With the advent of technologies on modern structural and building materials, adaptation of such technological features has been the pivotal concern of researchers. Ceramic tiles attained a distinctive focus due to its decorative feature for both indoor and outdoor conditions and also for their ease of implementation. The purpose of exploiting ceramic tiles is not only to impart structural strength but also the aesthetical characteristics that seem to matter a great deal. When it comes to the adhesion of these tiles onto the substrate wall, traditional method exerts the use of sand-cement grout. This faces some drawbacks including poor water retention property, hard and brittleness of the surface, much higher drying time, no flexibility, higher thickness of the paste and so on. These difficulties can be overcome by the addition of redispersible polymer powder (RPP) along with other cementitious constituents. The blended polymers interact with cement components to improve the physical and mechanical properties such as increased adhesion strength, reduced shrinkage and lower water absorption. This review article made an effort to provide the generalized idea about the cementitious tile adhesive (CTA) and its components. Focus was made onto the commercially available RPP and formulation of CTA with the inclusion of RPP. Critical analysis of the repercussions of RPP fortification was also carried out based on different researcher's findings.
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Being the oldest form of decorative art, ceramic tiles, from its inception to evolution through ages, is laden with history, from ancient Egyptian artifacts to modern household decoration. Their enthralling beauty and structural permanence made them treasured for centuries. The oldest evidence of ceramic tiles usage was dated from 4700BC in Egypt; subsequently, Romans and Greeks also started decoration with ceramic tiles. During the Islamic period, decorative ceramic tiles were frequent in mosques. Decorative tiles got admiration at Europe during the middle age and it was the industrial revolution of Britain which made the mass production possible as well as the affordability for the middle class people [1].
External cladding of tiles, also known as tiling system [2] has become the most picked construction choices not only for its aesthetic purposes but also for the patronage of wall from aggressive environmental conditions [3]. Ceramic tiles are the most adopted tiles around the world due to its versatility [2] and affordability [4]. The ceramic tiling system comprises of three interactive layers; the substrate layer, the tile layer and most importantly the adhesive layer [3, 5]. Adhesives refer to the substance, which has the ability of holding minimum two surfaces substantially in an indissoluble way. Adhesives that are used in structural purposes must possess high shear strength and a very strong resistance from aggressive environment [6].
Traditionally for the external cladding of ceramic tiles, a cheaper form of adhesive which is nothing but a mixture of cement and sand, is being used widely and much popular in Indian subcontinent. Traditional grout preparation involves mixing of ordinary portland cement and sand with water and the application of this grout is done by following the thick bed method, where the grout bed occupies 10–25mm of thickness from the base to the adherent [7]. This is a very time consuming process and requires much effort. Application of a polymer modified thin bed tile adhesive is a possible solution to all the problems raised by traditional cement-sand grout [8].
Polymer based adhesives are introduced to cement as a modifier for the betterment of adhesion, strength, waterproofing, durability, flexibility and deformity. Polymer modification of cement grout or paste for tiling and other purposes is not a contemporary conception; rather it had been performed since 1923. In 1924, the first patent regarding polymer modification in cement mortar was issued. Since then, polymer modified cement grout got the implementation in construction arenas for providing a decent performance compared to that of the ordinary cement-sand grout and eventually got the popularity [9, 10].
The basic principle of polymer modification involves the mixing of polymer or monomer in powdered or liquid form along with cement and other admixtures followed by curing. In case of monomers being used, in-situ polymerization of that monomer is required. The polymers or monomers with which polymer modification can be done are of four major types ( ), i.e., RPP, polymer latex, water soluble polymer and liquid polymer [9].
