Wet CO2 Mineralization of Red Mud–Carbide Slag Composites for Alkaline Solid Waste Co-Disposal and Preliminary Utilization of Carbonated Products

Wet CO₂ mineralization of alkaline industrial solid wastes offers a promising route for carbon sequestration and waste utilization. In this study, red mud (RM) and carbide slag (CS) were combined to prepare RMCS composite systems for wet carbonation. The effects of RM/CS ratio, liquid-to-solid ratio, stirring speed, temperature, and CO₂ flow rate on carbonation performance were investigated. Solution chemistry, phase evolution, microstructure, and Si–O structural changes were analyzed using pH monitoring, ICP-OES, XRD, TG–DTG, SEM-EDS, and FT-IR. With the incorporation of 20 wt % RM, the RM20CS80 composite system maintained favorable carbonation performance. Under the selected conditions of a liquid-to-solid ratio of 2 mL/g, a stirring speed of 500 rpm, a temperature of 25 °C, and a CO₂ flow rate of 1.0 L/min, its actual CO₂ uptake reached 57 g CO₂/100 g solid, with an apparent carbonation degree of 93%. Kinetic analysis revealed a two-stage behavior: an early apparent chemical-reaction-controlled stage characterized by rapid portlandite dissolution and CaCO₃ precipitation, followed by a later surface-coverage stage associated with progressive accumulation of carbonate products on particle surfaces. Si/Al-bearing components from RM underwent alkaline-promoted dissolution and Si–O structural evolution during carbonation, suggesting their possible participation in the reaction process. For preliminary utilization, 20% replacement of OPC with carbonated RM20CS80 achieved a 28 d compressive strength of approximately 26.5 MPa, with the leaching concentrations of regulated potentially toxic elements remaining below the corresponding leaching limits, although paste flowability decreased. Overall, this study provides a process-oriented basis for CO₂ sequestration, alkaline solid waste co-disposal, and the subsequent utilization of carbonated RMCS products.