Early Age Performance of OPC-GGBFS-Concretes Containing Belite-CSA Cement Cured at Sub-Zero Temperatures

This study determined how replacing sodium nitrate-based antifreeze admixture (AF) with belite-calcium sulfoaluminate (belite-CSA) cement affects the early age properties of ecological concretes based on ordinary Portland cement (OPC) and ground granulated blast-furnace slag (GGBFS). Concrete specimens were cured at −15 °C and treated in various ways before testing, i.e., no treatment, stored at 20 °C for 12 and 24 h. Generally, the addition of belite-CSA cement shortened the setting time due to the rapid formation of ettringite. The incorporation of 25 wt% of antifreeze admixture (AF) to the OPC-GGBFS concrete cured at −15 °C partially inhibited ice formation and enabled the continuation of hydration processes. This trend was observed for all samples, independent of the applied AF after freezing curing. On the contrary, the addition of 20 wt% of CSA failed to inhibit the ice formation and increased the risk of frost damage for concretes despite the treatment after freezing. These concrete specimens had lower hydration, lower strength, and a more porous binder matrix. The microstructure of the binder matrix was significantly affected by the amount of CSA and extreme negative curing, followed by no notable recovery post-curing at room temperature. Therefore, pre-curing at room temperature for at least 6 h has the potential to avoid frost damage. Concrete containing 25 wt% AF combined with 12 h and 24 h of curing at 20 °C after removal from freezing and prior to testing could enhance the compressive strengths of all concretes. The renewed hydration was indicated as the main influencing factor.