RPPs are simply the spray-dried powders which when mixed with water, produce stable dispersion just like the original dispersion and this is governed by the fact that it redisperse only once [11]. Originally, RPPs are synthesized from latex dispersions and this involves a two-step synthesis process. The first step includes preparation of polymer latexes via emulsion polymerization and the second step being the spray drying of this emulsion into fine powder. Prior to the spray drying, various admixtures are added to the formulation such as anti-foaming agents, accelerators, water retention agents etc. for improving the performance of the polymer. To prevent the cake formation of the polymer powder during storage, anti-blocking aids such as clay, silica and CaCO3 are added before or after the spray drying [9].
Polymer latexes or polymer dispersions are synthesized just like RPP by emulsion polymerization. Polymer latexes are characterized by the fact that they are of very small particles, typically possesses 0.05–5μm in diameter. Common examples of synthetic latexes include vinyl acetate, homo- and co-polymer latexes, acrylic copolymer latex, styrene butadiene copolymer latex etc. Due to chloride ion liberation and unsatisfactory resistance, polyvinyl acetate and poly (vinylidene chloride-vinyl chloride) latexes are not advised. In case of natural rubber and epoxy latexes, they are not compatible to be synthesized by emulsion polymerization, rather they are acquired naturally by tapping from the rubber trees, which is then concentrated and finally a solid mass is obtained. Cellulose derivatives, polyvinyl alcohol, polyacrylamide etc. are examples of water soluble polymer which are mixed with cement in powdered form with the expectation of preventing the dry out occurrence and to improve the serviceableness. This is due to the increasing viscosity of water by dissolving the polymer and the sealing property is exerted by the formation of a thin film. Overall, the modification with water soluble polymer doesn't contribute substantially to the strength of the system. Liquid polymers find the least of utilization due to its handling and storage difficulties. They are mainly epoxy and unsaturated polyester resins which are applied along with hardener or catalyst and accelerators [12].
CTA modified with RPP has got the special consideration because of its beneficial aspects. This whole concept is justified by the fact that, the hydration of cement and formation of polymer film occurs accordingly which begets a network structure of monolithic matrix phase through which the hydrated cement phase and polymer phase interpenetrate. Modification with RPP is akin to latex modification but with the advantage of the redispersion property [13]. Some of the features delivered by RPP are improved workability, improved adhesion, increased flexural strength, increased plasticity, improved abrasion resistance, improved water retention, reduced water absorption, increased viscosity and cohesion etc. Inclusion of RPP in the cement grout caused the formation of a film [14] as water evaporates and this acts as a binder. This polymer film interconnects all the cement particles and fillers [15]. Moreover, this is more suitable for household tiling than industrial or special type tiling and the low price for a high outcome is the pivotal reason for its popularity.
This article will give an introductory idea about the components that are being used in making ceramic tile adhesive, especially the inclusion of RPP.
Now the Redispersible Polymer powder commonly used is mainly vinyl acetate-ethylene (referred to as VCAE) and vinyl acetate-vinyl tertiary acid (referred to as Ve0Va). Improving the bonding tensile strength of cement mortar and various substrates is the most crucial characteristic of vinyl copolymers, especially its ability to withstand significant changes in ambient temperature. In addition, it is conducive to developing long-term tensile bond strength of cement mortar. After two and a half years of storage in the air, the bond tensile strength of modified cement mortar is still increasing, while the tensile bond strength of ordinary cement mortar is significantly lower than that of maintaining the strength for 28d. Studies have shown that after ten years of application of three redispersible polymer dry powder modified cement mortars, such as polyethylene vinyl acetate, styrene/acrylic acid copolymer, and ethylene/vinyl chloride/vinyl laurate terpolymer, the cement mortar have stable bond strength, flexural strength, and compressive strength. In a five-year-old finishing mortar, the terpolymer still performed well in an aqueous environment with an excellent hydrophobic effect.
