Mineral carbonation of industrial byproducts is a promising carbon capture and storage technique to abate global warming. Steelmaking slag is the main byproduct of the steelmaking industry and it is a potential source of alkaline oxides, which can be transformed into carbonates. The carbonation of this steelmaking slag has proven to be a great countermeasure to sequester significant amounts of CO2 emitted from the steelmaking process at the point sources. In this study, a supercritical carbonation process is developed to sequester CO2 using steelmaking slag. A response surface methodology is utilized to assess the effect of operating parameters on carbonation efficiency and to optimize the process. Under the optimum conditions, the maximum CO2 uptake of 213 gCO2/kgSlag is achieved. Fundamental investigations are performed to elucidate the carbonation mechanisms with emphasis on the diffusion barrier, rate-determining step, and reaction pathway.