Na2CO3–NaOH Regulation of Steel Slag Carbonation: Interfacial Activation and Kinetic Enhancement

CO2 mineralization of industrial solid waste has emerged as a promising method for both CO2 utilization and sequestration. However, the reaction depth and efficiency of this technology have been limited by the passivating effect of the surface product layer. Using steel slag as a model, this study investigates the interrelated effects of ion leaching, mass transfer, and product-layer development, with emphasis on the formation and evolution of CaCO3 and silica-rich layers that govern reaction progression. Kinetic and experimental results demonstrate that the silica-rich layer inhibits sustained Ca2+ leaching from steel slag in the early carbonation stage, while CaCO3 deposition increases Ca2+ diffusion resistance in the later stage. Molecular dynamics simulations combined with experiments further demonstrate that CO32- concentration regulates CaCO3 nucleation and densification, whereas OH- promotes interfacial activation and dissolution of the silica-rich layer. The coupled regulation of these processes enables deep carbonation of steel slag. Based on these mechanisms, a dual-alkali cycling carbonation strategy using NaOH and Na2CO3 is proposed. Compared with direct carbonation, this strategy enhances the carbonation degree from 41% to 69%, while realizing the co-production of nanoscale SiO2. The dual-alkali cycling system can be integrated with CO2 capture from flue gas or air, offering a scalable pathway for CO2 utilization and high-value conversion of industrial solid waste.