellulose fibres are regularly used to increase the viscosity of cementitious materials in order to prevent slumping and segregation of any aggregate 121

Polymers in Cementitious Materials [3, 4]. It is important that the fibres introduced do not corrode within the matrix and their physical properties do not deteriorate when exposed to the alkaline conditions experienced during the hydration of cement [5]. The rheology of the mixture can also be more carefully controlled by the addition of these types of fibres and they are utilised in a variety of construction products such as cementitious grouts, self-smoothing flooring compounds, adhesives, and sealants. The introduction of these fibres has also been observed to reduce microcracking upon the surface of the hardened cementitious matrix.

5.3 Synthetic Fibres

5.3.1 Fibre-Reinforced Composites Incorporating Carbon Fibres and Glass Fibres Synthetic fibres such as glass and carbon are commonly used as structural reinforcement within a variety of composites (resin, cementitious, etc.), which are manufactured and utilised within the construction, electronics, aviation and automotive industries to name but a few [6, 7]. The extensive usage is primarily due to their superior strength relative to their actual weight and good chemical resistance. Carbon fibres have a number of physical properties, which makes their addition into concrete favourable [8]. Their structural performance is not compromised when they are exposed to an high alkaline environment, whilst their strength characteristics are in many ways comparable with steel. An improvement in the mechanical performance, i.e., flexural and tensile strength, of a cementitious mortar was recorded when small carbon fibres (manufactured from pitch) were added. Carbon fibres have been also used in combination with an appropriate resin to produce a lightweight fibre reinforced composite suitable for use in sports car manufacture. This material is moulded into the appropriate shape to provide the body of the vehicle. Such products are designed to provide the greatest possible protection to the individuals located within [6]. Unlike the vast majority of synthetic fibres, the physical performance of glass fibres, in particular their ability to resist impact and tensile strength, is impaired when the material is exposed to varied environment conditions experienced when applied externally [9]. The reduction in strength characteristics has been attributed to the chemical nature of the hydrates surrounding the glass fibres and the local vicinity (0.07-0.1 m from fibre interface). These types of fibres are still quite capable of preventing shrinkage when subjected to such conditions. Yet their introduction into the matrices in a structural capacity is generally not favoured.

122

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials

5.3.2 Polymer-Based Fibres

5.3.2.1 Typical Chemical Composition The combination of synthetic fibres and a chosen binder, either a resin or a cement is readily used throughout the chemical and construction industry [10]. A multitude of synthetic fibres of differing chemical composition such as PP, PE and polyester and of varying lengths (i.e., microfibres in excess of 50 mm), can be produced mechanically. PP is certainly one of the most common synthetic fibres used in combination with cement-based products [10]. The effect of varying the proportion, length and surface treatment of fibres upon the strength and drying shrinkage of the final material within the early and final setting stages has been studied. The resistance of the final hardened cementitious microstructure has also been assessed over a period of several years. In certain circumstances these types of fibres are chemically treated with solutions such as linear alcohol alkoxylates to improve the bonding at the fibre/binder interface [11]. Such coatings are utilised to a greater extent when the fibres are combined with resin.

5.4 Long-Term Effect of Incorporating Synthetic Fibres into Cementitious Matrices upon their Physical Performance In addition to PP, a number of synthetic fibres have been combined with concrete and other commercial cementitious products [12]. These include polyvinyl alcohol (PVOH), PE, polyester and carbon to name just a few [3, 10, 13, 14]. The long-term stability of such fibres has been assessed, a significant deterioration or adverse change in their physical characteristics could certainly have an adverse effect on the performance of the cementitious host. In the case of PVOH, concrete specimens were subjected to either a natural environment, or a procedure which accelerated the degeneration process to simulate typical ageing [12]. X-ray diffraction analysis of the extracted fibres, which had been encapsulated within the cementitious matrix, showed minor signs of disorder associated with the hydrogen bonded sheets within the crystal structure of the PVOH. The general assessment of the study was that such an effect would have no significant impact on the tensile strength of the fibres and resultant composite. This study concluded that PVOH fibres maintain their physical properties within a cementitious material for in excess of seven years. The authors also indicated that significant degeneration in the physical properties of such fibres would be expected beyond this point.

123

Polymers in Cementitious Materials

5.5 Introduction of Fibres into Cementitious Materials Concrete has a number of advantages as a construction material, no emission of volatile organic compounds, high mechanical strength, good electrical resistance and can be cast into a multitude of shapes [15]. However, the use of this diverse material is restricted principally by two factors: weakness of the microstructure when under tension and its lack of flexibility. Hence these problems have to be addressed in order to expand the feasible uses of concrete and other cementitious materials. Developments in fibre technology have provided materials which, when added to concrete, reduce these deficiencies in the cementitious matrix even at low concentrations such as 0.5-1.5% by volume. The use of synthetic fibres and fibre composites within the construction industry certainly has advantages in terms of their superior strength characteristics, relative weight and good chemical resistance making their inclusion into concrete an attractive proposition [16]. PE, polyester, Nylon, PP and steel fibres of differing lengths have been combined with a standard Portland cement mortar and concrete containing various sized aggregates [3]. The initial and final set of these fibre reinforced cementitious materials was studied during the curing process as well as how the fibres influenced the drying and shrinkage characteristics of these cementitious materials.

5.5.1 The Effect of Introducing Fibres into a Cementitious Medium upon the Physical Properties

5.5.1.1 Influence of the Length and Physical Nature of the Fibre upon the Resultant Mechanical Properties Research has shown the physical nature of the fibres and the quantity of fibres incorporated, has an effect on the final physical performance of the reinforced cementitious matrix. The effects are seen in terms of compressive, flexural and tensile properties as well as impact resistance [14, 17]. The structure, proportion and length of the fibres used will influence the physical performance of the binder they are introduced into [10]. In one particular study, PE in the form of a pulp and a high modulus fibre was combined with several cementitious materials [17]. The greatest improvement in terms of microstructural toughness and flexural strength was observed when limited proportions of both types of fibres were used in combination. A common disadvantage of introducing fibres is a reduction in the overall compressive strength of the resultant cementitious microstructure, however, this was less pronounced when both fibres and pulp were incorporated into the mixture. Previous studies within this area have also investigated the effect of adding fibrillated PP fibres of varying lengths (i.e., 6.25-12.5 mm), at a ratio of 0.1-0.5% by volume [10]. 124

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials This type of fibre is commonly used throughout many countries within Europe and the USA to enhance the physical properties of the cementitious base such as the surface durability and crack resistance [18]. A slight improvement in the flexural strength and impact resistance was noted when the longer fibres were combined with the cementitious material at a lower dosage rate of < 0.3% by volume [10]. The greatest improvement in the parameters described previously was achieved when the shorter fibres were used at the highest dosage rate, i.e., 0.5% by volume. Incorporating short rather than long fibres does have the advantage of enabling a more uniform, homogeneous material to be achieved as they are more easily dispersed throughout the mixture [15]. The thickness to length ratio is important to achieve the best physical performance, if the fibres are long they tend to knot and ‘ball up’ resulting in an uneven distribution. Research has indicated that these fibres can be aligned in a certain direction within the cementitious microstructure during the manufacture and application of concrete. The fibres can therefore be aligned in the direction, in which the concrete is most likely to encounter stress or a load. An improvement in the tensile and flexural strength of the concrete can be gained by incorporating fibres. Thus aligning the fibres in a given direction can result in the concrete being less likely to suffer the adverse effects of stress of a load, which could induce cracking.

5.5.1.2 Effect of Varying the Shape of the Actual Fibre The design of fibres has also been observed to influence the physical performance of fibre reinforced composite [19]. The effect of using corrugated, straight and steel fibres with hooked ends along with ordinary straight PP fibres upon the impact resistance and flexural fatigue strength of the reinforced matrix has been studied. A definite improvement in these properties was observed for all fibre-reinforced specimens when compared to the physical performance of standard concrete. The greatest improvement was noted when the steel fibres possessed hooked ends, whilst the straight steel and PP fibres resulted in the smallest improvement in flexural fatigue strength being recorded. The inclusion of straight steel fibres had the least affect upon the impact resistance.

5.5.2 Slump Characteristics Plastic settlement and segregation of the aggregate contained within concrete or other cementitious materials such as self-levelling floor screeds can be prevented by the addition of a small quantity of fibres [5]. Hence fibres in one shape or form are regularly used in cementitious renders and adhesives to produce the desired rheology of the compound such as a non-slumping thixotropic paste. This in turn enables such mixtures to be applied to 125

Polymers in Cementitious Materials varying thickness dependent on the overall consistency in vertical and overhead situations [3]. Both synthetic and naturally occurring fibres such as PP, PE and cellulose are used for this purpose. It is important that the fibres used for this type of application do not deteriorate when exposed to alkaline conditions such as those experienced during the curing of materials containing cement [4].

5.5.3 Structural Shrinkage and Effect of Structural Deterioration on the Cementitious Matrix The addition of steel and resin-based fibres has certainly been observed to be beneficial in terms of reducing shrinkage of the cementitious microstructure [20]. Analysis has indicated that fibres in a pulp form are more effective in reducing the occurrence of cracks due to shrinkage when the cementitious mortar possessed a high cement content [3, 20]. Extending the length of the fibres utilised was the most effective at combating shrinkage when the proportion of aggregate to cement was relatively high. Preventing the susceptibility of cement to deterioration due to the penetration of chlorides and other contaminates into the microstructure by the addition of PP fibres has also been investigated [21]. In one particular study, fibre-reinforced and ordinary concrete specimens were subjected to wet and dry conditions alternated in a cycle. This was done to mimic the conditions of a hot climate and exposure to sea water, which is known to have an adverse effect on normal concrete. After exposure over a period of up to 85 weeks, the exterior surface of the samples containing the fibres showed less signs of degeneration when compared to normal concrete.

5.5.4 Microcracking and Deformation within the Cementitious Matrix Research has indicated that incorporating PP fibres into concrete does clearly reduce the extent and size of cracks, which can form within the cementitious matrix [22]. Cracking within concrete can occur due to shrinkage of the cementitious matrix during the curing process, or when subjected to a significant load or stress, which is capable of propagating, cracks within the microstructure [3]. Even at low concentrations, the introduction of synthetic fibres such as PP fibres into concrete has been observed to significantly reduce the degree of plastic shrinkage, which can result in microcracking [23]. The introduction of fibres is particularly advantageous when the concrete possesses a high concentration of silica fumes, as the latter tends to initiate cracking due to ‘autogeneous shrinkage’ [24]. This effect has been witnessed even when moisture is prevented from readily evaporating from the matrix. This phenomenon has been attributed to the use

126

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials of a low cement mix and in order to rectify this problem steel fibres were incorporated into the concrete mix. Although the problem was not completely eliminated a definite reduction and delay in the occurrence of cracking was observed. Deformation of concrete structures due to the effects of constantly being in a state of stress, i.e., ‘creep’, has also been reduced when fibres are combined with the wet mix [15]. A reduction in compressive and tensile creep in the region of 10-20% and in excess of 50%, respectively, has been observed between ordinary and fibre reinforced concrete specimens.

5.5.5 Other Factors Influenced by the Inclusion of Fibres Another benefit of fibre-reinforcement is an increase in the surface toughness and longterm durability of the modified material [25]. Such properties have been observed in fibre reinforced beams when high strength polyester fibres were introduced into the concrete before casting [26]. In the case of steel and PP fibres, increasing the proportion added into the concrete resulted in a greater degree of toughness being registered. A reduction in the general brittleness of certain high strength mortars has also been noted when synthetic fibres are used in conjunction with the material [27]. Furthermore the ability of fibre-reinforced concrete to absorb energy is certainly higher than that witnessed for ordinary concrete [28]. Research has also shown that the addition of synthetic fibres does not influence the ability of the fibre-reinforced material to withstand the adverse effect of frost [15]. This will ultimately be governed by the quality of the concrete utilised and the degree of air entrained within the cementitious matrix. The presence of fibres will however, reduce the affect of frost inducing cracking within the cementitious matrix due to their ability to prevent crack propagation.

5.5.6 The Introduction of a Latex into Fibre Reinforced Concrete The combination of a latex with reinforced concrete containing carbon and steel fibres has been observed to improve the bond adhesion between the fibres and cementitious microstructure [29, 30]. Reductions in the drying shrinkage and increased resistance of the microstructure to the effects of exposure to acids were also noted when the matrix contained carbon fibres [29]. However, as with all latex compounds a reduction in compressive strength was recorded, its addition may not be favoured if the fibre reinforced concrete is to be used in a structural capacity. It would appear that the addition of a polymer emulsion into a carbon fibre reinforced cementitious material provided no

127

Polymers in Cementitious Materials additional benefits in terms of impact resistance hence unless the particular improvements in the properties stated previously are important, its incorporation would have to be determined to be cost effective. However in the case of steel fibres, the addition of a latex had a positive influence upon the impact resistance [30]. The author indicated that this type of modification improved the compatibility of the cementitious microstructure with the steel fibres.

5.5.7 The Influence on Corrosion of Incorporating Fibres into a Cementitious Matrix The possibility of incorporating synthetic fibres into the steel reinforced concrete to prevent corrosion of the reinforcing bars has been investigated [31]. This was due to the improvements in durability and structural toughness of a given cementitious matrix when fibres are added. In one particular experiment, PP fibres were introduced at a rate of 0.2% by volume and the physical performance of concrete produced at different cement/water ratios (0.45-0.65) was assessed. The concrete specimens were then subjected to a variety of conditions, which would induce severe corrosion of the steel bars within. Analysis revealed that the corrosion process was not retarded in any way, hence there was no benefit to be gained in terms of preventing this process by simply incorporating fibres into the cementitious matrix.

5.6 Comparison of Steel and Polymer-Based Fibres upon the Physical Properties of Cementitious Materials One particular study commissioned by the Research and Development Branch of the Alberta (Canada) Transportation Department was undertaken to compare what benefits incorporating steel and PP fibres had upon the physical characteristics of certain cementitious materials [32]. The products assessed during this study were concrete and a mortar ‘shotcrete’ used to repair bridge supports. Silica fume was also introduced into a proportion of the fibre reinforced cementitious systems analysed. When silica fume was present within the cementitious matrix an increase to the degree of force required to perpetuate cracking was noted. From the data collated from this study, it would appear the use of steel fibre reinforcement was favoured over those containing PP fibres. Subsequently a number of cementitious structures (i.e., bridges, beams etc.), were repaired using an appropriate steel reinforced cement-based system.

