Mechanism for rapid hardening of cement pastes under coupled CO₂-water curing regime

A coupled CO₂-water curing regime was employed on Ordinary Portland cement (OPC) paste samples immediately after casting, which allowed carbonation and hydration of OPC to proceed simultaneously. The strength development and microstructural evolution was evaluated by using multiple-techniques. The results indicated that, compared to the normal hydrated counterpart, a lower porosity, higher amorphous phase content and overall reaction degree can be achieved in the coupled CO₂-water cured OPC sample. By combining with the morphological observations, a new mechanism was proposed for the rapid hardening of OPC. It is shown that the carbonation reactions led to the formation of calcite particles, which provided more nucleating sites for C–S–H gel growth; and thus, an increase in the overall reaction degree of the cement paste can be achieved within the first 24 h compared to the conventional water curing process.