<p>In line with the dual-carbon objectives, the study promotes high-value applications of steel slag. Steel-slag powder was carbonated by a semi-dry route and used to produce concrete at a fixed reference mix. Mixtures with uncarbonated powder and with powders at different carbonation ages were prepared. The carbonation characteristics of the powder and their effects on workability, compressive strength, chloride ingress, and freeze–thaw durability were evaluated. Results show that a dense microstructure develops as carbonation proceeds, and a good linear correlation exists between carbonation mass gain and CO₂ uptake. Incorporating carbonated steel slag reduces the flowability of fresh concrete but markedly improves mechanical properties as well as resistance to chloride ingress and freeze–thaw damage. At 7&#xa0;days, the compressive strength of C-7d is slightly lower than C-1.5d and C-3.5d; at 28&#xa0;days, compressive strength increases with carbonation age. Carbonation of the slag significantly lowers the chloride migration coefficient and enhances impermeability. After 105 freeze–thaw cycles, specimens with carbonated slag show lower mass-loss and strength-loss rates and smaller changes in ultrasonic pulse velocity than the uncarbonated group, indicating superior freeze–thaw resistance. Microstructural analysis indicates that carbonation suppresses pore connectivity and crack propagation, thereby improving resistance to chloride transport and freeze–thaw damage. Consequently, the longer-age group (C-7d) is more suitable for projects requiring high long-term durability, whereas shorter-age groups (C-1.5d and C-3.5d) combine early strength benefits with resource-saving potential.</p>