Temperature-dependent behavior of polymer latex in calcium sulfoaluminate cement: critical role of glass transition temperature

Polymer latex modification is highly temperature-sensitive in cement-based systems, yet its governing mechanisms remain unclear, particularly under sub-zero conditions. This study systematically investigates the temperature-dependent behavior of a controlled model polymer‑calcium sulfoaluminate (CSA) cement system to elucidate how polymer glass transition temperature (Tg), as a descriptor of polymer chain mobility, governs the coupling among polymer film formation, CSA hydration, pore structure, rheology, and fracture behavior. Three synthesized latexes with Tg values of −20 °C, 10 °C, and 40 °C were incorporated into CSA pastes and mortars cured at 25 °C and − 10 °C to resolve the underlying multi-scale mechanisms. The results demonstrate that modification efficiency is governed by the relative relationship between environmental temperature and Tg. Continuous polymer film formation occurs only when the environmental temperature exceeds Tg, enabling effective crack bridging and fracture energy enhancement. Otherwise, restricted polymer chain mobility and steric hindrance result in fragmented membranes or isolated particles. Latex incorporation induces significant air entrainment and increases rheological resistance, leading to a looser pore structure, lower matrix density, and reduced compressive strength. The latexes also inhibit ettringite formation, with stronger suppression at lower Tg. At −10 °C, ye'elimite and anhydrite are still consumed, whereas crystalline ettringite is nearly undetectable, suggesting that the hydration products are mainly poorly crystalline or XRD-amorphous phases. These findings establish a Tg-governed mechanistic framework for designing polymer-modified CSA repair materials for cold-region infrastructure.