The low hydration reactivity of steel slag （SS） significantly limits its application as supplementary cementitious materials (SCMs). In this study, SS was converted into carbonated steel slag (CS) with high activity by carbonization, and the effect of CS on the hydration process and mechanical properties of cement composites was systematically studied. The results show that CS markedly accelertes the cement hydration reaction, enhances the setting rate, and substantially enhances the compressive strength of cement composites. When the CS content is 20% (mass fraction), the compressive strength of the cement composite increases by approximately 0.9% and 14.4% compared with the reference sample at 1 and 28 d, respectively. The enhancement in hydration process and mechanical properties of cement composites with CS is primarily attributed to the highly reactive silica gels and calcium carbonate, which accelerate the formation of hydration products. In addition, the ultrafine particles in CS play a filling effect and enhance the compactness of the microstructure, thus effectively improving the compressive strength.