Adding Redispersible Polymer Powder into the mortar is a modification, also known as Polymer Modified Cement Mortar， to improve the weaknesses of traditional cement mortar, such as brittleness and high elastic modulus. Therefore, mortar with better flexibility and tensile bond strength can resist or delay the generation of cement mortar cracks. Since the polymer and cement mortar form an interpenetrating network structure, a continuous polymer film is formed in the pores, which strengthens the bonding between the aggregates and blocks some pores in the mortar, so various properties after hardening are much better than ordinary cement mortar. It is mainly manifested in the following characteristics:
(1) Strength
Generally speaking, the tensile strength and flexural strength of Polymer Modified Cement mortar are significantly improved compared with ordinary cement mortar, while the compressive strength is not, or even decreased. The increase is mainly attributed to the higher tensile strength of the polymer itself and the improvement of the bond between the cement hydration product and the aggregate.
The cracking will occur when the tensile stress exceeds its tensile strength value, mainly caused by tensile failure. Therefore, having a high tensile strength value is a necessary condition for crack resistance.
Studies have shown that with the increase of the poly-ash ratio, the tensile strength of the polymer-modified mortar generally increases first and then declines, indicating an optimal dosage range and the decline is typically the addition of excess. The Redispersible Polymer Powder introduced excessive air bubbles, resulting in a downward trend in compressive strength. In addition, since the elastic modulus of the polymer is lower than that of cement mortar, and the polymer chain segment is more flexible, it cannot provide rigid support when the composite is under pressure, resulting in a decrease in the compressive strength of the modified mortar as the poly-ash ratio increases. Therefore, the primary means to improve the compressive strength is to adjust the lime-sand ratio, water-cement ratio, aggregate gradation, and aggregate type. It can enhance tensile and flexural strength, flexibility, crack resistance, and hydrophobicity by adding a certain amount of Redispersible Polymer Powder, but not as much as possible. Adding silica fume can replace part of cement and Redispersible Polymer Powder, reducing the cost and improving the compressive and flexural strength.
(2) Elongation
The elongation and toughness of the mortar added with Redispersible Polymer Powder are much better than that of ordinary cement mortar; the fracture energy is more than twice that of ordinary cement mortar and the impact toughness increases with the higher of the aggregate-ash ratio. With the rise of the content of Redispersible Polymer Powder, the flexible cushioning effect of the polymer can inhibit or delay the development of cracks. At the same time, it has a better stress dispersion effect when the stress is generated by the deformation of the substrate, which will be transmitted or dispersed to avoid cracking due to stress concentration.
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In addition, it should also be noted that there is a specific matching relationship between the tensile strength and elongation of the material. The high content of flexible Redispersible Polymer Powder will increase the elongation and make the polymer in a continuous phase. The polymer dominates in the hardening cement mortar, showing a decrease in tensile strength and water resistance. However, in the initial stage of the hydration heat reaction, the low tensile strength will make it quickly approach the ultimate tensile strength value, which causes the material to be prone to fatigue and fracture. So while ensuring sufficient elongation at break, it should also have the highest possible tensile strength value.
The external thermal insulation system should be placed in a flexible environment. The polystyrene board adhesive and plastering mortar should first choose flexible Redispersible Polymer Powder with a low glass transition temperature to release various deformation stresses in time.
Experiments have shown that the amount of flexible Redispersible Polymer Powder in the polystyrene board adhesive is about 3%, the amount of crack-resistant plastering mortar is controlled between 4% and 5%, and the amount of adhesive for exterior wall tiles is about 3%. The folding ratio is less than 3, which can effectively control the expansion, hollowing, and cracking caused by deformation.
(3)Adhesive property
The bonding ability of cement mortar with Redispersible Polymer Powder is much better than that of ordinary cement mortar, mainly due to the bonding ability shown by the characteristics of the polymer, and the bonding strength increases with the rise of the content. However, there is an optimal dosage range and cost considerations.