128

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials

5.7 Typical Applications of Fibre Reinforced Materials

5.7.1 Construction Fibres have been introduced into a variety of construction materials to improve certain properties and enhance the physical performance [32, 33]. These include cementitious overlays and mortars for renovating bridge decks and repairing areas where the concrete is showing signs of deterioration. Other applications include the formation of foundation piles and coatings for pipework in the case of propylene, or the formation of boat hulls from resin-based materials containing carbon fibres [15]. Fibre reinforced concrete can be cast into a variety of shapes, moulded or even sprayed and has been used in the formation of beams, pavement stone as well as the more traditional uses such as floor slabs for building [15, 34].

5.7.2 Introduction of Fibres into Cementitious Overlays

5.7.2.1 Repair of Concrete Road Surfaces Damaged and deteriorated concrete road surfaces can be costly to repair as well as difficult to maintain. The road cannot be ‘out of action’ for a lengthy period of time particularly on a busy route [35]. One method commonly adopted is to overcoat the damaged section with a cementitious overlay. Studies have shown if cracking within the surface of the concrete is not repaired, these cracks tend to reflect through the overlay, hence causing problems once more. However, the necessity of rectifying these cracks prior to the actual repair would ultimately increase the overall cost and time required to repair the damaged section. Introducing a small proportion of steel fibres (by volume) into the cementitious overlay has been observed to reduce this problem due to the extra flexibility these fibres impart into system. There is also the added capability of reducing the thickness of the overlay due to the enhanced performance of the cementitious material when steel fibres are added [15].

5.7.3 Comparison of Different Types of Reinforcement upon the LongTerm Performance of Cementitious Overlays A long-term study of reinforced concrete overlays indicated that after 15 years of continuous use the products had generally maintained their physical characteristics barring

129

Polymers in Cementitious Materials some areas of minor deterioration on the exterior surface of the material [36]. The effect of inserting dowels, a non-fibrous mesh and individual fibres of varying sizes upon the physical performance of cementitious overlays has been analysed. The objective being to determine which of the various methods utilised was the most effective at reinforcing the cementitious matrix. The greatest overall product performance was recorded for concrete sections 100 mm thick containing the non-fibrous mesh which resulted in the best structural durability, toughness and crack resistance [36]. In terms of the physical characteristics assessed during this exercise, inserting dowels into the concrete medium (approximately 125 mm cementitious overlay sections analysed) resulted in a material which came second in terms of performance. Of the sections reinforced with various sizes of fibres an improvement in performance was noted as the thickness of the actual overlay increased (approximately 50.4 mm to 75.6 mm). No significant differences in structural performance were noted by altering the length of the fibre to between 25.2 mm to 50.4 mm. Increasing the proportion of cement utilised in the concrete incorporating these individual fibres produced no significant improvements in terms of the physical performance. In this particular study, the sections reinforced with a non-fibrous mesh and steel dowels out performed the thinner sectioned overlays containing individual synthetic fibres. Hence, good long-term physical performance of concrete could be achieved using other methods of reinforcement other than the typical introduction of individual fibres into the matrix due to the reinforcing properties of dowels and a non-fibrous mesh.

5.7.4 Introduction of Synthetic Fibres into Cementitious Repair Mortar The introduction of collated PP fibres into the cementitious repair mortar ‘shotcrete’ has been investigated to determine whether there were any benefits to be gained in terms of physical performance [37]. This type of product is typically used to reinforce tunnels and other environments where there is a significant risk of subsidence. These fibres were added to wet mixes and compared with the traditional, wire mesh and steel reinforced systems. All specimens were subjected to the same load and degree of crack formation. This exercise determined that certain properties similar to the more traditional ‘shotcrete’ systems could be achieved introducing high quality collated PP fibres. This product would certainly have its benefits where a durable, ductile support material was required. By combining a high proportion of fly ash (> 50% by weight) with concrete and PP fibres a mortar suitable for use as an alternative for ‘shotcrete functions’ can be produced [38]. Through the addition of an appropriate superplasticiser sufficient workability can be achieved along with a suitable strength by limiting the water addition. Hence, an equivalent

130

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials to this commercial product was synthesised. Other advantages include a high degree of durability, structural toughness in conjunction with minimal drying shrinkage.

5.7.4.1 Pre-Cast Concrete Beams The introduction of steel fibres into pre-cast concrete beams in order to improve the strength characteristics have also been studied, the objective being to reinforce the cementitious matrix and dissipate energy that prevents cracking when exposed to a load [20]. A number of experiments were undertaken to determine the shear strength of the fibre-reinforced beams in comparison to ordinary cast concrete beams. Combining steel fibres with concrete effectively improved the ability of the structure to absorb energy and withstand a higher load before cracking of the microstructure was induced.

5.7.5 Fibre-Reinforced Resin Composites

5.7.5.1 Natural Fibre Resin Composites Glass fibres are regularly used in the formation of fibre reinforced resin composites. However, these fibres along with carbon fibres are expensive [39]. In recent years research has assessed the feasibility of designing fibre reinforced components using natural fibres, which provide similar physical properties to those containing glass fibres. Naturally occurring fibres have the advantage that their manufacturing processes are in many ways less intense than their synthetic equivalents and they are classed as environmentally friendly: an issue that certainly is a major concern for a vast array of modern day manufacturing processes. In one particular study the effect of using a naturally occurring fibre such as flax as an alternative to glass in the formation of fibre-reinforced composites was determined. In order to achieve the best performance it is important that the bond adhesion between the fibre and the resin is assessed within the resultant fibre reinforced composite. If good adhesion is not achieved between the fibre and resin interface, the physical performance of the composite will ultimately be compromised and in many cases act as a point of weakness within the material. The bond adhesion between the epoxy/phenolic resin and different types of fibres was determined. Bundles and strips of fibres were used where the outerlayer of the fibre remained intact (i.e., unretted), or had been removed (i.e., retted). The application method, quantity and size characteristics of such fibres and their affect on the physical performance of the resultant resin composite were assessed during this research.

131

Polymers in Cementitious Materials A more complex manufacturing process had to be utilised than would normally be required to maintain an even suspension and distribution of the flax fibres throughout the resin binder. This problem was primarily due to the differences in bulk density between the glass and the natural fibres used which were obviously lighter and hence it was more difficult to achieve a homogeneous mix. The wetting of fibres involving a pre-treatment process or the formation of a mat of fibres was suggested as possible ways of overcoming such issues. High strength and tensile modulus characteristics were noted when unretted and retted bundles of flax fibres were combined with a low viscosity epoxy resin. Improvements in the region of 300-400% were recorded in relation to the original epoxy resin when the fibre volume fraction was 0.5%. A similar and in some cases better physical performance was noted for the composites containing strips of retted flax fibres. A reduction in the tensile modulus of the composite was, however, observed when the curing process of the epoxy resin was accelerated. This was most likely due to the inability of epoxy to absorb into the fibre sufficiently during curing. The authors assumed that the actual manufacturing process used to produce these fibrous bundles in some ways damaged the fibres and hence reduced their stress capabilities. No improvement in structural reinforcement was observed when these types of flax fibres were combined with the phenolic resin compared to the physical properties of the original resin. It was possible that the bonding between the resin and fibres at the interface was poor, resulting in no significant improvement in physical performance relative to that achieved by the resin alone. Pre-treating these fibres with PVOH however improved the strength and stiffness of the resultant composite due to the improved bond adhesion between the fibres and this substance.

5.7.5.3 Highly Flexible Polymers PE fibres possessing a substantial modulus have been manufactured. However, these fibres tend to have a poor resistance to creep [2]. By controlled electron irradiation of the drawn fibres this problem can virtually be eliminated. Utilising this process effectively results in the formation of a yarn comprising a number of interconnected fibres. The physical characteristics of such fibres were maintained until the melting point of the polymer was attained. The researchers who worked on this project saw great potential for incorporating these crosslinked fibres into cementitious materials to prevent drying, shrinkage and reinforcing the hardened matrix. However, during this particular project they did not directly focus on introducing these crosslinked PE fibres into such a medium. Attention was focused

132

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials instead on assessing the effect of combining these fibres with other resin binders to produce resin-fibre composites. Great potential for the use of these fibres in the manufacture of ‘prepregs’ (i.e., fabrics produced commonly from glass and carbon fibres impregnated with a resin constituent such as an epoxy), was also noted due to the exceptional ability of these high modulus fibres to absorb energy upon impact. The surface of these PE fibres was treated with a plasma of oxygen and helium gas, which improved the adhesion between the fibre and the particular epoxy resin binder utilised in this study. A variety of uses were proposed for these irradiated flexible polymer including the formation of rope because they do not exhibit creep. The tensile and specific strength of these fibres is in certain substances comparable to that achieved by Kevlar and glass fibres.

5.7.5.4 Low Temperature Moulding Systems Fibre-reinforced composites are frequently used within the aerospace, automotive, marine and wind industries. Within the automotive industry, composites are used to manufacture the bodywork of sports cars and racing cars [6]. Radomes and other fittings within aircraft are also manufactured from composites [7]. The machinery and tools required to produce high performance composites along with energy demanding manufacturing processes such as autoclaving have certainly contributed to the overall cost of making such products expensive [11]. Ways of reducing the cost and time involved in such manufacturing procedures have been investigated to make such composites more desirable for the larger market place. One factor, which has been considered, is lowering the temperature at which the resin binder cures. The objective being to maintain the physical performance and quality of the final fibre reinforced material whilst making the manufacturing process more cost effective by reducing the necessity for autoclaving and high intensity tooling processes. Epoxy resins, which cure at high temperatures, i.e., above 100 °C, have conventionally been utilised in the formation of ‘prepregs’ in order to achieve the performance criteria required. However, research has suggested that the use of such resins is not always completely necessary to produce these types of composites. High temperature curing resins do have the advantage that they possess a sufficient open time at room temperature to enable them to be easily used in the manufacture of ‘prepreg’ products. A ‘low temperature moulding system’ has been developed. This commonly uses glass or carbon fibres in conjunction with formulated resins capable of curing at temperatures below those traditionally used. Reducing the curing temperature does affect the workability and

133

Polymers in Cementitious Materials period over which the system maintains its workability at ambient temperature. However a period of six days was still achievable. Due to the nature of the resin, components could be handled whilst post-curing proceeded without the necessity for additional support. Tools and other moulded components have subsequently been manufactured where the temperature of the curing process was reduced and in some cases the necessity for autoclaving removed.

References 1.

R. Hussein and N.P. Cheremisinoff in Elastomer Technology Handbook, Ed., N.P. Cheremisinoff, 1993, CRC Press, Boca Raton, FL, USA, p.875-907.

2.

I.M. Ward in Intregration Of Fundamental Polymer Science & Technology, Eds., L.A. Kleintjens and P.J. Lemstra, 1986, Elsevier Applied Science Publishers Ltd, Barking, UK, p. 634-647.

3.

P.N. Balaguru, ACI Materials Journal, 1994, 91, 3, 280.

4.

Functional Filler Fibres, Adhesive Age, 1992, 35, 9, 8.

5.

Concrete Toughness Up, in Materials Engineering (Cleveland), 1991, 102, 1, 38.

6.

Advanced Materials & Composites News, 1999, 21, 16, 7.

7.

C.F. Lewis, Materials Science and Engineering A, 1988, 105, 6, 37.

8.

Q. Zheng, and D.D.L. Chung, Cement and Concrete Research, 1989, 19, 1, 25.

9.

M. Kawamura and S. Igarashi in Proceedings of the Second ACI International Conference - Durability of Concrete, Montreal, Canada, Ed., V.M. Malhotra, 1991, Detroit, MI, USA, 1991

10. Z. Bayasi and J. Zeng, ACI Materials Journal, 1993, 90, 6, 605. 11. A. Potts, British Plastics and Rubber, 1997, November, 4. 12. S.A.S. Akers, J.B. Studinka, P. Meier, M.G. Debb, D.J. Johnson and J. Hikasa, International Journal of Cement Composites & Lightweight Concrete, 1989, 11, 2, 79. 13. S.H. Ahmad, P. Zia, T.J. Yu and Y. Xie, Concrete International, 1994, 16, 6, 36.

134

The Use of Synthetic Fibres to Reinforce Cementitious and Resin-Based Materials 14. H. Nakagawa, S. Akihama and T. Suenaga in Proceedings of the International Conference on Fibre Reinforced Cements And Concrete: Recent Developments, Eds., R.N. Swarmy and B. Barr, Elsevier Applied Science, London, UK, 1989. 15. J.J. Beaudoin, Fibre-Reinforced Concrete, Canadian Building Digest No.223, Institute for Research in Construction, National Research Council Canada, Ottawa, Canada, 1982. 16. H. Saadatmanesh, ACI Structural Journal, 1994, 91, 3, 346. 17. P. Soroushian, A. Tlili, A. Alhozaimy and A. Khan, ACI Materials Journal, 1993, 90, 2 182. 18. C. Ellis, High Performance Plastics, 1989, 6, 6, 1. 19. V. Ramakrishnan, G.Y. Wu and G. Hosalli in Transportation Research Record No.1226, 1989, 17. 20. S.W. Shin, J.G. Oh and S.K. Ghosh in Fiber Reinforced Concrete: Developments and Innovations, SP-142, 1994, American Concrete Institute International, Farmington Hills, MI, USA. 21. A-H.J. Al-Tayyib and M. M. Al-Zahrani, ACI Materials Journal, 1990, 87, 4, 363. 22. M.A. Taylor in Structural Materials, Proceedings of the ASCE Structures Congress 89, Ed., J.F. Orofino, San Francisco, CA, USA, 1989, p.12-20. 23. P. Soroushian, F. Mirza and A. Alhoziamy, Transportation Research Record, 1993, No. 1382, 64. 24. A.M. Paillere, M. Buil and J.J. Serrano, ACI Materials Journal, 1989, 86, 2, 139. 25. F. Benaiche and B. Barr, Proceedings of the International Conference on Fibre Reinforced Cements And Concrete: Recent Developments, Eds., R.N. Swarmy and B. Barr, Elsevier Applied Science, London, UK, 1989. 26. R.Y. Liang and E. Galvez, Proceedings of the First ACSE Materials Engineering Congress, 1990, Denver, CO, USA. 27. V.C. Li, S. Backer, Y. Wang, R. Ward and E. Green, Proceedings of the International Conference on Fibre Reinforced Cements and Concrete: Recent Developments, Eds., R.N. Swarmy and B. Barr, Elsevier Applied Science, London, UK, 1989.

