Mechanical properties of polymer modified mortars using polymer latexes with varied glass transition temperature and surface charges

Mechanical properties of polymer modified mortar (PMM) were investigated using self-synthesized polymer latexes with varied glass transition temperature (Tg) and colloidal surface charges to build correlations between the mechanical properties of PMMs and the intrinsic properties of the polymers. Generally, compressive strength and elastic modulus of mortar are decreased, but ductility and toughness are enhanced by incorporation of the polymers. Impacts of the lower Tg polymers on the mechanical properties of PMMs are more pronounced. With increasing polymer content, flexural and tensile strength of mortar first increase and then decrease when using low Tg polymers that form continuous film during hardening of the mortars, while steadily decrease in the case of high Tg polymers that exist as individual particles in the hardened PMMs. For the high Tg polymer modified mortars, it is interestingly found that a subsequential heat treatment effectively promotes the film formation of the polymers in the hardened PMMs and thus greatly increases their flexural and tensile strength. This indicates that polymer film formation is essential to the enhancement of the PMMs, because the polymer films play a crack-bridging effect while the individual particles cannot do such job. The high Tg polymers lead to higher tensile strength of PMMs after heat treatment due to the higher inherent strength of the polymer film. Higher colloidal surface charge benefits the mechanical properties of the PMMs by (1) promoting a more homogenous polymer distribution in the hardened PMMs and/or (2) improving the bonding between the polymer phase and cement hydrates.