Regimes in wet wollastonite carbonation: A comparative kinetics and multi-technique investigation

Calcium silicate minerals, including wollastonite, are considered promising feedstocks for mineral carbonation, particularly for applications in low-carbon building and construction materials; however, the mechanisms governing their behavior under mild wet carbonation conditions are not yet fully understood. While previous studies have primarily focused on enhancing carbonation through elevated temperatures, pressures, or chemical additives, the direct influence of operational conditions on reaction pathways and product evolution under additive-free mild conditions remains insufficiently explored. Here, two representative carbonated samples, exhibiting low (approximately 6%) and high (approximately 50%) carbonation efficiencies, were selected from a systematic screening for detailed analysis. These samples exemplify contrasting regimes of gas-liquid and solid-liquid mass transfer, primarily influenced by CO₂ flow rate and agitation speed. An integrated characterization strategy, incorporating XRF, TGA, XRD, BET, Pycnometry, and SEM-EDX, along with time-resolved kinetic model fitting, was employed. Phase and microstructural analyses reveal limited carbonate formation and sparse, discontinuous precipitates under mass-transfer-limited conditions, whereas extensive calcite precipitation and coalescing intergranular carbonate domains develop when mass transfer is enhanced. Variations in specific surface area, skeletal density, and thermal decomposition behavior correspond to progressive carbonate maturation during the reaction. Kinetic analysis of independent batch experiments at varying carbonation times indicates that both the Avrami model and the surface-reaction-controlled shrinking-core model provide the most suitable descriptions of the conversion data, suggesting that carbonation is mainly associated with surface-related dissolution, precipitation, and carbonate nucleation/growth. The diffusion-controlled shrinking-core model shows weaker agreement across the full reaction range, although transport limitations may become more relevant at later stages as carbonate-silica products develop. These findings provide a coherent mechanistic framework that links operational conditions to the evolution of reaction pathways during wet wollastonite carbonation under mild conditions.