135

Polymers in Cementitious Materials 28. M. Nagabhushanam, V. Ramakrishnan and G. Vondran in Transportation Research Record, 1989, No.1226, 36. 29. P. Soroushian, F. Aouadi and M. Nagi, ACI Materials Journal, 1991, 88, 1, 11. 30. P. Soroushian and A. Tlili, Transportation Research Record, 1991, No.1301, 6. 31. A-H.J. Al-Tayyib and M.M. Al-Zahrani, ACI Materials Journal, 1990, 87, 2, 108. 32. C.D. Johnston and P.D. Carter, Transportation Research Record, 1989, No.1226, 7. 33. K. Verhoeven in Proceedings of the 4th International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, IN, USA, 1989. 34. K-H. Kwakk, J. Suh and C-T. T. Hsu, ACI Structural Journal, 1991, 88, 2, 155. 35. G. Chanvillard and P.C. Aitcin, Canadian Journal of Civil Engineering, 1990, 17, 4, 521. 36. V.J. Marks, A Fifteen Year Performance Summary of Fibrous PC Concrete Overlay Research in Greene County, Iowa, 1989, Iowa Department Of Transporation, Highway Division, Ames, IO, USA. 37. D.R. Morgan, N. McAskill, B.W. Richardson and R.C. Zellers, Transportation Research Record, 1989, No.1226, 78. 38. V.M. Malhotra, G.G. Garette and A. Bilodeau, ACI Materials Journal, 1994, 91, 5, 478. 39. D.G. Hepworth, D.M. Bruce, J.F.V. Vincent and G. Jeronimidis, Journal of Materials Science, 2000, 35, 2, 293.

136

Adhesives and Coatings

6

Adhesives and Coatings Author

6.1 Introduction A vast variety of adhesives are commercially available either as a liquid or thixotropic paste and can be purchased over the counter from high street shops or through specialist distributors and manufacturers [1-3]. The vast majority of adhesives consist of a resin base and a curing agent (hardener) which when mixed in various proportions produce a hard matrix capable of provide an excellent bond between substrates such as metal, wood and ceramics [1]. Epoxy resins are utilised in the formulation of adhesives due to their exceptional mechanical properties whilst polyurethanes (PU) are favoured due to their flexibility. Many adhesives are designed to cure either at ambient temperature or require a heat source to promote crosslinking of the pre-polymers contained within, and hence curing. In certain circumstances single components are used where the resin component such as an epoxy pre-polymer and curing agent are combined together and curing is activated when a certain temperature is achieved. Typical curing agents used for these types of adhesives include boron trifluoride salts (BF3) and dicyandiamides, the reactions of which are initiated at high temperature [1, 2]. A cured matrix may also be achieved by the interaction of the pre-polymer and moisture. This type of reaction occurs for PU. Nowadays there is practically an adhesive capable of bonding any two materials together whether it be concrete to metal, metal to metal, glass, carbon fibres, the list is endless [4]. Structural stability, negligible shrinkage as well as superior bond adhesion are important characteristics which adhesives must possess in order to be suitable for use particularly within the construction, aerospace and automotive industries [1, 2]. In the case of concrete, the repair of damaged sections normally requires a compound, which rapidly cures in order to limit the time the area is out of action. Adhesives are used in vast quantities within the automotive and aerospace industries to adhere structural components removing the necessity for riveting or welding [1, 5].

137

Polymers in Cementitious Materials

6.2 Types of Adhesives Available Resin adhesives are ideal for structural and non-structural applications throughout a multitude of manufacturing processes due to the longevity of their physical performance and the strength characteristics attained [6]. Certain factors have to be taken into consideration when choosing the correct adhesive, such as its end use and the environment it will be applied in. If electrical machinery is operating, the flashpoint of the constituents, such as the solvents, within the adhesive have to be taken into consideration within such an environment during the application. Solvent, solvent-free and water-based adhesives are commercially available, as well as hot melt adhesives, and are utilised in a vast variety of industries and applications. Pressure sensitive adhesives are sticky to the touch and form a strong bond easily to an array of substrates with hardly any pressure except that exerted by a human finger or hand [7]. The cohesive nature of such an adhesive means they tend to stick together to such an extent that no residue remains when they are removed from the material they are bonded to.

6.2.1 Solvent-Based Adhesives Adhesives containing solvents have been used for several years in a variety of applications within the industrial, commercial and domestic (DIY) markets [8]. Typically, the solvent elements used consist of chlorinated and aromatic hydrocarbons, ketones and esters. A variety of polymers are used in the manufacture of solvent-based adhesives, these include acrylics, PU and polychloroprenes which are combined with the necessary fillers, pigments and antioxidants. With such a range of polymers available the physical properties of the adhesive can be tailored to meet specific requirements. Health and Safety as well as environmental legislations have been implemented to restrict the use of such solvents. A restriction on the emission of volatile organic compounds (VOC) applies to industries, which utilise five tons or more of such materials annually according to the Environmental Protection Act 1990 [9]. The quantity of solvents, which can be released into the atmosphere can be reduced by utilising the appropriate equipment within the manufacturing facility such as condensers. Alternatives to traditional solvents are now available and readily used, these are co-solvents which are described as more ‘environmentally friendly’ because they incorporate a high degree of solids. Water-based along with hot melt adhesives are also viable alternatives to solvent-based products depending on the physical properties required [10].

6.2.2 Solvent-Free and Water-Based Adhesives The requirement for solvent-free adhesives has certainly escalated over the last decade as industry has generally moved towards limiting or eliminating the use of solvents due to

138

Adhesives and Coatings the demands of the public [11, 12]. The necessity for special handling, storage as well as health and environmental consideration are the driving forces behind the change over. Particular growth in adhesives possessing 100% solids has been observed as they have the added advantage that they have drying times that are normally less than those commonly observed for water-based adhesives [12]. Thus meeting the market requirements for products, which can cure quickly and are easy to handle. Polymer dispersions (latexes) are commonly used in the manufacture of products such as contact tape and multipurpose adhesives. In order to achieve the required physical properties the polymer is combined with appropriate fillers, antifoamers and plasticisers [8]. Styrene butadiene rubber (SBR), and acrylic and vinyl acetate ethylenes possess a number of physical properties (i.e., bond adhesion, peel strength, resistance to yellowing and hydrolysis), favoured for the use of adhesives [7, 13]. These types of polymers are easy to store and handle, have a low toxicity, require no solvent precautions, have a good drying time, film formation and ideal bond adhesion. The manufacturing costs are relatively low due to reduced storage and handling requirements necessary when compared to utilising solvents and machinery, e.g., flame proof mixers. PU, polychloroprene and starch have also been used to manufacture water-based adhesives [8].

6.2.3 Hot Melt Adhesives Hot melt adhesives have a solids content of 100% and hence their manufacture does not involve the use of solvents which is advantageous in terms of environmental and health issues [14]. Such compounds have found favour within industry as an alternative to traditional solvent-based adhesives [8]. Hot melt adhesives are applied to substrates, which require bonding in a molten state such as wood, glass, rubber and paper. These adhesives do not lose water or solvents during the curing process, simply removing the heat source encourages the adhesive to solidify due to cooling which produces a permanent tough bond as a consequence of the crosslinked matrix formed [8, 14]. Such products harden quickly without the necessity of a curing agent, producing a strong durable bond capable of withstanding adverse temperatures as well as being resistant to a variety of chemicals [14]. Once an homogeneous mix has been achieved in a heated vessel the hot melt adhesive is filtered and discharged. Upon cooling the hot melt solidifies and is typically discharged as pellets which vary in colour (white to amber) depending on the raw materials used [14]. The type of polymer utilised will determine the cohesive strength, flexibility and durability of the resultant hot melt adhesive [8, 14]. Hot melt adhesives are commonly manufactured from thermoplastics such as polyolefines, ethylene-vinyl acetate co-polymers and thermoplastic rubbers (styrene/isoprene, SBR) [8, 14]. These thermoplastics are blended with other components to achieve the desired properties. Waxes, rosin, hydrocarbon resins, plasticisers

139

Polymers in Cementitious Materials and antioxidants are also used in one capacity or another [14]. Polyamides and polyesters have also been used as hot melt adhesives due to their own natural adhesion - it is not necessary to blend such compounds with other thermoplastics [8]. Hot melt adhesives can be heated to 160-180 °C before application proceeds [14]. It is important to maintain the required application temperature during the discharging process. Simply, the hot melt adhesive is contained with a vessel heated to the correct temperature, which is then dispensed through a hose and gun nozzle maintained at a specific temperature. One of the key advantages of using hot melt adhesives is their fast bonding time, which can accelerate the manufacturing efficiency as the time to assemble a particular product within industry can be reduced in relation to the use of alternative adhesives. Another advantage of hot melt adhesives is they are not flammable and hence do not release toxic fumes when burnt [10, 14]. Since there is practically no variation in the dimensions of the adhesive due to shrinkage during the curing process this type of adhesive is ideal for filling gaps [14]. The development of moisture-curing hot melt adhesive has provided an opportunity for this type of adhesive to be used in applications previously restricted to other adhesives such as solvent-based systems [15]. One example is the formation of ‘structural bonds’ which in the past have been achieved by the use of solvent-based adhesives, riveting or welding, etc. This type of adhesive has been used to produce insulated garage doors. Hot melt adhesives are generally used in a variety of industries such as the automotive sector where they are used to bond a number of fittings together or within the general consumer market to assembly goods such as freezers, washing machines [14].

6.3 Brief Summary of Composition, Properties and General Uses of Adhesives

6.3.1 Epoxy Resins Epoxy resins are commonly utilised as adhesives due to their exceptional chemical resistance, negligible shrinkage along with the ability to bond to a vast array of substrates [1-4, 16]. This includes glass and concrete, even when the surface is contaminated with materials such as oil. Epoxy adhesives are available which are capable of achieving a superior degree of bonding, yet maintain their physical performance when subjected to high temperatures [17]. In the case of epoxy resins a variety of curing agents are used including aliphatic amines, polyamides, aromatic amines, polymercaptans, phenol and anhydrides depending upon

140

Adhesives and Coatings the physical properties of the adhesive required and its application, e.g., aerospace or automotive industries [2]. Combining a polysulfide resin with an epoxy resin improves the flexibility of the resultant resin, accelerates the setting, thus reducing the open time [1]. Epoxy-based single component adhesives are used throughout the aerospace and automotive industries for a variety of applications [2, 6]. This component contains a curing agent (boron trifluoride salts, dicyandiamide) which is ‘dormant’ until a certain temperature is achieved then rapid curing is initiated. The coefficient of thermal expansion (CTE) also has to be taken into consideration in the case of epoxies when used in conjunction with concrete and other cementitious materials. Distinctive differences in the CTE will result in stress developing in the region where the two materials meet hence the possibility of cracking [1]. Fillers such as barytes and kiln dried sand are combined with epoxy resins to bring the CTE in line with that of concrete and steel. The introduction of fillers lower the exotherm which develops during curing and, depending upon the particle size distribution, enables the formulation to achieve the required thixotropy or flow characteristics [18].

6.3.2 Polyurethane PU adhesives are utilised extensively within a vast array of industries. Typical examples include the bonding of sheet moulding compounds within the automotive industries, the attachment of shoe soles and the formation of artificial sports surfaces by blending a PU with rubber particles [19, 20]. PU adhesives, which are capable of bonding rubber to metallic surfaces, are also available [21]. Flexible PU with a high bond adhesion can be produced by the reaction of a diisocyanate with a polyol (polyether/polyester). In fact the reaction of 4, 4´-diphenylmethane diisocyanate and castor oil has been observed to produce PU systems with good adhesive properties. The versatility of PU enables the formulator to target the properties of the products to particular requirements [20].

6.3.3 Acrylic Acrylic adhesives are commonly used to bond everyday items such as wood, aluminium as well as a multitude of other materials. These types of adhesives are used for a number of applications including the bonding of fixtures and fittings within ships [6, 22]. Waterbased acrylics have also been used in the development of bi-oriented polypropylene tapes in order to reduce the noise level produced during winding which is a characteristic of traditional solvent-based systems [23]. These types of adhesive are used extensively within the USA for sealing cartons and packaging, etc. Pressure sensitive adhesives are used in the manufacture of adhesive tapes, permanent application labels along with those that

141

Polymers in Cementitious Materials can be removed and reapplied a number of times. These products are produced from acrylic polymers combined with a co-monomer such as ethyl, hexyl or butyl acrylate acrylonitrile [24].

6.3.4 Urea-Formaldehyde/Phenolic Resins An ability to resist degeneration of the physical structure when exposed to water is a necessary characteristic of modern glues [25]. Urea-formaldehyde based resins are commonly used in wood adhesives and formed by a condensation reaction. The time it takes to achieve a hard structure is dependent upon the quantity of hardener incorporated and the surrounding temperature. Furfuryl or benzyl alcohol can be included in such adhesives to modify their properties and make them more resistant to cracking on the surface as well as increasing the flexibility of the hardened structure. Such properties make this type of adhesive ideal for filling gaps and hence they are commonly used within the upholstery business to assemble furniture. Phenolic resins are also commonly used in the formulation/manufacture of adhesives for bonding plywood.

6.3.5 Other Polymers used in the Manufacture of Adhesives A variety of vinyl acetate dispersions are utilised in the manufacture of adhesives to produce economical, rapid setting and water resistant materials which adhere to wood and most common types of packaging. This type of polymer adhesive is also used to bond paper and in the manufacture of pressure sensitive/permanently adhered tapes and labels [24]. Polyvinyl alcohols, natural gums and dextrin are commonly used as stabilisers in these types of emulsions. A wide range of products such as primers and low odour adhesives for flooring applications capable of bonding carpet and linoleum to concrete and other substrates have been manufactured containing SBR [26, 27] This type of polymer is also used in the manufacture of tapes, carbonless copy paper and magazine paper [28]. Depending upon the type of latex being utilised the tape can be designed to bond permanently to the required surface or be removed and replaced on several occasions. PVC co-polymer dispersions are commercially available and suitable for use in the formulation of laminating adhesives as well as backing material for carpets. These dispersions exhibit a good storage stability and stable viscosity [28].

