Influence of dispersible polymer powder on cement-based materials

Redispersible latex powder is a commonly used organic gelling material. It is a powder obtained by spray drying a polymer emulsion with polyvinyl alcohol as a protective colloid. This powder can be re-dispersed in water evenly after encountering water. , forming an emulsion. The addition of dispersible polymer powder can improve the water retention performance of fresh cement mortar, as well as the bonding performance, flexibility, impermeability and corrosion resistance of hardened cement mortar. The following introduces the mechanism of redispersible polymer powder in cement mortar and its influence on the performance of cement mortar.

Effects on cement hydration process and paste structure

As long as the cement-based material added with latex powder contacts water, the hydration reaction starts, the calcium hydroxide solution quickly reaches saturation and crystallizes, and at the same time, ettringite crystals and hydrated calcium silicate gel are formed, and the polymerization in the emulsion The cement particles are deposited on the gel and unhydrated cement particles. With the progress of the hydration reaction, the hydration products increased, and the polymer particles gradually aggregated in the capillary pores and formed a close-packed layer on the surface of the gel and the unhydrated cement particles. The aggregated polymer particles gradually filled the capillary pores, but could not completely fill the inner surface of the capillary pores. As hydration or drying reduces moisture further, the polymer particles tightly packed on the gel and in the pores coalesce into a continuous film, forming an interpenetrating mixture with the hydrating cement slurry and improving hydration Bonding of product to aggregate. Because the hydration product with polymer forms a covering layer at the interface, it may affect the growth of ettringite and coarse calcium hydroxide crystals; also because the polymer coagulates into the film in the pores of the interface transition zone, which makes the polymer cement-based material The transition zone is denser. The active groups in some polymer molecules will also have a cross-linking reaction with Ca2+, A13+, etc. in the cement hydration product, forming a special bridging bond, improving the physical structure of the cement-based material hardened body, relieving internal stress, reducing generation of microcracks. As the cement gel structure develops, the water is depleted and the polymer particles are gradually confined in the capillary pores. With further hydration of the cement, the water in the capillary pores decreases, and the polymer particles aggregate on the surface of the cement hydration product gel/unhydrated cement particle mixture and aggregates, forming a continuous tightly packed layer with large pores Filled with sticky or self-adhesive polymer particles.

The role of dispersible polymer powder in mortar is controlled by the two processes of cement hydration and polymer film formation. The formation of a composite system of cement hydration and polymer film formation is accomplished in 4 steps:

(1) After the redispersible latex powder is mixed with the cement mortar, it is uniformly dispersed in the system;

(2) polymer particles are deposited on the surface of the cement hydration product gel/unhydrated cement particle mixture;

(3) The polymer particles form a continuous and tight stacking layer;

(4) During the cement hydration process, the closely packed polymer particles aggregate into a continuous film, and the hydration products are bonded together to form a complete network structure.

The dispersion emulsion of redispersible polymer powder can form a water-insoluble continuous film (polymer network) after drying, and this low elastic modulus polymer network can improve the performance of cement; Some polar groups of the cement react chemically with the cement hydration product to form a special bridge bond, which improves the physical structure of the cement hydration product and alleviates and reduces the generation of cracks. After the redispersible polymer powder is added, the initial hydration rate of the cement is slowed down, and the polymer film can partially or completely wrap the cement particles, so that the cement can be fully hydrated and its various properties can be improved.

Influence on the bond strength of cement-based materials

Emulsion and dispersible polymer powder can form high tensile strength and bond strength on different materials after film formation. They are combined with inorganic binder cement as second binder in mortar. Cement and polymer are respectively Give play to the corresponding specialties, so that the performance of the mortar can be improved. By observing the microstructure of the polymer-cement composite material, it is believed that the addition of redispersible polymer powder can make the polymer film and become a part of the pore wall. aggregate strength, thereby increasing the failure stress of the mortar and increasing the ultimate strain. The long-term performance of the redispersible polymer powder in the mortar was studied. It was observed by SEM that after 10 years, the microstructure and morphology of the polymer in the mortar remained unchanged, maintaining stable bonding, flexural resistance and compression resistance. strength and good hydrophobicity. Wang Ziming et al. [11] studied the formation mechanism of redispersible latex powder on the strength of tile adhesives, and found that after the polymer was dried to form a film, the polymer film formed a flexible connection between the mortar and the tile on the one hand, and the other On the one hand, in fresh mortar, the polymer increases the air content of the mortar and affects the formation and wettability of the surface, and subsequently during the setting process, the polymer also contributes to the hydration process and shrinkage of the cement in the binder. The best effect, all of which will have a better help to improve the bond strength.

