Additive-free direct CO₂ mineralization of carbide slag: Process behavior, reaction mechanism and product characteristics

The direct mineralization of CO₂ using carbide slag offers a promising route for simultaneously achieving carbon sequestration and solid-waste valorization. In this study, an additive-free direct mineralization process was developed to convert carbide slag into CaCO₃, and the effects of CO₂ flow rate, reaction temperature, and liquid-to-solid (L/S) ratio on mineralization performance were systematically investigated. Under the optimal conditions of an L/S ratio of 10:1, a reaction temperature of 25 °C, and a CO₂ flow rate of 400 mL/min, the Ca(OH)₂ conversion reached 90% under near-ambient conditions. Analysis of the coupled solid–liquid–gas system revealed that the mineralization rate was primarily governed by the reactive solid dosage and CO₂ supply, whereas the liquid phase exerted a complex influence. The mineralization proceeded continuously from the onset of CO₂ injection, while the pH remained relatively stable during most of the reaction owing to continuous Ca(OH)₂ dissolution and decreased sharply only as the reactive solid was nearly depleted. XRD analysis confirmed the formation of calcite. The resulting CaCO₃ consisted of nanoscale crystallites, whereas the aggregated powder exhibited an average particle size of 3.568 μm. The product shows considerable application potential in large-volume markets such as construction materials, rubber products, inks, and asphalt, where high whiteness is not a critical requirement.