6.3.6 The Effect of Incorporating Rubber into Adhesives Today adhesives have to have a number of properties in order to be commercially attractive, i.e., toughness, excellent bonding capabilities, high impact resistance as well as the ability

142

Adhesives and Coatings to prevent peeling from the surface to which they are applied if a permanent bond is required [6]. The use of epoxy resins in the formulation of adhesives is favoured due to their exceptional physical properties. However, the resultant crosslinked structure formed as a consequence of the reaction between the epoxy pre-polymer and a suitable curing agent is also brittle. Toughening of the epoxy matrix has been observed to improve the impact strength and hence broaden the end use of such systems [29]. Thermoplastic and thermosetting resin can be toughened by the addition of nitrile rubber (i.e., polybutadiene/acrylonitride), into the resin [30]. Research has shown that an improvement in the toughness of epoxy and acrylic-based resins can be achieved by incorporating a low molecular weight rubber [6]. If the rubber constituent possesses the correct functional group a reaction can occur between the rubber and the epoxy present. Research has shown that the adduct formed can react in the same manner as conventional epoxies with a typical curing agent such as amides or amines. The rubber essentially reacts and becomes part of the polymer particles, which have previously formed. These in turn enhance the ability of the cured resin to dissipate and withstand stress. Introducing rubber into epoxy resins has not been observed to effect or influence the overall curing mechanism. In one particular study various proportions of rubber were incorporated into single pack epoxies, which required heat to cure. These compounds remained brittle until the rubber content exceeded 15%. However as the quantity of rubber exceeded this level an increase in the resistance of the adhesive to impact and peel was noted. It should also be noted that although rubbers are an efficient toughening agent a reduction in the ‘end-use’ temperature of epoxy-based systems has also been noted following their introduction [29]. Alternatively siloxane-based rubbers have been utilised, the introduction of this type of rubber in certain circumstances does not have such an effect on the working temperature. Analysis has also determined that an improvement in the toughness of epoxy systems can be achieved by the addition of thermoplastic polyimides. An improvement in the toughness has also been noted when poly(ether)sulfones are used.

6.4 The Use of Adhesives within the Construction Industry As with any products it is important that adhesives achieve a sufficiently strong bond in an appropriate length of time and that the product is capable of adhering to a variety of substrates [1]. Negligible or limited shrinkage should be registered throughout the curing process. Yet the hardened compound should be tough enough to withstand varying climatic conditions hence not be susceptible to structural deterioration due to UV light, acid rain, frost and in some applications significant changes in temperature.

143

Polymers in Cementitious Materials Concrete has been used in a structural capacity onto which a decorative finish is applied in order to achieve an aesthetically pleasing appearance [1]. Epoxy adhesives are commonly used to bond such finishes due to their exceptional bonding properties and strength characteristics. Similarly, steel dowels and anchor bars are secured into concrete using epoxy resin based compounds - in such circumstances the pull out strength is one of most important properties. Significant bonding between the concrete, steel and epoxy resin is required in order to prevent dowel or anchor bars becoming dislodged during service. It is also important that the component used to repair the concrete meets and surpasses the compressive and tensile strength of the concrete. When cured, epoxy products such as bonding agents have the added advantage of being capable of achieving a compressive and tensile strength higher than ordinary concrete. Hence epoxy-based mortars and grouts are regularly used to repair cracks and damaged sections of concrete. Polymers (i.e., acrylics, SBR and ethylene-vinyl acetate), in the form of emulsions or redispersible polymer powders are commonly blended with cementitious materials containing a blend of cement constituents, fillers such as sand, plasticisers and antifoamer [31]. When cementitious materials are mixed with either polymer emulsion or the correct quantity of water, if the cementitious component contains a redispersible polymer powder, a tile adhesive can be produced which is capable of bonding floor and wall tiles. A combination of cement and polymer emulsion or simply the latter only can also be used to provide a bonding coat between a concrete substrate and subsequently applied cementitious mortar or self-smoothing flooring compound [31, 32]. The application of such bonding coats or the application of the polymer emulsion alone as a primer also decreases the possibility of the newly applied cementitious material debonding from the substrate [31]. However is it important that the bonding coat does not dry before the application of the required screed or mortar as the benefits of applying in terms of bonding are diminished.

6.5 Automotive and Aerospace Applications An important factor for a number of adhesives used in particular within the aerospace and automotive industries is fire resistance as well as producing minimal smoke and fumes [33]. Tragic incidents have all too clearly demonstrated the devastating effect fire can have especially within a confined space [6]. In order to prevent the repetition of such effects, self-extinguishing materials, which do not continuously burn have been developed [33]. Minimising the generation of toxic fumes such as carbon monoxide (CO) and sulfur dioxide (SO2) within a confined area is also a critical factor. Several adhesives are now commercial available, which produce minimal levels of toxic compounds in a fire. Due to their application it is also important that the bond strength of adhesion is maintained when exposed to heat. These types of adhesives are used in aircraft to bond the

144

Adhesives and Coatings internal fittings such as luggage compartments, removing the necessity to rivet or weld of components [1, 2]. Typically, multipurpose flooring adhesives tend to lose their adhesion and bonding capabilities when subject to the high temperatures normally experienced in a fire [33].

6.6 Resin Coatings

6.6.1 Introduction Coatings are extensively used throughout a wide variety of industries such as construction, aerospace and automotive to provide a decorative, hygienic, chemically resistant and hard wearing surface. Typically, coating systems are based on epoxy, acrylic or PU technologies depending upon the type of application and physical properties required [3, 31, 34]. Within the construction industry, coatings are applied to old or (depending upon the chemical nature of the polymer utilised) new concrete in order to seal the surface and prevent subsequent dusting of the concrete underneath [34]. Coatings can provide a durable, easy cleanable surface which is resistant to a variety of chemicals which would normally attack and degenerate the substrate underneath [10, 34]. These systems are commonly supplied as two separate components (i.e., base and hardener). Other coatings used to varying degrees within the construction industry include oil-based, rubber- and bitumenbased systems [32]. Generally, the types of coatings available can be separated into two categories: those that cure at ambient temperature and those which require heat to promote curing [2]. Ambient temperature curing coatings are generally favoured throughout the construction and other industries as a heat source is not required in order to apply the product. These coatings provide a durable surface capable of withstanding a high degree of wear and tear, impact and abrasion. In the case of epoxy resins, crosslinking commonly occurs via the epoxide ring and is initiated by the introduction of a suitable curing agent such as polyamines in the case of ambient curing systems. Alternatively, acid anhydrides or polycarboxylic acids are used in heat curing systems where the reaction can proceed by both the epoxy and hydroxyl groups present. Numerous epoxy resins are used in the formulation of bonding agents and coatings [2, 16]. These resins tend to have a superior resistance to a vast range of chemicals, good tensile and impact strength. Aliphatic diisocyanates are commonly used in the manufacture of the majority of PU coatings, although the use of aromatic diisocyanates still occurs depending upon the

145

Polymers in Cementitious Materials required physical properties of the resultant PU coating [35]. The former is generally favoured due to its stability towards discoloration when exposed to UV light. The polyol utilised normally possesses carbonate, ester or ether functionality [19, 21, 36, 37]. PU dispersions based on polyester are the most diverse and general products commercially used due to their resistance to UV radiation and hydrolysis [35]. The resultant resin is capable of enduring exposure to a wide range of chemicals as well as exhibiting good adhesion and colour stability when exposed to UV rays. When the polyol contains a polyether group, the resultant PU dispersion exhibits a good degree of flexibility even at low temperatures and is not susceptible to hydrolysis. However, the resultant polymer film does have a tendency to be affected by UV light. Whilst superior adhesion and a tough exterior are noted when the polyol used in PU manufacture contains a carbonyl functionality. The use of acrylic coatings is favoured within the construction industry where a high physical performance is required [38]. This is attributed to the exceptional wearing characteristics and chemical resistance of this type of polymer [31]. Coatings manufactured from this type of polymer are ideally suited to exterior applications due to resistance to deterioration from UV radiation, hydrolysis and oxidation [13]. Clear acrylic sealers designed to seal and harden concrete surfaces are also commercially available [34]. The application of this type of sealer improves the resistance of the concrete to abrasion and subsequently enhances the durability and reduces the necessity to repair the surface due to general wear and tear. Acrylic emulsions are also used to seal the surface of concrete before the application of self-smoothing floor screeds. This provides a barrier between the concrete surface and newly applied screeds preventing the transport of air contained within the pores of the concrete into the wet screed. Any air entering the screed from the concrete will cause bubbling and the formation of pinholes. These pinholes cannot be removed once the screed begins to harden resulting in a porous finish.

6.6.2 Solvent-Based Coatings Solvent-based coatings have been extensively used throughout the construction industry as concrete sealers and paints [39]. Typically the solvents initially utilised were manufactured from petroleum. The use of solvent-based coatings was favoured within general industry due to the high degree of chemical and abrasion resistance obtained [40]. Indeed, the degree of impact and abrasion resistance a coating possesses are important considerations when determining whether it is suitable for a particular application. An example is the application of coatings in industrial situations such as factories, warehouses where the floor areas will be exposed in some cases to a significant degree of traffic, i.e., foot and mechanical. Another key advantage of solvent-based coatings is their natural ability to wet the surface onto which they are being applied and thus achieve a good degree of adhesion [41]. Surfactants or alcohol have been added to water-based coatings in order to reduce the

146

Adhesives and Coatings surface tension and hence achieve a performance on a par with those containing solvent. Solvent-based epoxy coatings have been traditionally used to prevent metal corrosion [39]. However, the necessity to reduce the use of VOC due to environmental and health concerns has focused the formulator to provide solvent-free and water-based alternatives

6.6.3 Conversion from Solvent-Based to Water-Based Compounds The conversion from solvent-based resin systems to water-based compounds is not necessarily as easy as it sounds and there are a number of difficulties or issues associated with such a process [40]. At the latter end of the 20th century a survey of numerous resin coating manufactures revealed that the vast majority preferred to continue current production methods. This option was directly related to the equipment, manpower, training and disposal methods currently in place. The disposal of water-based solutions containing compounds such as isocyanates, which cannot be disposed of by normal drainage methods or recycled completely, is also a problem, which has to be dealt with. The other core difference between water and solvent-based resins is the general physical properties, which in many cases are far superior for a solvent containing system. This is particularly revalent in terms of their resistance to a diverse range of chemicals, although advances in polymer technology have certainly enhanced the performance of water-based materials, the ultimate object is to obtain a similar performance. This has already been achieved for certain acrylic containing compounds however another alternative is to produce high solids coating containing diluents which provide the necessary viscosity yet limit the use of solvents. Oxazolidine diluents were invented for this purpose and they have been used in combination with low viscosity polyols and isocyanate compounds to produce high solids coatings. This includes the manufacture of clear lacquers such as those used in the automotive industry where the VOC content was below the acceptable level for this industry. The raw materials used have the lowest viscosity possible, yet maintain the necessary physical performance of the coating upon its final application. The advantage being that the solvent required to achieve the desired flow and consistency is minimised.

6.6.4 Solvent-Free and Water-Based Coatings In response to increasing concerns regarding the use of solvents and solvent-based products, the application of water-based, solvent-free coatings and membranes have increased within the general construction industry [2]. A number of these coatings are pigmented and unlike the cementitious systems they have the added advantage that practically any available colour can be incorporated. Coatings of this type have certainly increased in

147

Polymers in Cementitious Materials popularity due to the aesthetic finish that can be achieved in conjunction with the physical performance and the protection such coatings can provide [2, 16]. Eliminating the presence of any solvent also enables the application of solvent-free or water-based coatings to proceed during normal working hours. This prevents the need for any disruption to the working environment such as ‘closed off’ areas which in certain circumstances such as key manufacturing areas within factories can be quite expensive [10]. These types of coatings can be applied to floors, walls and ceilings to provide protection against chemicals, graffiti and produce a hygienic surface.

6.6.4.1 Water-Based Epoxy Coatings As well as providing an hygienic surface for walls and floors, two pack epoxy coatings have been used to protect steel from corrosion over the last 30 years [39]. The development of water-based coatings as substitutes for the traditional solvent-based products has escalated over this period of time. This is primarily due to the necessity of lowering the consumption or release of VOC within the workplace and surrounding atmosphere. The first breakthrough in the formulation of water-based products occurred during the 1970s with the development of an aqueous polyaminoamide system, which was suitable, as a curing agent yet did not contain any solvent. Polyaminoamides are formed by the reaction of a fatty acid (dimer) with a polyamine such as diethylenetriamine [42]. Either a solid or thixotropic liquid is produced depending upon the chemical nature of the polyamine utilised along with the molar ratio of each component. Polyaminoamides are comparable to a surfactant molecule, curing occurs via the reaction of the epoxide group present on the epoxy pre-polymer and the amine groups present within the amide [43]. Curing epoxies with an polyaminoamide can produce coatings with an excellent resistance to a multitude of chemicals. Corrosion prevention has also been noted in conjunction with an excellent bond adhesion making this resin ideal for use within heavy industry situations [42]. Polyamine adducts were subsequently invented in order to broaden the uses of such compounds enabling the manufacture of a more diverse range of end products containing a higher solids content or low viscosity than previously achieved [39, 43]. These types of polyamine adducts can be used in combination with epoxy resins producing a material which is suitable for use on ‘green concrete’ (i.e., still curing), as a curing membrane or surface sealer [43]. Water-based epoxies containing a high molecular weight resin dispersed within, can be produced using a minimal quantity of co-solvent [39]. These resin systems cure by evaporation and the polymer molecules coalesce to produce a polymer film [39, 43]. However, they can also be combined with aqueous curing agents (i.e., amine based), resulting in the formation of a tougher more durable film due to interaction of the amine

148

Adhesives and Coatings and epoxy. As research into this area progressed, the combination of different resins was assessed (i.e., acrylic with epoxies). This involved manufacturing a water-based acrylic with an amine or carboxyl functionality which when combined enabled the epoxy and functional group to interact, resulting in a polymer film which is capable of undergoing further crosslinking. The formation of such films occurs via evaporation followed by coalescence of the polymer particles. However, crosslinking proceeds over a number of days. Commercially available water-based epoxy resins are regularly used throughout the construction industry as decorative, hygienic coatings and clear sealers [43].

