The production of cement and concrete using carbonated steel slag as a supplementary cementitious material achieves the dual benefits of efficient steel slag utilization and CO₂ fixation. In this study, a combination of microbial technology and a rotary kiln process was employed to expedite the carbonation of steel slag for CO₂ fixation from cement kiln flue gas. This approach resulted in a significant increase in the CO₂-fixation rate, with a CO₂-fixation ratio of approximately 10% achieved within 1 h and consistent performance across different seasons throughout the year. Investigation revealed that both the CO₂-fixation ratio and the particle fineness are pivotal for increasing the soundness and reactivity of steel slag. When the CO₂-fixation ratio exceeds 8% and the specific surface area is at least 300 m²∙kg⁻¹, the soundness issue of steel slag can be effectively addressed, facilitating the safe utilization of steel slag. Residual microbes present in the carbonated steel slag powder act as nucleating sites, increasing the hydration rate of the silicate phases in Portland cement to form more hydration products. Microbial regulation results in the biogenic calcium carbonate having smaller crystal sizes, which facilitates the formation of monocarboaluminate to increase the strength of hardened cement paste. At the same CO₂-fixation ratio, microbial mineralized steel slag powder exhibits greater hydration activity than carbonated steel slag powder. With a CO₂-fixation ratio of 10% and a specific surface area of 600 m²∙kg⁻¹, replacing 30% of cement clinker with microbial mineralized steel slag powder yields an activity index of 87.7%. This study provides a sustainable solution for reducing carbon emissions and safely and efficiently utilizing steel slag in the construction materials sector, while expanding the application scope of microbial technology.