Hydration–carbonation coupling in wollastonite-modified belite-rich cement: CO2 uptake, matrix densification, and performance evolution

Wollastonite, a naturally occurring calcium silicate mineral, is considered a promising additive for enhancing CO 2 sequestration and improving the densification of cementitious matrices owing to its carbonation reactivity and nucleation effect. However, the role of wollastonite in the carbonation curing of belite-rich cement remains insufficiently understood. This study investigates the effects of varying wollastonite replacement levels on the carbonation curing behavior of belite-rich cement, with particular attention to hydration-carbonation coupling, CO 2 uptake, mechanical performance, and microstructural evolution under different curing conditions. In addition, the residual performance of specimens under both curing regimes after exposure to elevated temperatures is evaluated. The results indicate that wollastonite accelerated the early and intermediate stages of the carbonation process and promoted the formation of CaCO 3 and silica-rich products. The direct carbonation of wollastonite enhanced the CO 2 uptake capacity of the composite cementitious material; at a wollastonite replacement level of 30%, the CO 2 uptake reached 17.03%. At the same curing age and WOL content, carbonation-cured specimens generally exhibited higher electrical resistivity and ultrasonic pulse velocity than normally cured specimens, consistent with improved continuity of the solid matrix. After 28 days of curing, the compressive strength of BRC15WOL-C was 95.5% higher than that of the corresponding BRC15WOL-NC specimen. Upon high-temperature exposure, wollastonite incorporation reduced visible surface cracking, particularly at 30% replacement. For most specimens, the residual compressive strength increased at 300 ℃ but decreased significantly at 600 ℃ and 900 ℃; meanwhile, the ultrasonic pulse velocity (UPV) values essentially all decreased with increasing temperature. The deterioration was mainly associated with C-S-H dehydration and structural degradation, Ca(OH) 2 dehydroxylation, and carbonate decomposition. Overall, this study investigated the hydration-carbonation synergistic behavior between wollastonite and belite-rich cement and its high-temperature performance, providing a potential approach for developing low-carbon cement-based materials with enhanced CO 2 uptake capacity and balanced mechanical and high-temperature performance.