6.6.3.2 Water-Based Polyurethanes The necessity to restrict the use of VOC has resulted in the increasing popularity of waterbased PU [35, 44]. Removing the solvent from these types of products has a number of advantages. The emission of VOC and their impact on the environment and human health is considerably reduced. Preventing the use of solvents with low flash points also alleviates concern regarding the flammability of such products, and reduces hazards during their manufacture [44, 45]. Interest in the formulation of water-based PU has certainly escalated in recent times due to the flexibility, chemical properties and the resistance to impact and abrasion of such a compound [35, 43]. Commercially available water-based PU systems have been on the market since the latter end of the 20th century [44]. These PU dispersions can be tailored to meet the specific requirements of the end user hence the consumption of such dispersions has increased. A variety of adhesives and coatings with no or minimal quantities of co-solvent have been manufactured [35]. Technological advances within this area have produced aqueous urethanes, which in certain circumstances achieve similar physical properties to those of solvent containing systems. As previously discussed (in Section 2.2.11.7) PU have a unique morphology as the polymer backbone comprises hard and soft segments attributed to the diisocyanate and polyol, respectively [45, 46]. It is this structural characteristic which is responsible for PU’s versatile properties including superior film development. Many of the products manufactured from PU possess a high degree of impact and abrasion resistance, which makes them ideal for use in coatings. Waterborne PU commonly possess linear pre-polymers hence the crosslinking of such polymer molecules by the addition of a suitable curing agent is normally required in order to achieve a physical performance on a par with that noted typically for solvent containing systems [35]. Developments in PU technology have included the introduction of different monomers into the urethane backbone including the formation of ‘interpenetrating polymer networks’. There are a number of ways of manufacturing PU dispersions:

149

Polymers in Cementitious Materials 1. Pre-polymer mixing process, 2. Ketanine/ketazine process, 3. Melt dispersion process, 4. Acetone process. A study by Manock and co-workers focused on the pre-polymer mixing process due to the fact that no significant volume of solvent was required [35]. This process generally results in the formation of a ‘hydrophilic modified NCO terminated PU pre-polymer’. Aromatic (i.e., 2,4-toluene diisocyanates), or aliphatic diisocyanates (isophorone diisocyanate) can be used in this process, however the latter is generally favoured due to the resistance to deterioration when exposed to UV radiation and limited reaction with water. The diisocyanate is combined with a polyol which is either polyether- or polyester-based. The actual physical properties of the water-based PU dispersion will determine whether a polyester or polyether is used. For example polyesters are used if the water-based PU dispersion must exhibit a high degree of resistance to UV radiation. A pre-polymer (NCO terminated) is formed by the reaction of the diisocyanate and polyol component. Further steps in the pre-polymer mixing process involve the neutralisation of the pre-polymer before dispersion into the aqueous phase followed by the extension of the polymer chains by the addition of an appropriate compound such as an aliphatic diamine. This process results in the formation of a water-based PU dispersion (high molecular weight). In order to reduce the cost of the PU dispersion other monomers have been introduced into the urethane backbone, i.e., acrylics, alkyds and polyesters. With all polymer blends it is important that the different monomer species are compatible, if this is not the case then coagulation will tend to occur upon mixing, resulting in the formation of a milky coloration or the separation of components during storage. By combining PU and acrylic compounds the resultant dispersion can be used as a coating where the main physical properties are an ability to rapidly harden, a good degree of flexibility and resistance to a wide range of common chemicals. Coatings manufactured from water-based PU are suitable for use on concrete walls, floors and tiles as well as other types of masonry. However if the resin coating is to be used in any external environment it is important that it does not discolour or deteriorate when exposed to UV light.

6.6.3.3 Other Polymer Dispersions A film is produced from a polymer dispersion (SBR, acrylics or ethylene vinyl acetate copolymer) by the polymer particulates coalescing together following the evaporation of the aqueous constituent [31, 47, 48]. Depending upon the chemical nature of the polymer,

150

Adhesives and Coatings the resultant polymer film can possess a degree of chemical resistance, resistance to hydrolysis and a certain degree of flexural/tensile strength [13, 48]. The physical properties of these polymer films are, however, in many ways inferior to those produced when the final resin film is formed by a crosslinking process. The introduction of compounds to promote chemical crosslinking of polymer dispersion has been researched over a number of years. One particular study has focused on the crosslinking of an acrylic dispersion with a cycloaliphatic epoxide and the effect of introducing a catalyst. The catalyst used to promote crosslinking was either sulfonic or phosphonic acid in origin or sulfonic/phosphonic acrylic acid monomer. The initial acrylic latexes were produced from butyl acetate/methyl methacrylate (MMA). 2-Hydroxyethyl methacrylate or methacrylic acids were also incorporated during the formation of the acrylic latex in order to produce hydroxyl or carbonyl functionality. Cycloaliphatic epoxides were utilised as they have a high affinity for carbonyl and hydroxyl groups hence crosslinking can be initiated in the presence of compounds containing these functionalities. It was determined that for an acrylic possessing a hydroxyl functionality, an acid catalyst was required to initiate crosslinking of the polymer chains. In the absence of such a catalyst the physical properties of the resultant coating were poor. This was not necessarily the case when the acrylic polymer possessed a carboxyl functionality. In fact the addition of an acid catalyst increased the susceptibility of the coating to water and solvent. Crosslinking between the acrylic dispersion and cycloaliphatic epoxide was promoted by the addition of the acrylic acid monomers (sulfonic and phosphonic based). Neutralisation of the acid catalyst did result in an overall improvement in the physical properties of the resultant coating. The authors attribute this phenomenon to the presence of the acid catalyst resulting in the cycloaliphatic epoxide hydrolysing [49].

6.6.4.4 Powder Coatings Advances in powder technology have resulted in the development of powder coatings capable of providing a protective and decorative finish on metal surfaces [50]. These coatings contain thermosetting resin powders and can be produced in a variety of colours resulting in protective and decorative finishes. The absence of solvent compounds also makes this type of product a favourable alternative to traditional paint finishes. The degree of toughness and resistance to chipping of the resulting finish produced from these types of coatings has been noted to exceed that achieved from the typical liquid paints utilised within the automotive industry. An ability to retain colour definition, UV stability, moisture, and chip resistance as well as being capable of resisting a variety of chemicals (i.e., acid, petrol, etc.), are important factors. Research has focused on improving the durability of

151

Polymers in Cementitious Materials such coatings to the effect of weather and UV radiation, which will expand the uses of such coatings particularly within the automotive industry.

6.6.5 Common Applications

6.6.5.1 Construction Coatings are normally available as single or two component systems, which are cured by heat, moisture or a crosslinking process resulting from the addition of a curing agent [34]. These coatings can be water-based, solvent-free or solvent-based in origin and are normally used to provide a surface which is hard wearing, resistant to a variety of chemicals, hygienic and easy to clean particularly in food preparation areas [10, 32, 34]. Generally, excellent chemical resistance can be achieved with solvent-based coatings and hence they are applied in areas where contact with aggressive chemicals (i.e., corrosives), regularly occurs. An improvement in freeze thaw resistance has also been noted and thus can prevent damage of the concrete as a consequence of this cycle [34]. These materials are frequently used on brickwork, breeze blocks and paving stones in order to help to maintain the physical properties and extend overall lifetime of such products as early or continual replacement can be a costly exercise [16].

6.6.5.2 Protection of Metal Surfaces Corrosion of metals by exposure to chloride ions, acid rain and other chemicals can be a particular problem throughout a number of industries including the construction, aerospace and automotive industries [32]. A significant amount of time and effort is therefore spent throughout these industries in order to prevent the corrosion of metals from occurring. Coatings have commonly been used to protect metals such as steel and aluminium against corrosion [51]. The physical properties of the coating required will vary depending upon the application, metal and the environment in which it is used. Protective coatings within the aerospace industry have to be capable of maintaining their performance when exposed to extremes in temperature as well as being resistant to a variety of chemicals utilised within this industry such as oil and skydrol (aviation hydraulic fluid). Epoxy chromate primers have been used to prevent the corrosion of metals such as aluminium alloys. However, alternative coatings are now being favoured due to the toxicity issues associated with chromate-based pigments. Zinc phosphate pigments are regularly used in the development of corrosion preventive coatings [39]. Corrosion prevention has

152

Adhesives and Coatings also been noted when barium sulfate and ‘micaceous iron oxide’ have been introduced into coating systems. Corrosion of steel reinforcement bars housed within concrete can occur primarily by two key routes, carbonation or chloride exposure due to either the porosity of the concrete or alternatively the initial level of chlorides present within the concrete mix [32]. Carbonation of concrete and hence degeneration of the steel reinforcement within can be reduced by the application of a coating which can prevent the penetration of carbon dioxide into the cementitious matrix. However, it should be noted that in these circumstances corrosion of the steel reinforcement would only be prevented if no contact with carbonation has occurred prior to the application of the coating. Alternative methods to the actual manufacture of coatings have also been determined. In one particular study, the feasibility of polymerising the actual polymer on the metal surface has been assessed [52]. This process would have the obvious advantage that the manufacture and application of the coating occurred at the same time thus making the process more cost effective as the time and energy required to actually produce the coating beforehand is removed. Certain resin coatings have been produced containing hydrophilic polymer molecules, which will absorb a small quantity of water yet prevent enough passing through to damage any material situated in the underlying structure. A 4-carboxyphenyl maleimide/styrene or MMA system was studied. The material to be coated was submerged into the solution containing primarily the desired monomer, which was then polymerised. The reaction time, reaction rate and initial monomer concentration governed the quantity of polymer produced. Extending the reaction period and quantity of monomer used will enhance polymer development. Analysis has shown that a polymer film (50 μm thick) can be achieved with 4-carbonylphenylmaleimide/styrene after 10 minutes.

6.6.5.3 Overcoating of Metal Structures The renovation and redecoration of motorway bridge structures is a demanding and in some cases a very costly process [53]. To the end of the latter half of the 20th century an area of concern was raised relating to the ability to overcoat traditional lead-based coatings without the necessity to remove such materials first. The ability to undertake such a process would reduce the time required for renovation. The application of a sealer capable of penetrating into any rust present is applied first. Epoxy-based sealers have been designed to provide ideal wetting characteristics, yet produce a sound bond between subsequent topcoats. It is important that the overcoat should be capable of withstanding a high degree of weathering over several years. UV stability is also important in order to prevent the sealer becoming brittle.

153

Polymers in Cementitious Materials Water-based acrylics, high solids alkyds and epoxy sealers have been used for this purpose and are capable of providing a good bond, however they also have a number of disadvantages. The acrylics are not suitable if the previous coating is in a poor condition, an ability to induce stress in the previous coatings and low temperature stability are also an issue with high alkyds and epoxies, respectively. Research has been undertaken to assess the performance of a variety of coating systems when exposed to cyclic freeze thawing, ‘UV radiation/condensation cycle’ and ‘salt-fog’. The coatings analysed were based on acrylics, acrylic-epoxy, zinc rich epoxies and PU. Restricting the level of VOC utilised in the manufacture of these coatings is also favoured. The most favourable results in terms of the physical performance of the coatings when exposed to such conditions, were found when using a zinc rich PU.

6.6.5.5 General Applications Styrene acrylics and vinyl acetates are used in coatings designed for protecting paper and release agents for labels, tapes, and so on [54]. Melamine-based formaldehydes are used to increase the strength of materials such as tissues and paper towels even when in a wet state. Whilst resin coated paper and filters are other products, which commonly incorporate acrylics of vinyl ethylene acetates in one form of another. Acrylic resins are also frequently used in the formulation of sealants, paints and coatings [22]. They are available as water-based products and in certain circumstances the physical properties are comparable with those observed for the more traditional solvent-based products. These compounds have traditionally be used in the formulation of paints.

References 1.

P. Maslow in Plastic, Mortars, Sealants and Caulking Compounds, Ed., R.B. Seymour, ACS Symposia Series No.113, ACS, Washington, DC, USA, 1979, p.39-60.

2.

R.S. Bauer in Applied Polymer Science, 2nd Edition, Eds., R.W. Tess and G.W. Poehlein, ACS Symposium Series No.285, ACS, Washington, DC, USA, 1985, p.931.

3.

Urethanes Technology, 1992, 8, 6, 2.

4.

European Adhesives & Sealants, 1989, 6, 2, 46.

5.

D. Basque and K. Bitar in Proceedings of the SPI Hybrid Technologies for the Epoxy Resin Formulator Conference, Hollywood, FL, USA, 1990, Paper No.12.

154

Adhesives and Coatings 6.

K.W. Harrison in Proceedings of Adhesion 12, the 25th Annual Conference on Adhesion and Adhesives, London, UK, 1987, p.121.

7.

R.M. Mohsen, H.E. Nasr and A.S. Badran, Pigment & Resin Technology, 1995, 24, 3, 9.

8.

M. Cook, Pigment & Resin Technology, 1996, 25, 1, 12.

9.

W.M. McLauchlan, Pigment & Resin Technology, 1996, 25, 4, 25.

10. A. Cargill, Pigment & Resin Technology, 1996, 1996, 25, 4, 29. 11. Pigment & Resin Technology, 1996, 25, 1, 30. 12. Adesione, 1992, 3, 3, 15. 13. Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 186. 14. P.G. Frankling, Pigment & Resin Technology, 1996, 25, 1, 4. 15. M. Brannen and J. Broeder in Proceedings of TAPPI Hot Melt Symposium Conference, Hilton Head Island, SC, USA, 1992, p.31. 16. Chemical Industries Newsletter, 1988, March/April, 4. 17. Rubber World, 1992, 206, 2, 62. 18. P.I. Ku, Advances in Polymer Technology, 1988, 8, 1, 81. 19. B.K. Howe in Proceedings of Rubberplas 84 Conference, Singapore, 1984, Volume 1, Paper No.2. 20. J. Johnston, Shell Chemicals Europe Magazine, 1995, 2, 8. 21. M.S. Bhatnagar, Popular Plastics and Packaging, 1992, 37, 7, 43. 22. M. Reisch, Chemical and Engineering News, 1994, 72, 5, 13. 23. G. Pedala, Pigment & Resin Technology, 1996, 25, 1, 21. 24. Harco Dispersions: A Guide To Their Properties And Applications, Harlow Chemical Company Ltd., Harlow, UK. 25. F.F. Abd El Mohsen, R.M. Mohsen and Y.M. Abu Ayana, Pigment & Resin Technology, 1996, 25, 1, 17.

