A stepwise carbonation strategy for steel slag-based concrete: Mechanistic insights into coupled CO₂ sequestration and durability enhancement

The utilization of steel slag as a supplementary cementitious material is limited by its low reactivity and poor volumetric stability. This study proposes a stepwise carbonation strategy, integrating slag pre-carbonation with subsequent carbonation curing, to simultaneously enhance CO₂ sequestration and durability. Results show that stepwise carbonation increases the CO₂ sequestration capacity by up to 147.8 wt%, significantly higher than single-step carbonation (∼66.3 wt%). Although early strength decreases with slag incorporation, satisfactory long-term strength is maintained (∼36–38 MPa at 180 d for 40% slag). Meanwhile, durability is markedly improved, with the chloride migration coefficient reduced by ∼37% and drying shrinkage effectively suppressed. Mechanistically, pre-carbonation introduces initial carbonates and regulates slag reactivity, while carbonation curing promotes in-situ CaCO₃ formation via Ca(OH)₂ consumption, leading to pore filling and matrix densification. However, excessive carbonation may cause carbonate over-accumulation and structural heterogeneity, indicating that optimizing the carbonation regime is crucial. Microstructural analyses reveal a core-shell structure in slag particles and abundant calcite (∼1–5 μm), accompanied by reduced pore volume (∼0.051 cm³/g), increased CaCO₃ (∼12.4 wt%), and enhanced silicate polymerization. Overall, the combination of pre-carbonation and carbonation curing provides a synergistic effect through coupled mechanisms of carbonate formation, pore refinement, and silicate restructuring, offering a promising route for low-carbon and high-durability cementitious materials.