A compounded system of fly ash (FA) and carbide slag (CS) was proposed for CO₂ mineralization using the aqueous approach to prepare supplementary cementitious material. Influence of CS dosage on the morphology, particle size distribution, and chemical phases of the carbonation products were characterized. It is found that the mineralization products (calcite) cover on the surface of FA leading to a remarkable synergistic effect in which the CO₂ uptake is improved by about 50%. Furthermore, FA with calcite attached effectively mitigates the set retardation in OPC/FA blends by about 30% and improved the 3- and 28-day compressive strengths by 37.2% and 24.3% respectively due to combined physical and chemical effects. The results indicate that a high-volume cement replacement can be achieved using the carbonated FA produced by the proposed synergy CO₂ mineralization.