Hydration mechanism and early frost resistance of calcium sulfoaluminate cement concrete

This study investigated the hydration mechanism and mechanical properties of ordinary Portland cement (OPC) blended with calcium sulfoaluminate (CSA) cement. Heat evolution, hydration products, pore size distribution, and microstructure were investigated for OPC-CSA blends concrete with different contents of CSA cement. Macroscopic properties, such as internal temperature, dynamic elastic modulus, and compressive strength, are also studied through concrete subjected to early frost conditions. The results show that the OPC-CSA blended cement displayed a higher early strength and exhibited enhanced resistance to the early frost damage compared to OPC. The OPC-CSA blended cement also exhibits a higher hydration rate and a larger amount of heat of hydration than that in the OPC at the early stage. The increased heat of hydration can effectively prolong the hydration duration at sub-zero temperatures. However, incorporating CSA delayed the hydration of C3S at the late stage, thus affecting the development of compressive strength and dynamic elastic modulus. On the other hand, the hardened blended cement exhibited an higher porosity, which was corresponding to the increasing proportion of macropores (diameter over 1000 nm). If concrete directly is suffered from early frost after casting, blended cement with 20% of CSA can effectively reduce strength loss from frost damage by 100% at −5 °C, and that from frost damage by 80% at −15 °C respectively. Furthermore, when the calcium nitrite is incorporated as the antifreeze admixture with OPC-CSA blended concrete, the early stage frost resistance of concrete infrastructures can be significantly improved.