Mechanism of Polymer-Modified Cement Mortar

When a polymer emulsion is added to cement mortar, as the cement hydrates and absorbs water, the dehydrated polymer particles gradually aggregate to form a three-dimensional, spatially continuous network of polymer film. This film adheres to the cement hydration products and aggregate surfaces. The cement stone also penetrates through the mesh pores of the polymer network, forming its own spatially continuous network structure. The two network structures interpenetrate, interlock, and entangle with each other, forming a continuous and dense matrix structure. This structure enhances the density of the interfacial transition zone and improves the bond between aggregates and cement hydration products, thereby increasing the strength of the cement mortar.

In this structure, the polymer network penetrates the air voids and cracks in the cement stone, reducing and bridging them to form an elastic 'hinge' structure. This not only disperses stress concentrations but also increases the resistance to deformation. Even if cracks form under stress, the polymer bridges across the cracks and inhibits their propagation, thereby improving the fracture toughness, deformability, and crack resistance of the hardened body.

In this structure, the polymer fills and connects the voids and cracks in the cement stone, reducing the porosity of the hardened body. This improves the impermeability, frost resistance, water resistance, temperature resistance, and corrosion resistance of the hardened body.

This structure involves not only physical adsorption and bonding but also chemical interactions in the form of chemical bonds. Ionic chemical bonding occurs between polymer particles and cement hydration products, which affects both the film-forming process of the polymer and the hydration process of the cement. Hydrogen bonding and van der Waals forces may also exist between polymer particles and cement hydration products. These chemical bond interactions exert a beneficial effect on the structure of the hardened body.

Polymer-modified cement mortar exhibits different properties with varying polymer content. When the polymer content is too low, the polymer only acts as a plasticizer for the cement mortar, with no significant strengthening effect. When the polymer content is moderate, it not only increases the deformation resistance, tensile strength, and bond strength but also improves impermeability, demonstrating excellent crack resistance. When the polymer content is excessively high, the polymer becomes dominant in the cement mortar, disrupting the continuity of the cement-sand matrix. The polymer film cracks due to excessive shrinkage, forming inclusions weaker than the cement stone, which ultimately leads to a decrease in strength.