155

Polymers in Cementitious Materials 26. More Than Just Cement, Synthomer GmbH, Frankfurt am Main, Germany. 27. Synthomer The Synthetic Latex Specialists, Synthomer GmbH, Frankfurt, Germany. 28. Plastics World, 1982, 40, 11, 75. 29. J.N. Hay, B. Woodfine and M. Davies, High Performance Polymers, 1996, 8, 1, 35. 30. R.S. Drake in Proceedings of ACS Polymeric Materials Science & Engineering, 1983, Cleveland, OH, USA, p.220. 31. J.D.N. Shaw, P.J. Brown, R. Cather, R. Dennis, P.C. Hewlett, R.A. Johnston, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Technical Polymers No.39, Polymers In Concrete, 2nd Edition, Concrete Society Working Party, The Concrete Society, 1994. 32. T.P. O'Brien, W.B. Long, P.C. Hewlett, J.D.N. Shaw, L.J. Tabor and P. J. Winchcombe, Repair of Concrete Damaged by Reinforcement Corrosion, The Concrete Society in association with The Federation of Resin Formulators and Applicators, and The Association of Gunite Contractors, London, UK, 1984. 33. E. Rietkirk, Aerospace Composites and Materials, 1991, 3, 5, 19. 34. SBD Flooring Products Guide, SBD Construction Products Ltd., Flitwick, UK, 1991. 35. H.L. Manock, Pigment & Resin Technology, 2000, 29, 3, 143. 36. Urethane Plastics and Products, 1992, 22, 8, 1-3. 37. K.K. Oster, Rubber and Plastics News, 1995, 24, 18, 30. 38. I. Nelson in Proceedings of the ConChem Conference, 1997, Dusseldorf, Germany, p.1-8. 39. G. Howarth, Pigment Resin & Technology, 1995, 24, 6, 3. 40. N.G. Carter, Surface Coatings International, 1999, 82, 10, 497. 41

V.P. Janule, Pigment & Resin Technology, 1995, 24, 1, 7.

42. Paint Manufacturer, 1979, 8, 20.

156

Adhesives and Coatings 43. G.A. Howarth in Waterborne Coatings and Additives, Eds., D.R. Karsa and W.D. Davies, RSC, Cambridge, UK 1995, p.58. 44. D. Dieterich in Proceedings of the 6th International Conference in Organic Coatings Science and Technology, Athens, Greece, Advances In Organic Coatings Science And Technology Series, Volume 4, 1982, p.51. 45. B.K. Kim and J.C. Lee, Journal of Polymer Science: Part A Polymer Chemistry, 1996, 34, 6, 1095. 46. H. Janik and J. Foks, in Proceedings of Utech '92 Conference, Hague, The Netherlands, 1992, p.170. 47. J.A. Manson, Materials Science and Engineering, 1976, 25, 1-2, 41. 48. D.Z. Zurbriggen, Influence Of Redispersible Powders On Shrinkage, Hydration Behaviour And Microstructure Of Tile Adhesives, 1999, Elotex AG, Sempach Station, Switzerland. 49. S. Wu, J.D. Jorgensen, A.D. Skaja, J.P. Williams and M.D. Soucek, Progress in Organic Coatings, 1999, 36, 1-2, 21. 50. M.S. Osmond, Pigment & Resin Technology, 1995, 24, 6, 10. 51. B. Norton, Pigment & Resin Technology, 1995, 24, 6, 14. 52. X. Zhang and J.P. Bell in Proceedings of the 20th Annual Anniversary Meeting Of The Adhesion Society, 1997, Hilton Head Island, SC, USA, p.413. 53. A. Edwards, Pigment & Resin Technology, 1995, 24, 6, 16. 54. R. Shereff, Chemical Business, 1987, 9, 6, 30.

157

Polymers in Cementitious Materials

158

Summary of the Applications and Benefits of Utilising Polymers in Construction

7

Summary of the Applications and Benefits of Utilising Polymers in Construction Author

The discovery of Portland cement in the 18th century represented a turning point in the history of construction. Products readily utilised before the conception of Portland cement such as lime and clay, though malleable and easy to work, did not achieve a high strength when cured. Thus limiting, to an extent, the maximum height and design which could be achieved whilst maintaining structural integrity. Portland cement provided a solution to this problem. Blended with a suitable aggregate and mixed with water it produced a product with a far superior mechanical strength. The advantages of such a product were soon realised and a revolution in building construction began. Rather than using bricks and stones, bonded by a mortar, large sections of the buildings’ outer-structure could be pre-cast and assembled on-site. Cost savings in terms of the labour and time required to assemble new buildings were realised. The use of an inner steel structure provided additional structural support enabling the construction of building to reach new heights and the development of modern skyscrapers. Although the benefits of Portland cement can clearly be seen there are certain limitations. One of its disadvantages is its rigidity when cured. The cure matrix has a limited capability to deform as a result of movement. If the force of this movement exceeds the natural flexibility of the cured material, cracking is induced. Depending upon the extent, location within the building and type of structure it can be a costly process to rectify as well as looking aesthetically unpleasing. A classic example is cracking within concrete renders applied onto the exterior of buildings. Incorporating polymers into cementitious materials proved a way of overcoming or improving the disadvantages of cement-based materials. Analysis of polymer modified cementitious materials has clearly shown an improvement in flexural strength and impact resistance of the final product [1-3]. This in turn results in an increased tolerance to movement and deformation. An insight into the potential benefits of combining Portland cement and a polymer were observed during the experiments conducted throughout the 1920-1930s. By blending a natural polymer with Portland cement, an improvement in the workability and ease of use of the resultant mix was identified [4, 5]. Natural in origin, this polymer could be readily harvested and processed to produce the desired polymer product. The disadvantage

159

Polymers in Cement was, however, that the properties could not be modified. Compared to modern synthetic polymers the chemical resistance of natural rubber is poor, particularly when exposed to oils and solvents. In order to improve the physical performance of polymer-modified cement further, new polymers were required. The fundamental driving force behind the development of synthetic polymers was, however, not directly the construction industry but other industries such as the automotive and aerospace industries (i.e., plastics, adhesives, and tyres) [6]. As new synthetic polymers became commercially available their potential benefits when combined with cementitious materials were assessed. As the key advantages of combining polymer into cementitious materials became clearer, the demand for polymer modified cementitious materials increased. This has ultimately resulted in the wealth of polymer modified cementitious materials, which are used within the construction industry today. The number of manufacturers producing polymers specifically designed for use in cement-based materials has grown out of this demand. The type of polymer used in a cement modifier will depend on its properties, the end-use of the product and cost. All polymers used in direct combination with cement are water dispersible, either as a liquid or redispersible polymer powder. Hydration of the cement component and film formation of the polymer within the matrix must occur in unison in order to achieve the maximum benefits. As with any product only certain types of polymers are suited for specific applications. Typically the polymers utilised in cementbased products purchased from DIY stores are suitable for internal use and only where the exposure to chemicals of any sort is absolutely minimal. Such products include tile adhesives, underlayments and self-levelling compounds. In these circumstances, a good bond adhesion, workability and ease of use are the key properties. Polyvinylacetates (PVA), for example, are used extensively as a primer and bonding agent in internal applications [4, 5, 7]. It is not suitable for external use as the polymer film breaks down when exposed to moisture. Vinyl acetate ethylenes are also used as primers/bonding agents, yet have the added advantage of being less sensitive to moisture than PVA [4]. If the application for which PVA is being used for is not going to come in contact with moisture this polymer is ideal and cost effective. In addition, tile adhesives contain a hydrophobic polymer, which prevents the penetration of water into the cured microstructure. Products formulated for industrial use (mortars, self-levelling floor screeds) tend to contain polymers, which are moisture resistant, durable, and also have a good abrasion and chemical resistance. Depending upon the type of industrial and commercial situation there may be a requirement for a higher chemical resistance than these products can provide alone. Advances in polymer technology resulted in the synthesis of epoxy resins and polyurethanes (PU) [7]. These resins possess a superior bond adhesion, flexibility and chemical resistance compared to those typically incorporated directly into cementitious products. If one considers where formulated polymer products, or polymer/cement products are utilised in construction, two of the largest product areas are screeds and coatings.

160

Summary of the Applications and Benefits of Utilising Polymers in Construction These products are used to refurbish an existing concrete surface or seal a newly laid concrete section. Today, many industries due to the nature of their business have to be able to manufacture and supply products globally in the shortest time frame possible. The fundamental requirement for the majority of industries is that the maintenance or renovation should be completed in the minimum amount of time and with little disruption. The need to close areas within the working environment due to the necessity to repair or revitalise walkways, walls and so on, should ideally be limited to periods outside the daily operation. However, this becomes a more difficult task if the facility and area in question is utilised 24 hours a day during the week. The solution is either to carry out the repairs during the weekend when the facility is not in operation or close the area in question for as short a time as possible. The type of product ideally suited for the refurbishment requirements will depend upon the area under renovation, the type of industry, and the products the surrounding environment is exposed to. The most economical and fast track method of renovating a floor area is to use a self-levelling floor screed [8, 9]. These are cementitious in origin and were originally developed during the 1980s as a fast-track alternative to polymer-based screeds. The key advantage of this type product is its workability - the material is so fluid it levels of its own accord although it does require some skill to ensure the correct thickness is applied. Up to 1500 m2 can be applied daily using an appropriate floor screed pump. Before the application of the screed can commence, the substrate surface has to be properly prepared. This normally means the substrate is scabbled or shot-blasted to remove a thin section of the top surface. If this is not done any latency or contamination already within the substrate surface could adversely effect the bond between the newly applied surface and substrate. Secondly, the substrate surface has to be sealed. Acrylic, and styrene butadiene rubber emulsions are commonly used as concrete primers due to their moisture resistance, particularly in industrial applications [7, 10]. If expansion joints need cutting into the newly laid screed they are normally wet-sawn. If the primer is not moisture resistant the polymer film can hydrolyse and thus result in the screed de-bonding and lifting in the areas which was cut. Polymers in the form of redispersible powders (e.g., acrylics, vinyl ethylene acetates, vinyl ester of versatic acid), are used in combination with a blend of cements, aggregates and additives [1]. The introduction of a water dispersible polymer improves the abrasion resistance, durability and resistance to deformation, relative to a cementitious screed. The addition of cellulose ethers enables the fluid consistency to be achieved whilst preventing sedimentation. The cellulose molecules form an inter-penetrating network throughout the mixed product and interact (hydrogen bonding) with the surface groups on the aggregate and cement grains. A highly fluid consistency is desired as the material is self-levelling, thus removing the necessity for the applicator to distribute the material accordingly in order to achieve a flat level surface. One of the critical factors to achieving the best surface finish

161

Polymers in Cement when using self-levelling will depend on the applicator achieving the correct consistency. Too thick and the product will not self-level and therefore produce an uneven finish upon curing. Too thin and the product will be susceptible to separation with the formation of a latency layer on the surface. The latency layer can reduce the bond adhesion between the recently applied screed and any additional surface treatment such as a coating. Incorporating a polymer into the mix also enhances the bond adhesion between the screed and substrate [10, 11]. This should reduce the possibility of the screed de-bonding from the substrate. This is the reason why polymers with a high degree of bond adhesion are used in this type of application. The application and curing characteristics of this type of product mean it is ideal for fast track renovation. Typically the newly laid surface can accept foot traffic in 24 hours [9]. If the self-levelling screed is to be the final finish (i.e., the addition of lino or tiles), then the application of polymer coating onto the surface is required. This maintains the aesthetic appearance by preventing dirt becoming ingrained into the cementitious surface and provides a degree of chemical resistance (i.e., oils, solvents), and so on. Water-based coatings are commonly applied to seal and maintain the aesthetics of the floor. Solvent-based coatings are the most expensive and hence are used when a high degree of chemical resistance is required. Depending upon the type and quantity of solvent utilised there is the potential for the release of volatile organic compounds within the local environment during the curing process. The applicators therefore require sufficient personal protection to avoid unacceptable levels of exposure and appropriate ventilation within the building. The coating will wear over a period of time due to surface traffic and hence will require re-applying in order to maintain the appearance and functionality of the floor. Alternatively, a resin-based screed (commonly epoxy and PU-based) may be applied. The wear and abrasion resistance of these types of screeds are superior when compared to a polymer modified cementitious screed [7]. Water-based, solvent-free and solvent-based screeds are commercially available. Typically the screed will be applied in a layer of between 3-5 mm onto a suitably prepared and primed substrate. Application of this type of product requires a degree of skill in order to achieve the best surface finish, as this is all done by hand. The quantity which can be mixed and applied by hand within the working life of the product is, however, the limiting factor which determines how many square meters can be applied within a day. The benefits provided have to be balanced with the cost of such products as the polymer will always be more expensive compared to cement. Underlayments are used throughout the construction industry to provide a flat level surface onto which the final floor finish can be applied (i.e., tiles and lino). In this type of situation the abrasion and wear characteristics of the screed are less critical as it is predominately being used to provide an even finish. In these circumstances the polymer may be supplied as an emulsion, which is mixed with the cement-based component (typically