Adding redispersible latex powder to mortar can significantly improve the bond strength with other materials, because the hydrophilic latex powder and the liquid phase of the cement suspension penetrate into the pores and capillaries of the matrix together, and the latex powder penetrates into the pores and capillaries. The inner film is formed and firmly adsorbed on the surface of the substrate, thereby ensuring a good bond strength between the cementitious material and the substrate.

The optimization of the working performance of the mortar by the latex powder is that the latex powder is a high molecular polymer with polar groups. When the latex powder is mixed with the EPS particles, the non-polar segments in the polymer main chain of the latex powder are Physical adsorption will occur with the non-polar surface of EPS. The polar groups in the polymer are oriented outward on the surface of the EPS particles, so that the EPS particles change from hydrophobicity to hydrophilicity. Floating, the problem of large mortar layering. At this time, adding cement and mixing, the polar groups adsorbed on the surface of EPS particles interact with the cement particles and combine closely, so that the workability of EPS insulation mortar is significantly improved. This is manifested in the fact that the EPS particles are easily wetted by the cement slurry, and the binding force between the two is greatly improved.

Influence on the flexibility of cement-based materials

Redispersible latex powder can improve the flexural strength, adhesion strength and other properties of mortar because it can form a polymer film on the surface of the mortar particles. There are pores on the surface of the film, and the surface of the pores is filled with mortar, so that the stress concentration is reduced. And under the action of external force will produce relaxation without damage. In addition, the mortar forms a rigid skeleton after the cement is hydrated, and the polymer in the skeleton has the function of a movable joint, similar to the tissue of the human body. The film formed by the polymer can be compared to joints and ligaments, thus ensuring the elasticity and toughness.

In the polymer-modified cement mortar system, the continuous and complete polymer film is intertwined with the cement paste and sand particles, making the whole mortar denser as a whole, and at the same time filling the capillaries and cavities to make the whole into an elastic network. Therefore, the polymer film can effectively transmit pressure and elastic tension. The polymer film can bridge the shrinkage cracks at the polymer-mortar interface, make the shrinkage cracks heal, and improve the sealability and cohesive strength of the mortar. The presence of highly flexible and highly elastic polymer domains improves the flexibility and elasticity of the mortar, providing cohesive and dynamic behavior to the rigid skeleton. When an external force is applied, the microcrack propagation process is delayed due to the improvement in flexibility and elasticity until higher stress is reached. The interwoven polymer domains also play a role in hindering the merging of microcracks into through-cracks. Therefore, the dispersible polymer powder increases the failure stress and failure strain of the material.

Influence on the durability of cement-based materials

The formation of polymer continuous films is extremely important to the properties of polymer-modified cement mortars. During the setting and hardening process of cement paste, many cavities will be generated inside, which become the weak parts of cement paste. After the redispersible polymer powder is added, the polymer powder immediately disperses into an emulsion in contact with water, and accumulates in the water-rich area (ie, in the cavity). As the cement paste sets and hardens, the movement of the polymer particles becomes more and more restricted, and the interfacial tension between water and air causes them to gradually align together. When the polymer particles begin to contact each other, the water in the network evaporates through the capillaries, and the polymer forms a continuous film around the cavity, strengthening these weak spots. At this time, the polymer film can not only play a hydrophobic role, but also will not block the capillary, so that the material has good hydrophobicity and air permeability.

The cement mortar without adding polymer is very loosely connected. On the contrary, the polymer modified cement mortar makes the whole mortar very closely connected due to the existence of the polymer film, thus obtaining better mechanical properties and weather resistance. sex. In the latex powder-modified cement mortar, the latex powder will increase the porosity of the cement paste, but reduce the porosity of the interface transition zone between the cement paste and the aggregate, so that the overall porosity of the mortar is basically unchanged. After the latex powder is formed into a film, the pores in the mortar can be better blocked, so that the structure of the transition zone between the cement paste and the aggregate interface is more compact, the permeability resistance of the latex powder modified mortar is improved, and the ability to resist the erosion of harmful media is enhanced. It has a positive effect on the improvement of mortar durability.

At present, dispersible polymer powder plays an important role as an additive for construction mortar. Adding redispersible latex powder to the mortar can prepare various mortar products such as tile adhesive, thermal insulation mortar, self-leveling mortar, putty, plastering mortar, decorative mortar, joint grout, repair mortar and waterproof sealing material. Application scope and application performance of building mortar. Of course, there are problems of adaptability between dispersible polymer powder and cement, admixtures and admixtures, which should be given enough attention in specific applications.