In Situ Generation of Calcium Carbonate via Wet Carbonation of Sulfoaluminate Cement: Improving the Long-Term Strength

Sulfoaluminate cement (SAC) with a low-calcium mineral composition exhibits significant advantages in low-carbon emission and aligns with the green development paradigm of the construction materials industry. However, this cement suffers from a technical bottleneck of long-term strength retrogression and a lack of supplementary cementitious materials. To address these issues, limestone powder is commonly incorporated as a reactive admixture in the SAC production. Nevertheless, the inherently low reactivity of natural limestone often leads to a suboptimal performance enhancement. Therefore, this study presented a wet carbonation restirring process to in situ introduce newly formed highly active CaCO3 into SAC. Through phase analysis and liquid-phase environment characterization, the carbonation reaction mechanism of this system was revealed. Experiments confirmed that the carbonation activity order of mineral phases is AFm > AFt > C2S > C4AF > C4A3Ṡ. After prehydration treatment converted the hardly carbonated mineral C4A3Ṡ into easily reactive hydration products, the carbonation degree of the system significantly increased by 230%. The gradient carbonation process of AFm and AFt formed a pH buffer system, promoting the formation of polymorphic CaCO3; aluminum gel dehydrates to form the AHx phase during the reaction, and part of the AHx phase reacted with silica gel to generate polymerized silica–alumina (PSA). This process optimized the heat release distribution of cement hydration, achieving sustained and healthy development of long-term strength. The introduced CaCO3 fully participated in the hydration reaction of C4A3Ṡ to form a stable CO3-AFm phase; the displaced SO42– further inhibited the formation of unstable phase CAH10, effectively blocking the long-term strength degradation caused by the AFt and AFm phase transitions in the control group. This study provides a new approach for solving the strength retrogression problem of the SAC and CO2 storage technology.