High bond strength can have a specific inhibitory effect on shrinkage, and the stress generated by deformation is easy to disperse and release, so bond strength is essential to improve crack resistance. Studies have shown that the synergistic effect of methylcellulose and Redispersible Polymer Powder is beneficial to improve the bond strength of cement mortar. The bonding mechanism of polymers depends more on the adsorption and diffusion of macromolecules on the bonding surface. At the same time, the polymer has a specific permeability, and together with methylcellulose acyl, it can fully infiltrate the surface of the base material, which is close to that of freshly applied mortar, thereby improving the adsorption of freshly applied mortar and base material, and significantly strengthening the bonding performance. In addition to improving the adhesion to the smooth substrate, the polymer-modified mortar also has the effect of closing the surface pores of the substrate, especially for those porous substrate surfaces, which can improve the waterproof, impervious, and freeze-thaw resistance of the base.
(4)Elasticity modulus
The elastic modulus of Redispersible Polymer Powder is low, in the range of 0.001-10GPa, while the elastic modulus of cement mortar is high, in the range of 10-30GPa. Therefore, the elastic modulus of cement mortar modified by adding Redispersible Polymer Powder is smaller than that of ordinary cement mortar. The type and amount of Redispersible Polymer Powder added also affect the elasticity modulus, which usually increases the polymer-cement ratio, the modulus of elasticity, and the deformation ability.
(5) Abrasion resistance
The wear resistance of the modified mortar is related to the type of Redispersible Polymer Powder and the polymer-cement ratio. Generally speaking, increasing the polymer-cement ratio raises the wear resistance mainly due to a certain amount of organic polymers on the wear surface. The polymer acts as a bond so that the mesh structure formed by the polymer can pass through the cement mortar. The pores and cracks in the cement improve the bonding between the aggregate and the cement hydration product, therefore the wear resistance is improved.
In the polymer-modified mortar, the mesh structure formed by the polymer filling seals the holes and cracks and reduces the porosity in the cement mortar, thereby improving the cement mortar’s impermeability, water and freezing resistance, which along with the increase of polymer-cement ratio.
Different types of Redispersible Polymer Powder have significant differences in water resistance. Usually, vinyl acetate powder (VAC, commonly known as Redispersible Polymer Powder formed by spraying white latex) modified mortar has poor water resistance. Therefore, except that the occasion that does not have water resistance requirement also may use VAC, basically do not use VAC for mortar modification. On the other hand, VAC is prone to saponification reactions in a strong base and loses its bond strength, so it is unsuitable for use with cement mortar. VAC can usually be used in gypsum-based products and putty powder without cement.
(6) Permeability
The water vapor permeability of building materials is of great significance. Waterproof and air permeability are desired on many occasions. Polymer-modified mortar has low water absorption and improved water resistance but still has a specific water vapor permeability which will decrease with the increase of polymer content. At the same time, it can be compounded with some Hydrophobic materials to improve its all-around performance.
(7) Freeze-thaw resistance
The porosity is reduced due to the significantly reduced water absorption of the polymer-modified mortar and the filling effect of the polymer. The Redispersible Polymer Powder has a specific air-entraining impact, and its freeze-thaw resistance is better than that of ordinary mortar. At the same time, with the increase of Redispersible Polymer Powder, the bond tensile strength of mortar before and after freeze-thaw cycles also showed an increasing trend. During construction in winter, adding an appropriate amount of calcium formate and antifreeze can significantly improve the mortar’s early strength and antifreeze performance.
Many experiments have shown that the amount of Redispersible Polymer Powder in cement mortar is not the more, the better. Lime-sand ratio, water-cement ratio, aggregate gradation, type, and features will ultimately affect the product’s overall performance. The polymer-improved cement mortar exhibited different properties when the content of the Redispersible Polymer Powder was varied. When the amount is too low, the polymer only has some plasticizing effect on the cement mortar, but the reinforcing effect is not apparent. When the amount is too large, the polymer dominates in the cement mortar, interrupting the continuous phase of the cement mortar and eventually leading to a decrease in strength. When the polymer content is moderate, it increases the deformation resistance, tensile strength, and bonding strength, improves the impermeability, and shows good crack resistance.
