High-Efficiency Carbon Sequestration and Cement Replacement through Direct and Indirect Carbonation of Magnesium Slag–Fly Ash Blends: Process Optimization and Mechanistic Insights

The large-scale generation of industrial by-products, particularly magnesium slag and fly ash, presents significant environmental challenges but offers opportunities for carbon dioxide ( CO 2 ) mitigation through mineral carbonation. Although these materials demonstrate considerable potential, the optimal conditions and catalytic mechanisms governing their carbonation are not yet fully understood. This study systematically investigated both direct wet and indirect carbonation processes for magnesium slag–fly ash mixtures, with the aim of optimizing CO 2 sequestration and assessing their suitability as supplementary cementitious materials. Direct wet carbonation achieved a maximum carbon sequestration efficiency of 91.25% under optimized conditions: particle size less than 0.075 mm, a magnesium slag–fly ash ratio of 10 ∶ 1 , temperature of 25°C, reaction time of 60 min, and a liquid–solid ratio of 6 ∶ 1 . This process promoted the formation of stable carbonates, such as CaCO 3 and MgCO 3 , resulting in a denser particle microstructure. In indirect carbonation, CH 3 COONH 4 was identified as the most effective single catalyst, yielding an efficiency of 87.11%. The combined use of ( NH 4 ) 2 SO 4 and CH 3 COONH 4 increased the efficiency to 92.73%. Incorporation of the carbonated products into cement mortar improved paste fluidity and enabled a cement substitution rate of up to 44.44% without compromising mechanical strength. Considering economic feasibility and catalyst recyclability, water mediated carbonation is recommended for practical application. These findings offer an innovative and efficient strategy for valorizing industrial waste, supporting sustainable waste management, and advancing CO 2 reduction strategies.