There are different views on the interaction between polyvinyl acetate and cement in aqueous solution. Some scholars think that the modified cement mortar with polyvinyl acetate itself forms polymer film inside and the holes inside mortar are sealed or covered, so its impermeability is improved. Foreign scholars think that the interaction between polymer and Ca(OH)2 will form a large polymer which will reduce the porosity.
Researches found chemical reaction between cement and polymer polyvinyl acetate. The vinyl acetate of polyethylene under the hydrolysis of alkali releases acetate group which reacts with Ca2 in pulp to form organic salt (calcium acetate) and polyvinyl alcohol. Calcium acetate is a kind of strong hygroscopic salt while polyvinyl alcohol is water-soluble. Therefore polyvinyl acetate can consume part of Ca2+ resulting in the decrease of calcium hydroxide content in pulp . Polyvinyl acetate can delay the hydration process of cement and rod ettringite crystal and Hadley crystal will be formed in the modified cement paste. Polyvinyl acetate can significantly enlarge pore volume, threshold radius and pore volume of cement paste as well as the pore structure of cement paste is closely related to the curing system and its dosage.
Researches also found that EVA in cement slurry makes the slurry form a complex structure and changes the morphology, composition and quantity of cement hydration phase, especially calcium hydroxide phase. When polymer dry powder is added into cement , spherical polymer particles are filled in the middle of cement particles respectively, and the hydration products gather around the polymer particles after hardening. The dissolution and dispersion of polymer dry powder in cement is similar to polymer emulsion. Polymer particles dispersed in hardened cement paste have toughening effect, thus improving the bond strength of cement. The dispersion effect of polymer particles also reduces or even disappears the average free path between polymer particles in the slurry which makes the transmission of cracks in the hardened slurry more complex, the flexural strength ratio of the hardened slurry increases, and the toughness increases. The increase of bond strength between polymer modified cement mortar and other materials is due to the formation of polymer film on the interface. The formation of polymer film and the dispersion of polymer particles are conductive to the composite action of polymer cement mortar. It can be concluded that there are both chemical and physical reactions between polyvinyl acetate and cement paste, and the comprehensive effect of the two reactions affects the properties of cement paste.
Compared with the liquid polymer modified cement mortar, the polymer particles in the polymer dry powder cement mortar have poor connection, the polymer film formed is poor and the continuity of the polymer film is relatively poor. Different polymer dry powder modified cement mortar has different ability to form polymer film, but there is no such phenomenon in liquid polymer modified cement mortar. The polymer membrane structure of two kinds of modified cement mortar is different, which is related to the polymer type, mixing amount and solid-liquid form. It can be shown as network shape, linear shape, uneven shape, dense or thin round fibrous shape. When the ration of polymer dry powder modified cement mortar is more than 10%, the polymer dry powder modified cement mortar and the liquid polymer modified cement mortar have obvious continuous polymer film. The hydration products of C3A phase are closely related to the resistance of cement to sulfate attack and other erosions. Some scholars believe that the reason why cement is resistant to sulfate attack is that C3AH6 is converted into inactive hydrogarnet . The strength of cement is also related to the spatial network of hydrogarnet cubic crystals. By studying the hydrogarnet cubic crystal form of many kinds of polymer-modified cement mortar, it was found that some of the polymer dry-powder modified cement mortar had hydrogarnet cubic crystal, while others did not. At low magnification, the hydrogarnet cubic crystal of polymer-modified cement mortar is similar to sucrose crystal, with a very smooth surface, but at high magnification, the surface of hydrogarnet cubic crystal is a little uneven. No similar hydrogarnet cubic crystal was found in ordinary mortar, because the water ash of polymer dry-powder modified cement mortar is lower than that of ordinary mortar, which is conducive to the formation of hydrogarnet cubic crystal. With different properties of polymer, the process of cement hydration is also different. Therefore, the formation of hydrogarnet cubic crystal is affected to different degrees. In the dry powder modified cement mortar, the formation of hydrogarnet cubic crystal depends on the type and content of polymer.
The properties of redispersible powder in dry mortar products such as low temperature flexibility, hydrophobility, alkali resistance, adhesion, water retention, air entraining and workability, are mainly derived from the physical and chemical properties of the original polymer emulsion. These physical and chemical properties depend on which polymer monomers are selected to polymerize in the systhesis process. The hard monomers of redispersible powder are mainly vinyl acetate. The glass transition temperature is about 28 ℃. After film forming, the hardness is high and the adhesion is good but the weakness is that the brittleness is large and the alkali resistance is poor. In order to improve its weakness soft monomer ethylene is often used as internal plasticizer by adjusting the proportion of different monomers, a series of polymerized copolymerization emulsion with different hardness and flexibility can be obtained to reduce the rigidity of the film forming material, improve the film forming property at low temperature, alkali resistance and hydrophobicity and good adhesion of organic substrates such as polystyrene board, PVC and wood.
Therefore, we can see from the different brands of RDP produced in various countries that they all have different physical properties and are applicable to different application fields. Some brands have excellent low temperature flexibility and cohesiveness which are suitable for ceramic tile pasting and crack resistant plastering mortar with strong deformation ability; some have excellent alkali resistance and hydrophobicity which are suitable for external insulation and finish system, joint filler and waterproof mortar. In practical application the final effect of different brands of RDP for different purposes differ a lot. Adding different types of RDP mainly shows the following characteristics:
- Improve the bond strength, tensile strength and flexural strength of the material
- Make the rigid and brittle cement mortar have certain flexibility, reduce the elastic modulus of the molding material so it has high plasticity
- Improve the water resistance, alkali resistance, wear resistance and impact resistance
- Improve the fluidity and constructability
- Due to the existence of water-soluble protective colloid on the surface of RDP, it can improve the water retention of mortar, reduce water evaporation and improve the strength.
- Reduce shrinkage and cracking.