Carbonation of ettringite and monosulfate: Product evolution, microstructure, and comparison

Abstract

Carbon capture, utilization, and storage (CCUS) using cement-based materials offers significant potential for CO₂ sequestration. Integrating CCUS with calcium sulfoaluminate (CSA) cement, a promising low-carbon alternative, presents distinct advantages. However, the understanding of the carbonation mechanisms for ettringite (AFt) and monosulfate (AFm), the primary hydration products in CSA cement, remains inadequate. In this study, the carbonation processes of pure AFt and AFm minerals were systematically investigated, with a comparative analysis of their carbonation products. Both phases exhibited decreasing pH, size, and content, alongside increasing total pore volume over carbonation time. However, distinct carbonation mechanisms were observed. The carbonation of AFt proceeded rapidly, forming well-crystalline calcite and abundant plate-like gypsum, with a uniform pore volume distribution. In contrast, AFm carbonation progressed more slowly, forming larger quantities of CaCO₃, primarily as vaterite and amorphous calcium carbonate. Gypsum was formed as a secondary, later-stage product with prismatic morphology during AFm carbonation. Crucially, XRD, TG, FTIR and Raman analyses revealed that no crystalline or microcrystalline aluminum hydroxide (AH₃) was formed. The AH₃ with an amorphous nature was confirmed by TEM and ²⁷Al NMR characterizations, with both its content and disorder degree increasing progressively during carbonation. These findings illuminate the different carbonation behaviors of AFt and AFm and the microstructure of carbonation-derived AH₃, providing fundamental insights for advancing CCUS implementation in CSA cement systems.