With the advent of technologies on modern structural and building materials, adaptation of such technological features has been the pivotal concern of researchers. Ceramic tiles attained a distinctive focus due to its decorative feature for both indoor and outdoor conditions and also for their ease of implementation. The purpose of exploiting ceramic tiles is not only to impart structural strength but also the aesthetical characteristics that seem to matter a great deal. When it comes to the adhesion of these tiles onto the substrate wall, traditional method exerts the use of sand-cement grout. This faces some drawbacks including poor water retention property, hard and brittleness of the surface, much higher drying time, no flexibility, higher thickness of the paste and so on. These difficulties can be overcome by the addition of redispersible polymer powder (RPP) along with other cementitious constituents. The blended polymers interact with cement components to improve the physical and mechanical properties such as increased adhesion strength, reduced shrinkage and lower water absorption. This review article made an effort to provide the generalized idea about the cementitious tile adhesive (CTA) and its components. Focus was made onto the commercially available RPP and formulation of CTA with the inclusion of RPP. Critical analysis of the repercussions of RPP fortification was also carried out based on different researcher's findings.
Being the oldest form of decorative art, ceramic tiles, from its inception to evolution through ages, is laden with history, from ancient Egyptian artifacts to modern household decoration. Their enthralling beauty and structural permanence made them treasured for centuries. The oldest evidence of ceramic tiles usage was dated from 4700BC in Egypt; subsequently, Romans and Greeks also started decoration with ceramic tiles. During the Islamic period, decorative ceramic tiles were frequent in mosques. Decorative tiles got admiration at Europe during the middle age and it was the industrial revolution of Britain which made the mass production possible as well as the affordability for the middle class people [1].
External cladding of tiles, also known as tiling system [2] has become the most picked construction choices not only for its aesthetic purposes but also for the patronage of wall from aggressive environmental conditions [3]. Ceramic tiles are the most adopted tiles around the world due to its versatility [2] and affordability [4]. The ceramic tiling system comprises of three interactive layers; the substrate layer, the tile layer and most importantly the adhesive layer [3, 5]. Adhesives refer to the substance, which has the ability of holding minimum two surfaces substantially in an indissoluble way. Adhesives that are used in structural purposes must possess high shear strength and a very strong resistance from aggressive environment [6].
Traditionally for the external cladding of ceramic tiles, a cheaper form of adhesive which is nothing but a mixture of cement and sand, is being used widely and much popular in Indian subcontinent. Traditional grout preparation involves mixing of ordinary portland cement and sand with water and the application of this grout is done by following the thick bed method, where the grout bed occupies 10–25mm of thickness from the base to the adherent [7]. This is a very time consuming process and requires much effort. Application of a polymer modified thin bed tile adhesive is a possible solution to all the problems raised by traditional cement-sand grout [8].
Polymer based adhesives are introduced to cement as a modifier for the betterment of adhesion, strength, waterproofing, durability, flexibility and deformity. Polymer modification of cement grout or paste for tiling and other purposes is not a contemporary conception; rather it had been performed since 1923. In 1924, the first patent regarding polymer modification in cement mortar was issued. Since then, polymer modified cement grout got the implementation in construction arenas for providing a decent performance compared to that of the ordinary cement-sand grout and eventually got the popularity [9, 10].
The basic principle of polymer modification involves the mixing of polymer or monomer in powdered or liquid form along with cement and other admixtures followed by curing. In case of monomers being used, in-situ polymerization of that monomer is required. The polymers or monomers with which polymer modification can be done are of four major types ( ), i.e., RPP, polymer latex, water soluble polymer and liquid polymer [9].
