Usually, polymer-modified cement concrete or mortar is prepared by mixing polymer dispersions or redispersible polymer powders with the fresh mixture. The surface active agents, added to allow emulsification and stabilization of the dispersion during storage, hinder the cement hydration and the polymer film formation. Therefore, the addition of polymers in aqueous solution is aimed for. Water-soluble polymer molecules are supplied on a molecular scale, improving the approach of the relatively large cement grains by the polymers. This will allow the building up of composite polymer-cement microstructures on a nano-scale. In the absence of surface active agents, water-soluble polymers tend to require a lower proportion in order to be comparably effective as polymer dispersions. The addition of very small amounts of water-soluble polymers results in an improvement of the durability and the adhesion strength of the cementitious materials, which makes them appropriate as repair materials.
The effect of the addition of water-soluble polymers is twofold: the cement hydration reactions may be influenced and polymer film formation may take place. The microstructure building and the influence on the cement hydration reactions is studied by SEM investigation. An abundant crystal growth is noticed at the air void surfaces of the modified mortars where the presence of water-soluble polymers is expected because of their strong affinity to the gas-water phase.
Possible film formation is studied by examining the influence of the curing conditions on the flexural strength of mortar beams. Cement hydration requires a wet or moist curing, while a dry curing promotes polymer film formation. If film formation takes place, the contribution of the polymer film to the mechanical properties and the durability of the polymer film should be studied with respect to practical applications, in particular their resistance to re-dissolution in humid environment.

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