Hydration and strength of cement paste containing metastable vaterite derived from recycled concrete fines and CO2

Alkaline solid wastes, such as recycled concrete fines, can integrate with CO₂ to produce high-purity vaterite CaCO₃ via leaching‑carbonation mineralization. This study aims to examine the stability and nucleation effects of the metastable vaterite relative to stable conventional calcite in cement paste systems. To better elucidate the underlying mechanisms, their influence in the pure C₃S/C₃A phase systems was also comparatively studied. The results indicate that vaterite accelerates the hydration rate of C₃S and C₃A phases, resulting in the formation of both crystalline and amorphous calcium silicate hydrates, as well as carbonate-Am phases, within the first 12 h of hydration. The spherical morphology of vaterite facilitates a more uniform distribution of hydration products, leading to particle encapsulation and reduced nucleation effects at later ages (> 7 days). Overall, vaterite blended cement pastes exhibit 40% higher early strength at 7 days and comparable long-term mechanical properties to those made with calcite.