A Dry-Wet Cyclic Carbonation Scheme for Steel Slag Application and Study on Its Multifactorial Regulation Mechanism

• A novel dry-wet cyclic carbonation strategy achieves up to 114.8% enhancement in CO₂ uptake efficiency under ambient conditions, offering a highly practical and scalable solution for direct integration with existing steel slag stockpiles. • The process fundamentally addresses the volume instability of steel slag by efficiently promoting the conversion of expansive free CaO and MgO into stable calcite, as confirmed by a significant reduction in autoclaved powdering rate to below the 5.90% threshold. • The carbonated product demonstrates superior performance as a construction aggregate, maintaining a Class I crushing index while exhibiting excellent volume stability, with its advantages in low water consumption and cost-effectiveness clearly established against traditional carbonation methods. This study investigates a technical pathway for steel slag carbonation by proposing a dry-wet cycling strategy. Two types of basic oxygen furnace slags (BOFS) and their graded products were selected for systematic carbonation experiments conducted in a precision environmental control reaction system. A stepwise moisture intervention sequence, involving static pre-wetting and periodic dynamic water supplementation, was designed. The CO₂ uptake and phase transformation were quantitatively-qualitatively characterized using thermogravimetric analysis (TG) and X-ray diffraction (XRD). The regulatory mechanism of the dry-wet cycles on carbonation efficiency and product performance was elucidated by analyzing the crushing index, autoclave powdering rate, and micromorphology via scanning electron microscopy (SEM). Results demonstrated that the dry-wet cycling strategy significantly enhanced carbon sequestration efficiency through dynamic moisture regulation. Particle size and packing position significantly influenced carbonation uniformity. XRD confirmed that dynamic water supplementation promoted the conversion of calcium-bearing phases into calcite. The carbonated slag's crushing index met the Class I sand standard, its autoclave powdering rate was reduced below the specified limit, and its volume stability complied with standard requirements, confirming its suitability as an aggregate for mortar and concrete. SEM revealed that moisture conditions directly regulate carbonate crystal morphology and pore structure evolution, forming a dense network. By establishing a multi-scale interaction model of "moisture-particle size-structure-performance," this study provides a theoretical basis and technical pathway for optimizing industrial steel slag carbonation processes and its resource utilization as a construction material, particularly suitable for low-cost in-situ treatment at existing stockpiles.