RPPs are simply the spray-dried powders which when mixed with water, produce stable dispersion just like the original dispersion and this is governed by the fact that it redisperse only once [11]. Originally, RPPs are synthesized from latex dispersions and this involves a two-step synthesis process. The first step includes preparation of polymer latexes via emulsion polymerization and the second step being the spray drying of this emulsion into fine powder. Prior to the spray drying, various admixtures are added to the formulation such as anti-foaming agents, accelerators, water retention agents etc. for improving the performance of the polymer. To prevent the cake formation of the polymer powder during storage, anti-blocking aids such as clay, silica and CaCO3 are added before or after the spray drying [9].
Polymer latexes or polymer dispersions are synthesized just like RPP by emulsion polymerization. Polymer latexes are characterized by the fact that they are of very small particles, typically possesses 0.05–5μm in diameter. Common examples of synthetic latexes include vinyl acetate, homo- and co-polymer latexes, acrylic copolymer latex, styrene butadiene copolymer latex etc. Due to chloride ion liberation and unsatisfactory resistance, polyvinyl acetate and poly (vinylidene chloride-vinyl chloride) latexes are not advised. In case of natural rubber and epoxy latexes, they are not compatible to be synthesized by emulsion polymerization, rather they are acquired naturally by tapping from the rubber trees, which is then concentrated and finally a solid mass is obtained. Cellulose derivatives, polyvinyl alcohol, polyacrylamide etc. are examples of water soluble polymer which are mixed with cement in powdered form with the expectation of preventing the dry out occurrence and to improve the serviceableness. This is due to the increasing viscosity of water by dissolving the polymer and the sealing property is exerted by the formation of a thin film. Overall, the modification with water soluble polymer doesn't contribute substantially to the strength of the system. Liquid polymers find the least of utilization due to its handling and storage difficulties. They are mainly epoxy and unsaturated polyester resins which are applied along with hardener or catalyst and accelerators [12].
CTA modified with RPP has got the special consideration because of its beneficial aspects. This whole concept is justified by the fact that, the hydration of cement and formation of polymer film occurs accordingly which begets a network structure of monolithic matrix phase through which the hydrated cement phase and polymer phase interpenetrate. Modification with RPP is akin to latex modification but with the advantage of the redispersion property [13]. Some of the features delivered by RPP are improved workability, improved adhesion, increased flexural strength, increased plasticity, improved abrasion resistance, improved water retention, reduced water absorption, increased viscosity and cohesion etc. Inclusion of RPP in the cement grout caused the formation of a film [14] as water evaporates and this acts as a binder. This polymer film interconnects all the cement particles and fillers [15]. Moreover, this is more suitable for household tiling than industrial or special type tiling and the low price for a high outcome is the pivotal reason for its popularity.
This article will give an introductory idea about the components that are being used in making ceramic tile adhesive, especially the inclusion of RPP.
Now the Redispersible Polymer powder commonly used is mainly vinyl acetate-ethylene (referred to as VCAE) and vinyl acetate-vinyl tertiary acid (referred to as Ve0Va). Improving the bonding tensile strength of cement mortar and various substrates is the most crucial characteristic of vinyl copolymers, especially its ability to withstand significant changes in ambient temperature. In addition, it is conducive to developing long-term tensile bond strength of cement mortar. After two and a half years of storage in the air, the bond tensile strength of modified cement mortar is still increasing, while the tensile bond strength of ordinary cement mortar is significantly lower than that of maintaining the strength for 28d. Studies have shown that after ten years of application of three redispersible polymer dry powder modified cement mortars, such as polyethylene vinyl acetate, styrene/acrylic acid copolymer, and ethylene/vinyl chloride/vinyl laurate terpolymer, the cement mortar have stable bond strength, flexural strength, and compressive strength. In a five-year-old finishing mortar, the terpolymer still performed well in an aqueous environment with an excellent hydrophobic effect.
