Mineral carbonation with steel slag reduces greenhouse gas emissions and steel slag stockpiles. The mineral carbonation potential of steel slag was investigated by comparing fresh and weathered steel slag. Brownmillerite was discovered to be the most abundant inactive Ca-bearing mineral in steel slag, and the theoretical labile Ca was calculated using this information. The experimental labile Ca was studied using flow-through and batch experiments at ambient and CO₂-rich conditions; the theoretical and experimental labile Ca agree well and are both at around 80%, indicating high mineral carbonation reactivity. The experimental results are also well supported by PHREEQC modeling, charge balance, and mass balance. We discovered that each ton of steel slag can capture approximately 250 kg of CO₂. Furthermore, 450 Mt CO₂ could be directly captured via steel slag CO₂ mineralization. We offer dependable experimental and analytical methods for evaluating the mineral carbonation potential of steel slag.