The redispersible powder is a special water-based emulsion made by spray drying and a polymer binder prepared mainly based on the vinyl acetate-ethylene copolymer. After partial evaporation of water, polymer particles form a layer of polymer film by agglomeration and play the role of adhesive. When redispersible polymer powder is used together with inorganic cementitious minerals such as cement, it can have a modification effect on mortar. Such products are often referred to as polymer mortar. By polymer modification, it has improved the brittleness of the traditional cement mortar, enhanced the flexibility and tensile bond strength of cement mortar, and reduced the incidence of cracks in cement mortar. Since the polymer and cement mortar form an interpenetrating network structure and a continuous polymer film in the pores, strengthening the bond between aggregates and blocking some of the pores within the mortar, the polymer modified mortar after hardening is superior to ordinary cement mortar in all kinds of properties. The main functions of redispersible polymer powder are as follows.
(1) Improve the bond strength, tensile strength and flexural strength.
Redispersible polymer powder can significantly improve the bond strength of mortar. The greater the amount is added, the more it will be improved, but the compressive strength will be reduced. Therefore, there exists an optimal range of addition amount. Since the price of redispersible polymer powder is high, the greater the amount is added, the higher the cost of dry mortar will be. Therefore, the cost also needs to be taken into account. High bond strength can have a certain inhibitory effect on the contraction, while the stress generated by deformation is easily dispersed and released, so the bond strength is very important to improve the crack resistance. Studies have shown that the synergistic effect of cellulose ethers and polymer powders could help improve the bond strength of cement mortar.
(2) Reduce the elastic modulus of mortar, enabling the brittle cement mortar to have certain flexibility.
The elastic modulus of redispersible polymer powder is low, 0.001-10GPa; while that of cement mortar is high, 10-30GPa, so the elastic modulus of cement mortar will be reduced with the addition of polymer powder. But the type and dosage of polymer powders also have an effect on the elastic modulus. Usually, as the polymer-cement ratio increases, the elastic modulus decreases and the deformation capacity increase.
(3) Improve the water resistance, alkali resistance, abrasion resistance and impact resistance.
The network film structure formed by polymers closes the holes and cracks in cement mortar, and reduces the porosity of hardened body, thereby improving the impermeability, water resistance and frost resistance of cement mortar. This effect increases with the increase of the polymer-cement ratio. Improving the abrasion resistance is related to the type and polymer-cement ratio of polymer powder. In general, as the polymer-cement ratio increases, the abrasion resistance is improved.
(4) Improve the liquidity and constructability of mortar.
(5) Improve the water retention of mortar and reduce water evaporation.
The polymer emulsion formed after the redispersible polymer powder dissolved in water is dispersed in mortar and forms a continuous organic film in mortar after solidification. This organic film can prevent the migration of water, thereby reducing the water loss of mortar and playing a role in water retention.
(6) Reduce the cracking phenomenon
The elongation and toughness of polymer modified cement mortar are much better than those of ordinary cement mortar. Fracture performance is more than twice that of ordinary cement mortar; and impact resistance toughness increases with the increase of polymer-cement ratio. With the increase in the amount of polymer powder added, the flexible buffering effect of polymer can inhibit or retard the development of cracks, and meanwhile with good stress dispersion.
According to different mix ratios, redispersible polymer powder can be used for modification of dry-mixed mortar. It can not only improve the bond strength with various substrates, but also improve the flexibility, deformability, bending strength, abrasion resistance, toughness, adhesion, water-retaining capacity and workability of mortar.
A large number of tests show that the addition amount of polymer powder is not the more the better. When the addition amount of polymer powder is too low, it will only have some plasticizing effects but not obvious strength-increasing effects; when the amount added is too large, the strength will decrease substantially. Only when the amount added is moderate, can it not only increase the deformation resistance, tensile strength and bond strength, but also improve the impermeability and crack resistance. The cement/sand ratio, water/cement ratio, aggregate gradation and type, and aggregate properties will ultimately affect the overall performance of product.