Adding Redispersible Polymer Powder into the mortar is a modification, also known as Polymer Modified Cement Mortar， to improve the weaknesses of traditional cement mortar, such as brittleness and high elastic modulus. Therefore, mortar with better flexibility and tensile bond strength can resist or delay the generation of cement mortar cracks. Since the polymer and cement mortar form an interpenetrating network structure, a continuous polymer film is formed in the pores, which strengthens the bonding between the aggregates and blocks some pores in the mortar, so various properties after hardening are much better than ordinary cement mortar. It is mainly manifested in the following characteristics:
(1) Strength
Generally speaking, the tensile strength and flexural strength of Polymer Modified Cement mortar are significantly improved compared with ordinary cement mortar, while the compressive strength is not, or even decreased. The increase is mainly attributed to the higher tensile strength of the polymer itself and the improvement of the bond between the cement hydration product and the aggregate.
The cracking will occur when the tensile stress exceeds its tensile strength value, mainly caused by tensile failure. Therefore, having a high tensile strength value is a necessary condition for crack resistance.
Studies have shown that with the increase of the poly-ash ratio, the tensile strength of the polymer-modified mortar generally increases first and then declines, indicating an optimal dosage range and the decline is typically the addition of excess. The Redispersible Polymer Powder introduced excessive air bubbles, resulting in a downward trend in compressive strength. In addition, since the elastic modulus of the polymer is lower than that of cement mortar, and the polymer chain segment is more flexible, it cannot provide rigid support when the composite is under pressure, resulting in a decrease in the compressive strength of the modified mortar as the poly-ash ratio increases. Therefore, the primary means to improve the compressive strength is to adjust the lime-sand ratio, water-cement ratio, aggregate gradation, and aggregate type. It can enhance tensile and flexural strength, flexibility, crack resistance, and hydrophobicity by adding a certain amount of Redispersible Polymer Powder, but not as much as possible. Adding silica fume can replace part of cement and Redispersible Polymer Powder, reducing the cost and improving the compressive and flexural strength.
(2) Elongation
The elongation and toughness of the mortar added with Redispersible Polymer Powder are much better than that of ordinary cement mortar; the fracture energy is more than twice that of ordinary cement mortar and the impact toughness increases with the higher of the aggregate-ash ratio. With the rise of the content of Redispersible Polymer Powder, the flexible cushioning effect of the polymer can inhibit or delay the development of cracks. At the same time, it has a better stress dispersion effect when the stress is generated by the deformation of the substrate, which will be transmitted or dispersed to avoid cracking due to stress concentration.
In addition, it should also be noted that there is a specific matching relationship between the tensile strength and elongation of the material. The high content of flexible Redispersible Polymer Powder will increase the elongation and make the polymer in a continuous phase. The polymer dominates in the hardening cement mortar, showing a decrease in tensile strength and water resistance. However, in the initial stage of the hydration heat reaction, the low tensile strength will make it quickly approach the ultimate tensile strength value, which causes the material to be prone to fatigue and fracture. So while ensuring sufficient elongation at break, it should also have the highest possible tensile strength value.
The external thermal insulation system should be placed in a flexible environment. The polystyrene board adhesive and plastering mortar should first choose flexible Redispersible Polymer Powder with a low glass transition temperature to release various deformation stresses in time.
Experiments have shown that the amount of flexible Redispersible Polymer Powder in the polystyrene board adhesive is about 3%, the amount of crack-resistant plastering mortar is controlled between 4% and 5%, and the amount of adhesive for exterior wall tiles is about 3%. The folding ratio is less than 3, which can effectively control the expansion, hollowing, and cracking caused by deformation.
(3)Adhesive property
The bonding ability of cement mortar with Redispersible Polymer Powder is much better than that of ordinary cement mortar, mainly due to the bonding ability shown by the characteristics of the polymer, and the bonding strength increases with the rise of the content. However, there is an optimal dosage range and cost considerations.
