New insights into the role of early accelerated carbonation on the calcium leaching behavior of cement paste

Carbonation is generally considered beneficial for the calcium leaching resistance of cement-based materials. However, many studies have demonstrated that early accelerated carbonation has detrimental effects on the calcium leaching resistance. In this study, the calcium leaching behavior of an early-carbonated cement paste was investigated in detail. The results showed that 8–16 h of carbonation mitigated the reduction in compressive strength caused by calcium leaching; however, further prolongation of carbonation caused an increased decline in strength at the later leaching stage. In contrast to the calcium leaching results for the powdered samples, the decalcification level of calcium silicate hydrate during the leaching test for the cubic specimens was similar to or even higher than that of the early-carbonated specimens. CaCO3 and silica gel played key roles in these phenomena; the former provided ideal hardness and calcium sequestration, with the potential to generate protective films, while the latter acted as an ideal filler and binder. Nonetheless, considering the risk of rapid degradation during long-term calcium leaching, excessive carbonation curing of cement-based materials should still be avoided in aqueous environments.