<p>The effect of steelmaking slag composition on mineral phases and aqueous carbonation kinetics was investigated using synthetic basic oxygen furnace (BOF) slags with controlled basicity (CaO/SiO₂ = 3–5) and FeO–P₂O₅ reduction ratios (0–100%). Powder X-ray diffraction revealed that higher basicity and reduction ratios promoted the formation of Ca-rich phases, such as C₃S and CaO, while suppressing C₂S–C₃P-type solid solution and ferrite phases. Direct aqueous carbonation followed by thermogravimetric analysis (TGA) was conducted to evaluate the carbonation efficiency of the synthetic BOF slag samples. The maximum carbonation efficiency increased with the reduction ratio in the case of basicity-3 samples, associated with the phase transitions from both the phosphate and ferrite phases to more reactive Ca-rich phases. Further enhancement in CO₂ uptake was observed at higher basicity (e.g., basicity-5). Among the investigated samples, the fully reduced slag (100% reduction) exhibited the fastest initial carbonation rate and the highest final carbonation efficiency at a basicity of 5. Kinetic analysis with a surface-coverage model showed that the apparent rate constant increased from 0.003 to 0.246&#xa0;mol&#xa0;m⁻²&#xa0;min⁻¹, and the carbonation efficiency reached up to 42.4%, indicating a strong dependence on both basicity and reduction ratio. Unlike previous studies that primarily focused on process conditions, this study systematically isolates the role of slag composition in controlling phase assemblage and carbonation behavior. The findings clarify how slag composition governs carbonation efficiency, providing a basis for designing BOF slags with enhanced performance.</p> Graphical Abstract <p></p>