High bond strength can have a specific inhibitory effect on shrinkage, and the stress generated by deformation is easy to disperse and release, so bond strength is essential to improve crack resistance. Studies have shown that the synergistic effect of methylcellulose and Redispersible Polymer Powder is beneficial to improve the bond strength of cement mortar. The bonding mechanism of polymers depends more on the adsorption and diffusion of macromolecules on the bonding surface. At the same time, the polymer has a specific permeability, and together with methylcellulose acyl, it can fully infiltrate the surface of the base material, which is close to that of freshly applied mortar, thereby improving the adsorption of freshly applied mortar and base material, and significantly strengthening the bonding performance. In addition to improving the adhesion to the smooth substrate, the polymer-modified mortar also has the effect of closing the surface pores of the substrate, especially for those porous substrate surfaces, which can improve the waterproof, impervious, and freeze-thaw resistance of the base.
(4)Elasticity modulus
The elastic modulus of Redispersible Polymer Powder is low, in the range of 0.001-10GPa, while the elastic modulus of cement mortar is high, in the range of 10-30GPa. Therefore, the elastic modulus of cement mortar modified by adding Redispersible Polymer Powder is smaller than that of ordinary cement mortar. The type and amount of Redispersible Polymer Powder added also affect the elasticity modulus, which usually increases the polymer-cement ratio, the modulus of elasticity, and the deformation ability.
(5) Abrasion resistance
The wear resistance of the modified mortar is related to the type of Redispersible Polymer Powder and the polymer-cement ratio. Generally speaking, increasing the polymer-cement ratio raises the wear resistance mainly due to a certain amount of organic polymers on the wear surface. The polymer acts as a bond so that the mesh structure formed by the polymer can pass through the cement mortar. The pores and cracks in the cement improve the bonding between the aggregate and the cement hydration product, therefore the wear resistance is improved.
In the polymer-modified mortar, the mesh structure formed by the polymer filling seals the holes and cracks and reduces the porosity in the cement mortar, thereby improving the cement mortar’s impermeability, water and freezing resistance, which along with the increase of polymer-cement ratio.
Different types of Redispersible Polymer Powder have significant differences in water resistance. Usually, vinyl acetate powder (VAC, commonly known as Redispersible Polymer Powder formed by spraying white latex) modified mortar has poor water resistance. Therefore, except that the occasion that does not have water resistance requirement also may use VAC, basically do not use VAC for mortar modification. On the other hand, VAC is prone to saponification reactions in a strong base and loses its bond strength, so it is unsuitable for use with cement mortar. VAC can usually be used in gypsum-based products and putty powder without cement.
(6) Permeability
The water vapor permeability of building materials is of great significance. Waterproof and air permeability are desired on many occasions. Polymer-modified mortar has low water absorption and improved water resistance but still has a specific water vapor permeability which will decrease with the increase of polymer content. At the same time, it can be compounded with some Hydrophobic materials to improve its all-around performance.
(7) Freeze-thaw resistance
The porosity is reduced due to the significantly reduced water absorption of the polymer-modified mortar and the filling effect of the polymer. The Redispersible Polymer Powder has a specific air-entraining impact, and its freeze-thaw resistance is better than that of ordinary mortar. At the same time, with the increase of Redispersible Polymer Powder, the bond tensile strength of mortar before and after freeze-thaw cycles also showed an increasing trend. During construction in winter, adding an appropriate amount of calcium formate and antifreeze can significantly improve the mortar’s early strength and antifreeze performance.
Many experiments have shown that the amount of Redispersible Polymer Powder in cement mortar is not the more, the better. Lime-sand ratio, water-cement ratio, aggregate gradation, type, and features will ultimately affect the product’s overall performance. The polymer-improved cement mortar exhibited different properties when the content of the Redispersible Polymer Powder was varied. When the amount is too low, the polymer only has some plasticizing effect on the cement mortar, but the reinforcing effect is not apparent. When the amount is too large, the polymer dominates in the cement mortar, interrupting the continuous phase of the cement mortar and eventually leading to a decrease in strength. When the polymer content is moderate, it increases the deformation resistance, tensile strength, and bonding strength, improves the impermeability, and shows good crack resistance.