162

Summary of the Applications and Benefits of Utilising Polymers in Construction natural rubber, styrene butadiene rubber) or as redispersible polymer powder (acrylic, vinyl acetate ethylene and so forth) contained within the cementitious material [1, 10]. When utilising a polymer emulsion it is important that the volume mixed directly within the cementitious component contains a sufficient quantity of water. If this is not the case then the cement component will not be fully hydrated and an inferior product may be produced as a result. Today the vast majority of polymers mixed directly with cement are in a powder form. This avoids the complication of having to add additional water to that provided by the polymer emulsion to achieve the desired consistency. One or two applications still use a polymer in a liquid form. These products are typically used where the end use requires a high degree of flexibility and adhesion to the substrate enabling thin applications down to a featheredge (0-3 mm) to occur. The actual polymer content in these types of screeds is higher (10-15% by weight) when compared to self-levelling floor screeds which normally contain redispersible polymer powder (typically 5%) [7, 10]. This type of product is commonly used by professional flooring contractors to level the floor finish before carpet, tiles and lino are applied (used within the home, commercial building and shops and so on). In order to maintain a market attractive price, a low cost polymer is used in this type of screed - typically natural rubber. One key disadvantage when using natural rubbers is the release of a pungent odour (ammonia-like smell) when mixed with the cementitious component. One of the key benefits of coatings is that they can provide an impervious surface which is easy to clean. We are all aware of the necessity for hygienic, clean areas and surfaces within the home and workplace in order to prevent the spread of bacterial growth. The application of typically two to three coating layers can provide an impervious finish on top of a porous substrate/surface. This in turn prevents the build up of dirt and bacteria within the pores of the unsealed surface. Any dirt or spillage can be easily removed although it is important to ensure that the cleaning products utilised will not damage the coating surface. This is the reason why coatings are intensively used where hygiene and cleanliness are of critical importance such as in hospitals and food industries as well as general industry. The uses of raw materials (i.e., casein), which have the potential to biodegrade and thus provide the possibility for bacterial growth, are unacceptable for food and medical applications [9]. In fact research within the coatings industry is now focusing more on the potential of the applied surface to prevent bacterial growth. With this purpose in mind, coatings are now available which contain biocides. Thus, coatings enabling fast track renovation or revitalisation of concrete, brick and masonry surfaces, produce a durable and aesthetically pleasing finish. Coatings are commonly supplied as two-component systems (resin/hardener for epoxy and isocyanate/polyol for PU). There are however, commercially available one-component systems which cure by reacting with moisture within the atmosphere (PU) [7, 12]. Alternatively, the resin component is dissolved into a solvent medium and the polymer film forms as the solvent

163

Polymers in Cement evaporates. These types of clear sealant are used to seal the surface of newly laid concrete and dry shake finishes. The sealer is applied to prevent dusting of the concrete surface, which is unsightly and aids the curing of newly laid concrete, by preventing moisture loss through evaporation. These one-component sealers are typically sprayed onto the concrete or dry shake finishes. This enables large-scale coverage to be achieved with ease, thus resulting in time and cost saving compared to the application by roller. The physical performance achieved by such a sealer is good, however the key issue is the volatile organic compounds released into the atmosphere during the curing process. It is therefore very important that the required personal protection is adopted and ventilation is available during the application process. Since there is a drive to reduce the release of volatile inorganic compounds due to environmental and health concerns, the demand for solvent-free and water-based systems has increased. The key problem to overcome is trying to achieve the performance of these solvent-based sealers in the absence of any solvent. Polymers are also added to Portland cement renders and mortars to improve the workability of the mix, bond adhesion to the substrate and reduce the permeability of the cured product [5, 10]. This in turn is beneficial as it improves the resistance of the mortar to damage associated with the effects of frost and certain chemicals. In order to achieve the maximum benefits a hydrophobic polymer should be used. These types of polymers are used in tile adhesive and waterproofing compounds since they have a natural dislike of water. Other polymer-based products which are used within the construction industry but do not contain any actual Portland cement are polymer concrete and resin-based mortars [6]. Although these products have fundamental advantages over Portland cement containing products (superior mechanical properties) there are certain disadvantages, in particular their cost. Since the cost of any polymer far exceeds that of Portland cement these products will always be significantly more expensive. They are polymer rich, so the Tg of the polymer is also a consideration [7]. A reduction in mechanical strength is likely if the resinmortar is exposed to temperatures above the Tg of the resin component. Polymer concrete exhibits superior physical properties (exceptional strength characteristics) compared to standard Portland cement concrete in combination with a high stability towards corrosion and chemical attack. Such properties make the use of this type of product attractive in applications, which demand a high level of performance. Typical applications include the refurbishment and repair of motorways, bridges or the erection of supporting structures for buildings. More recent applications have seen this type of material used in the manufacture of precast beams, pipes and manhole covers. The key advantage is that the working life of these pipes are extended compared to those produced from traditional materials (i.e., concrete), and hence long-term are a more cost effective solution. So what does the future hold for the use of the polymer system within the construction industry? Clearly one of the largest uses of polymers is in the formulation of coatings to

164

Summary of the Applications and Benefits of Utilising Polymers in Construction provide a decorative finish as well as sealing and protecting the surface of bricks, blocks and concrete. The key benefits of these products within the construction industry have been described earlier, however, there are still certain areas, which would benefit from improvements such as the scratch resistance. The advantages of incorporating nanoparticle materials (alumina) into coatings are being intensively investigated [13]. Improvements in the scratch resistance and wear characteristics of coatings has been recorded when nanoparticle materials are dispersed throughout. This is turn will aid the longevity and the aesthetic appearance of the finish. Clear coatings which have been produced utilising this type of technology are ideal for sealing and protecting surfaces (i.e., brick, concrete), against external damage such as graffiti [14]. The need to minimise or eliminate release of volatile organic compounds into the atmosphere, has been the driving force behind improving the performance of water-based coatings. Clear coatings used predominately within the automotive industry are now available whose performance is close to that of the conventional solvent-based systems [15]. Whilst there are several two-component systems designed for use within the construction industry whose performance exceed the properties previously achieved for water-based systems, I can only surmise that the ultimate goal would be to achieve a ‘like for like’ performance compared to the traditional solvent-based coatings. Energy efficiency has become a primary consideration in construction projects with energy saving features, for example, materials utilised within the infrastructure. Developments within this field have resulted in the design of an internal plaster containing a synthetic resin and special materials designated 'microencapsulated phase change materials' [16]. Analysis of this type of plaster has indicated it possesses an equivalent thermal capacity of a brick (approximately 10 times the thickness of the plaster). The advantage of this type of plaster would be to reduce variations in internal temperature and hence maintain a more comfortable environment. The performance characteristics of composites are also an attractive proposition for the construction industry offering great potential for reinforcement and strengthening of existing structures. Potential uses suggested for reinforced composites include the infrastructure of bridges such as cables, girders (i.e., an alternative solution to the use of metals such as steel) [17]. The high strength along with chemical and corrosion resistance associated with reinforced composites has provided an effective solution to the problems related to deterioration and repair of pipe work and sewage systems [18]. Pipes and linings are produced from fibre-reinforced composites which achieved the required mechanical properties and chemical resistance required within such environments. Whether or not composites will be utilised in preference to conventional building products currently available will depend upon the properties, end-use, cost and fire resistance [19]. It is believed that the standards detailing testing and fire resistance will determine the use of

165

Polymers in Cement composites within the building construction. It is, however, clear that those polymers in the form of coatings, reinforced composites or in combination with the cement composites will be influential in future developments within the construction industry.

References 1.

J. Schulze, The Use of Redispersible Powders in Cement Mortars, Wacker-Chemie GmbH, Munich, Germany, 1985.

2.

Your Partner For Redispersible Powders, Elotex AG (Division of National Starch and Chemical Company), Sempach Station, Switzerland, 1998.

3.

D.Z. Zurbriggen, Influence of Redispersible Powders on Shrinkage, Hydration Behaviour and Microstructure of Tile Adhesives, Elotex AG, Sempach Station, Switzerland, 1999.

4.

D.G. Walters in Proceedings of the ACI Convention, 1997, Atlanta, GA, USA, p.1-5.

5.

Y. Ohama, Cement and Concrete Composites, 1998, 20, 2-3, 189.

6.

D. Feldman, Polymer News, 1993, 18, 9, 261.

7.

J.D.N. Shaw, P.J. Brown, R. Dennis, P.C. Hewlett, R.A. Johnson, M. Levitt, J.B. Newman, J. Raymond, B.W. Staynes, R.N. Swamy, L.J. Tabor and M.J. Walker, Technical Polymers No 39, Polymers In Concrete, Concrete Society Working Party, The Concrete Society, Camberley, Surrey, UK.

8.

D. Hogg, Concrete, 2000, September, 18.

9.

M. Roberts in Proceedings of Calcium Aluminate Cements 2001, Edinburgh, UK, Eds., R.J. Mangabhai and F.P. Glasser, 2001, Cambridge University Press, Cambridge, UK, p.605-614.

10. A. Blaga and J.J. Beaudoin, Polymer Modified Concrete, Canadian Building Digest No.241, Division of Building Research, National Research Council Canada, Ottawa, Canada, 1985. 11. I. Nelson in Proceedings of the ConChem Conference, 1997, Dusseldorf, Germany, p.1-8.

166

Summary of the Applications and Benefits of Utilising Polymers in Construction 12. M. Brannen and J. Broeder in Proceedings of TAPPI Hot Melt Symposium Conference, Hilton Head Island, SC, USA, 1992, p.31. 13. U. Nolte in Proceedings of the 5th European Congress on Construction Chemicals: Functional Surfaces – Innovations for Tomorrow, Nurnberg, Germany, 2005, Paper No.8. 14. S. Rastaetter in Proceedings of the 5th European Congress on Construction Chemicals: Functional Surfaces – Innovations for Tomorrow, Nurnberg, Germany, 2005, Paper No.6. 15. W. Paulus in Proceedings of the 5th European Congress on Construction Chemicals: Functional Surfaces – Innovations for Tomorrow, Nurnberg, Germany, 2005. 16. M.N. Fisch and L. Kühl in Proceedings of the 5th European Congress on Construction Chemicals: Functional Surfaces – Innovations for Tomorrow, Nurnberg, Germany, 2005, Paper No.11. 17. A. Jacob, Reinforced Plastics, 2004, 48, 6, 30. 18. G. Marsh, Reinforced Plastics, 2004, 48, 6, 20. 19. D. Westaway, Reinforced Plastics, 2004, 48, 6, 38.

167

Polymers in Cement

168

Polyols

8

Glossary Author

Co-polymer

Polymer produced by the polymerisation of more than one monomer of different chemical composition.

C-A-H

Calcium aluminate hydrate compounds which form as the result of high alumina cement or Portland cement (tricalcium aluminate) and water.

Cementitious

Material, which contains cement.

Crosslinking

Joining of two polymer chains into one longer polymer chain.

C-S-H

Calcium silicate hydrates - compounds that form as the result of the reaction between Portland cement and water.

Drying shrinkage

Shrinkage of the cementitious matrix, which occurs as a consequence of the curing process.

Homogeneous

Uniform, well distributed consistency.

Homopolymer

Polymer produced by the polymerisation of one monomer of a specific chemical composition.

Hydrophilic

Species that are water loving.

Hydrophobic

Species that dislike water.

Initiators

Chemical compounds used to initiate polymerisation of monomers (i.e., free radical) yet do not form part of the final polymer.

Latex

Polymer particles dispersed in an aqueous medium (i.e., water).

Latency

Residue, found on the surface of a concrete, cementitious product after curing.

Micro-cracking

Fine, small cracks which occur in the cured cementitious product.

169

Polymers in Cement Open time

The period of time when the cement remains in a useable, workable state, after this period the materials begins to set and stiffen.

Pea-gravel

Type of aggregate that is about the size of a pea.

Permability

Having pores or openings, which allow liquids or gases to pass through.

Pinholes

Small holes observed in the surface of a cementitious product (i.e., screed), as a consequence of air entrapment in the mix.

Polymer concrete

A blend of aggregates and thermosetting resins.

Polymerisation

Reaction of one or more monomer species under the required conditions to form a polymer.

Prepreg

Resin impregnated into a fabric, fibres (glass, carbon and so on).

Primer

A polymer solution applied to a prepared concrete substrate to seal the absorbent surface before the application of a new cementitious finish.

PU

Polyurethanes produced by the reaction of a diisocyanate and a polyol/amine based compound.

Retted

Removal of the outer surface of a fibre.

Screed

Mixture of cement and sand along with other materials which when mixed with water and applied to a concrete surface, provides a smooth level surface.

Slumping

Downward movement of a material applied to a vertical surface.

Thermoplastic

Material that softens whenever it is exposed to heat.

Thermoset

Once formed this material tends to maintain a rigid structure when exposed to heat repeatedly.

Underlayment

Screed used to provide a level surface onto which the final floor finish is applied.

Unretted

Outer surface of a fibre is intact.

Workability

Time period over which a product remains in a workable, useable condition.

170

Polyols

Abbreviations Author

AP

Acrylic polymer

CR

Chloroprene (2-chloro 1,3-butadiene)

C-S-H

Calcium silicate hydrates

CTE

Coefficient of thermal expansion

DGEBA

Diglycidyl ether of bisphenol A

DIY

Do-it-yourself

DNA

Deoxyribonucleic acid

MDI

Diphenylmethane diisocyanate

MFFT

Minimum film formation temperature

MMA

Methyl methacrylate(s)

NBR

Acrylonitrile butadiene copolymers

NBR

Nitrile butadiene rubber

NR

Natural rubber

PC

Polymer concrete

PE

Polyethylene

PIC

Polymer impregnated concrete

PMMA

Polymethyl methacryalate(s)

PP

Polypropylene

PPCC

Polymer - Portland cement concrete

PU

Polyurethane(s)

PVA

Polyvinyl acetate(s)

PVC

Polyvinyl chloride

171

Polymers in Cementitious Materials PVDC

Polyvinylidene chloride

PVOH

Polyvinyl alcohol

SA

Styrene acrylic(s)

SBR

Styrene butadiene rubber(s)

Tg

Glass transition temperature(s)

TMPTA

Trimethylolpropane triacrylate

TMPTM

Trimethylolpropane trimethyacrylate

UV

Ultraviolet

VA

Vinyl acrylics

VAc

Vinyl acetate

VAE

Polyvinyl acetate ethylene

VAE

Vinyl acetate ethylene(s)

VeoVa

Vinyl ester of versatic acid

VOC

Volatile organic compounds

172

Index

Index Page numbers in italic, e.g. 30, refer to figures. Page numbers in bold, e.g. 89, signify entries in tables. other polymers 142 polyurethanes 141 urea–formaldehyde/phenolic resins 142

A Abrasion resistance 108 Accelerators 10 Acid anhydrides, as curing agents 41–42 Acid resistance 111 Acrylic acid, rigidity for latex emulsions 89 Acrylic polymers 30 adhesives 141–142 applications 31 benefits when introduced into cementitious materials 100 common construction materials 112 methyl methacrylate monomer 30 physical performance 30–31 Acrylonitrile butadiene copolymers (NBR), polymer-Portland cement concrete (PPCC) 13 Adhesive properties 16 Adhesives 137 applications automotive and aerospace industries 144–145 construction industries 143–144

composition, properties and general uses acrylics 141–142 effect of incorporating rubber 142– 143 epoxy resins 141–142

types 138 hot melt 139–140 solvent-based 138 solvent-free and water-based 138–139

