5 Must-Have Features in a Acrylic Emulsion in Cement and Concrete Modification

CRETE-MOD&#; is a low-odor acrylic polymer emulsion designed for modifying Portland cement compositions. The superior adhesion, ultraviolet light resistance, water resistance, and increased mortar strength properties provided by previous acrylic emulsions have been complemented by improved sprayability in a low-odor formulation. Cement mortars modified with CRETE-MOD&#; are hard, tough, and durable to exterior exposure. Compared with unmodified mortars, acrylic polymer-modified mortars have superior flexural, tensile, and impact strengths, as well as excellent abrasion resistance and adhesion. These qualities are especially important in thin section applications (i.e. spray coatings, stuccos, and underlayments) and applications where excessive vibration and heavy traffic are encountered. CRETE-MOD&#; modified cement mortars have excellent adhesion to a variety of surfaces such as concrete, masonry, brick, wood, rigid polystyrene and polyurethane foam, glass, and metals. CRETE-MOD&#; modified mortars are uniform in color and resistant to yellowing or discoloration due to exposure to ultraviolet light. They also provide excellent water resistance to decrease spalling, cracking, and surface degradation caused by repeated freeze/thaw cycles. These mortars are also resistant to many industrial chemicals. CRETE-MOD&#; is ammonia-free, this low-odor feature is especially beneficial for interior construction or repair projects. CRETE-MOD&#; emulsion has also been optimized to provide a longer mortar pot life, an important feature for warm weather, low humidity application conditions. Further, the emulsion properties have been improved to enhance the flow characteristics of the modified cement for spray coat applications.

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CRETE-MOD&#; is a product with exceptional versatility. It can be used in a variety applications including: regular weight and lightweight spray-applied coatings, traditional trowel-applied stuccos, basement waterproof coatings, general purpose patching and repair mortars, floor resurfacing and underlayments, terrazzo, and heavy use industrial/commercial cement flooring. CRETE-MOD&#; also provides a superior binder for certain non-cementitious products such as one-package tile grouts.

The general procedure for preparing CRETE-MOD&#; modified mortar for evaluation begins by thoroughly premixing the sand and cement. The CRETE-MOD&#; and water are blended together and added to the premixed sand and cement. The entire composition is mixed thoroughly for about two to four minutes. To avoid overly fluid compositions, a portion of the water should be withheld and added gradually to the modified mortar mixture until the desired consistency is obtained. Although the procedures generally remain the same, the formulation varies according to the performance needs of the application situation. These formulations can vary in both the type and quantity of materials used. While numerous component relationships exist, some key formulation concepts are presented below: Choice of Filler - The type and particle size distribution of fillers are selected for reasons which include cost, density, color/texture of the final product, leveling characteristics and workability. Sand (filler)/Cement Ratio - For a typical modified mortar application, a 2:1 sand/cement ratio provides excellent mechanical strength properties. However, different ratios of filler to cement may be appropriate for applications with different performance requirements. Level of Water - Maximum density and strength are obtained when a minimum amount of water is used. This minimum water requirement refers to the lowest water/cement ratio which provides adequate workability. Level of Polymer Modification - An optimum balance of modified mortar properties (adhesion, tensile strength, flexural strength, compressive strength, impact resistance, water resistance and abrasion resistance) is obtained by incorporating 10 to 20 percent polymer solids, by weight, on cement. Higher modification will increase flexibility and water resistance. Lower levels of polymer will decrease the benefits of the polymer modification. RECOMMENDED MIXING RATIO: Cementitious materials should be pre-mixed to achieve a ratio of 2 Parts Sand and 1 part Cement. (Refer to the Data Sheet of the product to determine if sand is already in the mixture.) Use approximately 2 to 3 Quarts of CRETE-MOD&#; and 2 to 3 Quarts of Water to 50 pounds of pre-mixed cementitious material. Hold out some water to avoid an overly fluid mixture.

There are two views on the mechanism of acrylic acid lotion (PAE) modifying cement substrate: 1. During the hydration process, polyacrylic acid latex covers the surface of cement particles and hydration products or fills cracks in the cement hydration system. These physical effects can improve the porosity of cement. 2. PAE also has chemical reactions and physical behavior. They improve the performance of cement-based materials by connecting chemical bonds through chelation [76,77].

The modification of cement-based materials by polymer lotion occurs locally. Even if more polymer is added, the overall polymer film or network structure will not be formed, but the performance of cement-based materials can be modified at these local locations. This phenomenon is called polymer modification localization [ 49 , 79 ], as shown in Figure 3 . Initially, polymer particles, cement particles, water, and sand are mixed together without any physical or chemical reactions, but they are evenly distributed. As the cement hydration reaction progresses, some of the polymers start participating in chemical reactions, while others begin to fill up pores or adsorb onto the surface of cement-based materials, due to adsorption and various ion bonds, which is a physical reaction [ 80 ].

Polymer particles fill the pores of cement-based materials. As the hydration reaction proceeds, these particles accumulate between the pores and the interface transition zones, adsorbing and polymerizing in situ into thin, flexible films. This process forms a network structure that enhances the density and impermeability of cement-based materials, as observed in previous studies [ 7 , 78 ]. Additionally, the interface structures between organic and inorganic materials in polymer-modified cement-based materials create interactions between atoms and molecules in the polymers and the hydration products of cement-based materials through hydrogen bonds, van der Waals forces, and other mechanisms [ 20 ].

3.2.2. Chemical Mechanism of Acrylate-Lotion-Modified Cement-Based Materials

In the second stage of polymer-modified cement-based materials, as the hydration reaction between cement and water progresses, the cement hydration products and Ca2+ are released into the pore solution. Some polymers adhere to the surface of cement particles and hydration products, while the other portion combines with Ca2+ to form a flocculent structure.

