Stabilizing mechanisms of metastable vaterite in cement systems

Calcium carbonate exists in three anhydrous polymorphs: calcite, aragonite, and vaterite. Among these forms, calcite is considered the most thermodynamically stable, followed by metastable aragonite and vaterite. Vaterite, the least stable form, can easily transform to calcite in aqueous solutions through a dissolution–recrystallization process. However, vaterite has been observed to remain stable in cement systems even after a prolonged curing age. This study explored the mechanisms underlying the stabilization of vaterite in cement-based systems. Various factors that can potentially influence the polymorphic transformation of vaterite to calcite were investigated, including water availability, calcite seeding, cement pore solution, and inert fillers. Model systems were designed to decompose the complex cement systems to isolate the dominating mechanisms. Stabilizing mechanisms were proposed and supported with experimental evidence. Results indicated that the growth of cement hydrated phases on the surface of calcium carbonates was the dominating mechanism preventing vaterite from converting to calcite.