Aggregates, polymer concrete 65–66 Air bubbles 3, 10, 98–99 Anticaking agents 93 Antifoamers 91, 100 Antioxidants 91 Application techniques effects of polymer inclusion 108 polymer concrete 68–69 Argillaceous materials 5, 6

B Ball milling 6 Biodegradable polymers, epoxy resins 45 Bisphenol A/bisphenol F 39, 39 Bisquinazoline 44 Bleed water 104 Bond adhesion, polymer concrete 71 Bridge decking 111 1,3-Butadiene rigidity for latex emulsions 89 structure 32

173

Polymers in Cementitious Materials

C

solvent-based coatings 146–147

Caesin 15 C-A-H, definition 169 Calcareous materials 5, 6 Calcium carbonate 93 Calcium chloride 10 Calcium hydroxide 8, 9 Calcium ions, role in cement hydration 8–10 Calcium silicate hydrates (C-S-H) 10, 86 Calcium sulfate 7 Carbonation 110 Carbon-fibre reinforcement 122 Cellulose 11, 24–25 structure 24 Cement see also Portland cement historical perspective 4–5 manufacture 5–8 Cementitious, definition 169 Chemical resistance 111 N,N -bis-(3-chloro-2-oxypropyl)2,4,6tribromalin 44 Chloroprene (2-chloro-1,3-butadiene) 33–34 applications 35 common construction materials 112 physical properties 34 structure 34 synthesis 34 Citric acid 10 Clinker 6 Coatings 137, 163–164, 165 applications

solvent-free and water-based coatings 147–148

construction 152 general applications 154 overcoating of metal structures 153–154 protection of metal surfaces 153–154

powder coatings 151–152 resin coatings 145–146

174

conversion to water-based 147

epoxy coatings 148–149 other polymer dispersions 150–151 polyurethanes 149–150

Coefficient of thermal expansion (CTE) 141 Construction industry applications of polymers 159–166 future directions 164–165 incorporating polymers into cement mixes 162 polymer Portland cement concrete materials 111 ancillary 114 bridge decking 111 common polymers and their products 112 flooring 113 mortars and renders 112–113 primers 113–114

selection of materials 160–161 Copolymers, definition 169 Corrosion prevention 153–154 Crazing of surfaces 31 Crosslinking, definition 169 C-S-H, definition 169 Curing agents 40–41 acid anhydrides 41–42 benefits 43 catalysts 42 other constituents 43 phenol derivatives 42 polyamines 41 Curing of cement 8, 98

D Deformation 126–127 Degree of polymerisation 98 Dicalcium silicate 6, 7, 8

Index N,N-diglycidyl-benzimadazolene 44 Diisocyantes, formation of 48, 48 Diphenylmethane diisocyanate (MDI) 12, 47, 48 Drying characteristics 105 Drying shrinkage, definition 169 Durability 109

E Emulsions of polymers 87–88 latex emulsions free radical formation 90 general preparation 88–89 monomer composition 89 other ingredients 91 rigidity of common monomers 89 surfactants 90

Energy efficiency 165 Epichlorohydrin 39, 44 Epoxidised novolaks 40 Epoxy chromate primers 152 Epoxy resins 12, 37–38 adhesives 141–142 applications 12, 46–47 benefits when introduced into cementitious materials 102 biodegradable 45 chemical composition 38–39 alternative manufacturing methods 40 bisphenol A/bisphenol F 39, 39 epoxidised novolaks 40

curing agents 40–41 acid anhydrides 41–42 benefits 43 catalysts 42 other constituents 43 phenol derivatives 42 polyamines 41

curing conditions 43–44 epoxide ring structure 38 heat resistant 44–45 physical properties 45–46

use in polymer concrete 63–64 properties conferred 62

water-based coatings 148–149 N,N-bis-(2,3-epoxypropyl)-2,4,6tribromalin 44

F Ferrite 8 Fibre reinforcement see natural fibre reinforcement; synthetic fibre reinforcement Film formation of polymers 87 Flash setting 5 Flooring products 113 Flow aids 15 Free radical formation 90

G Glass transition temperature (Tg) epoxy resins 40 effects of curing agents 43

Glass-fibre reinforcement 122 Graft copolymerisation 89 Grouts, polymer concrete 74–75 Gutta-percha 25 Gypsum 6

H Heat-resistant polymers, epoxy resins 44–45 Hexahydrophthalic anhydride 44 Historical perspective 1 cement 4–5 hydration of Portland cement 8–10, 9 manufacturing principles 5–8

introduction of polymers into cementitious materials 2–3, 12–13 advantages 3–4

polymers 11–12

175

Polymers in Cementitious Materials applications 12

Homogeneous, definition 169 Homopolymer, definition 169 Hydration of cement 98 Portland cement 8–10, 9 Hydrogen peroxide 90 Hydrophilic species, definition 169 Hydrophobic species, definition 169

M

Impact resistance 109 Induction period of cement hydration 8, 9–10 Initiators of polymerisation 88 definition 169 Inorganic polymers 11 Interpenetrating polymer networks 149 Isoprene 26 Isopropyl alcohol 31

Magnesium carbonate 6 Magnesium oxide (magnesia) 6 Melamine, applications 12 Methyl methacrylate (MMA) 2, 30 cementitious composites 2 structure 30 use in polymer concrete 62 Microcracking 126–127 definition 169 Minimum film formation temperature (MFFT) 97 Mixing procedures for polymer concrete 65, 68 Modulus of elasticity 109 Mortars 112–113 polymer concrete 74–75 slumping prevention 3 strength enhancement 3

K

N

Kaolinite 6 Kiln manufacture of cement 5–6

Natural fibre reinforcement 121–122 natural fibre–resin composites 131– 132 Nitrile butadiene rubber (NBR) applications 37 manufacture 36–37 Nonyl phenol polyoxyethylene 90

I

L Latency, definition 169 Latex 25, 169 effect upon synthetic fibre-reinforced materials 127–128 emulsions 87–88 free radical formation 90 general preparation 88–89 monomer composition 89 other ingredients 91 rigidity of common monomers 89 surfactants 90

physical requirements 86–87 Lignin 40

176

O Open time, definition 170 Oxazolidine diluents 147

P Pea-gravel, definition 170 Permeability 109–110 definition 170 polymer concrete 72

Index Peroxide initiators 88 Persulfate initiators 88 Phenolic resins adhesives 142 applications 12 Phosphorus pentoxide 6, 7 Pinholes, definition 170 Polyamines, as curing agents 41 Polyesters use in polymer concrete 62–63 properties conferred 62

Polyethylene, slumping prevention in mortars 3 Polymer concrete (PC) 13, 14, 59–60 application techniques 68–69 applications 72–73 castable systems 76–77 overlays and coatings 73–74 resin grouts and mortars 74–75 road and bridge repair 73 sealants 75–76

chemical composition of thermosetting resins advantages of 60–61 commonly used materials 61–65, 61 properties conferred 62

decision model 69 definition 170 manufacturing principles 65 aggregates 65–66 mixing 65, 68 substrate priming 67–68 voids 67 workability and shrinkage characteristics 66

physical properties 70 bond adhesion 71 permeability 72 strength 70–71, 71

repairs 69 Polymer impregnated concrete 77–78 applications 80–81 disadvantages 81

manufacturing principles 78–79 physical properties 79–80, 79 Polymer integrated concrete (PIC) 13, 14 Polymer modification of cement and concrete 3, 23–24 commonly used polymers acrylic polymers 30–31 cellulose 24–25, 24 chloroprene (2-chloro-1,3-butadiene) 33–35 epoxy resins 37–47, 38 nitrile butadiene rubber (NBR) 36–37 polyurethanes 47–51 polyvinyl acetate (PVA) 27–28, 27 polyvinylidene chloride (PVDC) 35–36, 35 rubber, natural 25–27 styrene butadiene rubber (SBR) 31–33 vinyl acetate ethylene (VAE) copolymers 28–30

physical properties 15 adhesion 16 strength 16 viscosity 16 workability 15

Polymer Portland cement concrete 85 advantages 86 benefits of polymer inclusion 99–100 acrylics 100 epoxy resins 102 styrene acrylics (SA) 100–101 styrene butadiene rubber (SBR) 102 vinyl acetate ethylene (VAE) copolymers 101–102

common construction materials 111 ancillary 114 bridge decking 111 common polymers and their products 112 flooring 113 mortars and renders 112–113 primers 113–114

effects of polymer inclusion 103 abrasion and impact resistance 108–109 application method 108

177

Polymers in Cementitious Materials bleed water 104 carbonation 110 chemical and acid resistance 111 drying characteristics 105 durability and modulus of elasticity 109 permeability 109–110 stability 104–105 strength characteristics 106–108 voids 105 water demand 103 workability 104

matrix formation 97–99 physical requirement of latexes 86–87 polymer emulsions 87–88 manufacture of latex emulsions 88–91

redispersible polymer powders 91–92 commercially available products 96 conception and development 92 modern-day composition 96–97 particle size 94 rehydration 95 spray drying procedure 92–94, 93

Polymerisation definition 170 initiators 88 Polymer-Portland cement concrete (PPCC) 13–14 Polymers definition 11 historical perspective 11–12

properties 50 recycling 49 synthesis 47–48 formation of diisocyanates 48, 48 other ingredients 48–49

volatile organic compounds (VOC) 49 water-based coatings 149–150 Polyvinyl acetate (PVA) 23–24, 27 applications 12, 160 polymer-Portland cement concrete (PPCC) 13 redispersible polymer powders 92 vinyl acetate monomer 27 Polyvinyl chloride (PVC), applications 12 Polyvinylidene chloride (PVDC) 35–36 common construction materials 112 structure 35 Portland cement 1, 5, 159 hydration 8–10, 9, 98 manufacture 5–6 formation 7

setting characteristics 6–7 use of thermosetting resins 64–65 Pozzolana 4 Prepreg, definition 170 Primers 113–114 definition 170 Pyroillite 6

applications 12

Polymethyl methacrylate (PMMA) use in polymer concrete 62 properties conferred 62

Polypropylene, slumping prevention in mortars 3 Polystyrene applications 12 styrene monomer 32 Polyurethanes (PU) 12, 47 adhesives 141 applications 12, 50–51 definition 170

178

Q Quaternary ammonium salts 90

R Rayon 12 Recycling of polymers, polyurethanes 49 Redispersible polymer powders 91–92 commercially available products 96 conception and development 92 modern-day composition 96–97

Index particle size 94 rehydration 95 spray drying procedure 92–94, 93 Reinforced concrete 13, 14–15 Renders 112–113 Retarders 10 Retted, definition 170 Rubber, natural 23 applications 26–27 cementitious composites 2–3 chemical composition 25, 26 extraction and processing 25 physical properties 26

S Screed, definition 170 Sealants 75–76 Shrinkage, polymer concrete 66 Silica 93 Silk 11–12 artificial silk 12 Slumping definition 170 synthetic fibre reinforcement 125–126 Sodium dodecyl sulfate 90 Sodium gluconate 10 Sodium lauryl sulfates 90 Spray drying 29 Stability 104–105 Steel, protective coatings for 153–154 Steel-reinforced cementitious materials 128 Strength characteristics 106–108 polymer concrete 70–71, 71 polymer impregnated concrete 79 Strength modification 16 Styrene acrylics (SA) benefits when introduced into cementitious materials 100–101 common construction materials 112

polymer Portland cement concrete (PPCC) 13 Styrene butadiene rubber (SBR) 31–32 applications 12 benefits when introduced into cementitious materials 102 butadiene monomer 32 common construction materials 112 physical characteristics 32–33 polymer Portland cement concrete (PPCC) 13, 85 strengthening of mortars 2–3 styrene monomer 32 Styrene rigidity for latex emulsions 89 structure 32 Substrate priming, polymer concrete 67–68 Surface sealing of concrete 3 Surfactants 90 Synthetic fibre reinforcement 121 applications construction 129 highly flexible polymers 132–133 low temperature moulding systems 133–134 pre-cast concrete beams 131 repair mortars 130–131 repair of concrete road surfaces 129

carbon fibres and glass fibres 122 compared with steel-reinforced materials 128 introduction of fibres into cementitious materials 124 effect of fibre shape 125 influence of corrosion 128 influence of length and physical nature of fibres 124–125 latex, effect of 127–128 microcracking and deformation 126–127 other factors 127 slump characteristics 125–126

179

Polymers in Cementitious Materials structural shrinkage and deterioration 126

long-term effects 123 long-term performance comparison of different types of reinforcement 129–130 polymer-based fibres 123

T Tartaric acid 10 Tetrabromobenzimidazolone 44 Thermoplastics, definition 170 Thermosetting resins advantages 60–61 definition 170 use in polymer concrete 61, 61 epoxy resins 63–64 introduction into ordinary Portland cement 64–65 methyl methacrylates 62 other resin constituents 64 polyesters 62–63 properties conferred 62

Titanium oxide 7 Tricalcium aluminate 6, 8 Tricalcium silicate 5, 8 1,1,2-Trichloroethane 35 Trimethylolpropanetrimethylacrylate (TMPTM) 2 Trisodium citrate 10

U Underlayment, definition 170 Underwater applications 4 Unretted, definition 170 Urea-based polymers, applications 12 Urea–formaldehyde ahdesives 142 UV radiation stability 153 acrylic polymers 31

180

V Vinyl acetate 27 rigidity for latex emulsions 89 Vinyl acetate ethylene (VAE) copolymers 96–97 benefits when introduced into cementitious materials 101–102 common construction materials 112 effects upon cementitious materials 29–30 redispersible polymer powders 28–29 strengthening of mortars 2–3 synthesis 28 Vinyl ester of versatic acid (VeoVa) 96 common construction materials 112 Viscosity modification 16 Voids in matrices 99, 105 polymer concrete 67 Volatile organic compounds (VOC) 165 coatings 147 water-based polyurethanes 149

polyurethanes 49 solvent-based adhesives 138 vinyl acetate ethylene (VAE) copolymers 28

W Water demand 103 Workability of mixed cement 10, 15, 104, 164 definition 170 polymer concrete 66 World War II demand for polymers 12 latex production 31–32

Z Zinc phosphate pigments 152