The polymer particles that cover the cement surface will slow down the cement&#;s hydration process, and eventually, the cement hydrates will break through this barrier. As hydration progresses, the water phase in the pore solution is consumed, and cement hydrates continue to grow or embed into polymer flocs. The accumulated polymer particles will partially accumulate and coagulate into polymer network spatial structures or polymer membranes, as shown in Figure 4 and Figure 5 [81,82].

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Figure 4.

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Chemical modification simulation of polymer lotion in cement-based materials.

Figure 5.

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Second electronic image of ethylene vinyl acetate modified cement. (a)The absorption of polymer particles on the surface of cement particles; (b) porous C-S-H caused by polymer particles; (c) the local pore morphology of porous C-S-H; (d) formation of a polymer film on the surface of glass [78].

As early as , the three-step model proposed by Ohama [83] was the most popular in the principle of PMCs (polymer-modified cement-based materials). During the cement hydration process, the polymer forms a continuous and dense thin film on the surface of the cement particles. The adsorbed polymer particles changes the interface structure between the liquid and these particles, thereby affecting the performance of the interface. The interface performance will significantly affect the rheological properties of the cement and further determine the pumpability, self-compaction, and self-leveling of the cement, similar to the cement mixed with water-reducing agents [84,85]. Guo Yanfei et al. [86] studied the influence of polyacrylic acid lotion (PAE) on the fluidity of cement paste and found that when the content of PAE is less than 5.0%, the fluidity is reduced compared to when the content of PAE is more than 5.0%. The addition of PAE combines carboxyl groups with Ca2+ in the solution and Ca2+ on the surface of cement particles, while polymer PAE adsorbs on the surface of cement particles, both of which may affect the dispersion of the polymer and the rheological properties of PMCs [87,88,89].

The Konietzko model divides the mechanism of polymer-modified cement-based materials into four stages: the uniform dispersion of polymer particles, the accumulation of polymer particles, the aggregation of polymer particles into films, and the formation of spatial network structures by polymers in cement-based materials. The commonality between the Ohma model and the Konietzko model is that the formation of thin films causes the mechanism of polymer modification. But the Ohama model suggests that polymers form a spatial network structure in cement-based materials, and the hardened cement is encapsulated. The Konietzko model suggests that the products of polymer and cement, after hardening, penetrate each other to form a spatial network structure [90,91]. Therefore, both theoretical models are beneficial in explaining the mechanism of polymer-modified cement-based materials.

As further research reveals that the amount of polymer varies, the two theoretical models have different interpretations. When the polymer content is small, it cannot completely wrap the cement-based material, the polymer film forms a three-dimensional network structure with the cement hydration products. When the polymer content is high, the polymer forms a unique network structure to encapsulate cement-based materials. When the amount of polymer decreases, the polymer cannot form a continuous film but is dispersed in cement-based materials [92]. However, the current models established by researchers assume that polymer particles are uniformly dispersed in cement to form a polymer spatial network. The network structure ensures that polymer-modified cement-based materials have good toughness and corrosion resistance. After the polymer and cement paste are mixed, various ions generated by cement hydration impact the polymer&#;s stability, which may lead to the polymer particles being unable to form a continuous network structure [82,93,94]. Therefore, these models only apply to some of the explanations of the mechanism of polymer lotion-modified cement-based materials.

Wang Ming et al. [76] found three stages of chemical reactions in studying the mechanism of the PAE modification of cement-based materials. Ca(OH)2 produced by the hydration of cement leads to the whole system being alkali-rich and exothermic in the first stage, which is very helpful to the hydrolysis of ester groups in the acrylate chain, causing the carboxyl group to be formed in the second stage. In the last stage, the carboxyl group on the polyacrylate lotion chain reacts with Ca(OH)2, resulting in the final cross-linking network structure of the product [95]. Cement-based materials were modified using butyl styrene latex (SBR) and carboxyl butadiene styrene latex (XSBRI). After characterization, it was found that no chemical reaction occurred in the SBR latex-modified cement. However, the carboxyl groups in the XSBRI chain react with Ca2+ in Ca(OH)2, as shown in Figure 6. Hydration products connect the polymer lotion to obtain a three-dimensional network structure, improving polymer-latex-modified cement&#;s bending strength.

Figure 6.

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Chemical model of polymer-latex-modified cement. (a) The chemical reaction between polymer latex and hydration products. (b) A three-dimensional network structure obtained through chemical reactions in polymer-latex-modified cement systems [95].

At the same time, a possible physical modification mechanism model of polymer-latex-modified cement is also established, as shown in Figure 7. Polymer film and particles fill in cracks and pores and reflect some external forces during the fracturing process, improving the flexural strength of cement-based materials. Therefore, the polymer-modified cement-based materials mechanism includes physical and chemical aspects. When no active groups in the polymer can react with hydration products, the modification system only includes the physical modification mechanism. Polymer film covers the surface of crystals and fills the pores, improving the cement&#;s waterproof performance and bending strength. At the same time, polymer film occupies the position of hydration products, leading to a decrease in compressive strength. However, when the polymer chain of the polymer contains active groups, its mechanisms include physical and chemical modification mechanisms. The physical modification mechanism is the same as that of a polymer without active groups. In the chemical modification mechanism, active groups react with hydration products, connecting polymer latex chains to form a three-dimensional network structure, thereby improving the flexural strength of modified cement [75].

Figure 7.

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Physical model of polymer-latex-modified cement. (a) Polymer latex covers hydrated crystals (b) and polymer particles and films are filled in cracks and pores [95].

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