Phase modification of steel slag via direct carbonation for the utilization of construction material

Steel slag, specifically basic oxygen furnace slag (BOFS) generated from the iron and steel industry, is characterized by low resource utilization rates and high environmental risks from stockpiling. The poor volumetric stability of steel slag is a major constraint when used as a construction material. Carbonation treatment, which involves the reaction of CO2 with active phases in steel slag, enables phase transformation and microstructural modification, serving as an effective approach to improve the volumetric stability and construction suitability of steel slag. In this study, thermodynamic analysis, synthesis of phase in steel slag (C3S, C2S, C2F, CaO, and MgO), and carbonation experiments were conducted to investigate the carbonation behavior and transformation mechanisms of phases in steel slag. Carbonated steel slag powder was produced via an aqueous carbonation process and incorporated as a supplementary cementitious material in cement-based systems. The effects of its addition on the workability, mechanical properties, and microstructure of concrete and mortar were evaluated. Carbonation treatment achieves a 38% carbonation ratio and reduces free lime (f‑CaO) content, effectively improving the volumetric stability of steel slag. When used as a supplementary cementitious material, carbonated steel slag (CSS) delivers a 28‑day compressive strength of 50.1 MPa, comparable to plain cement (49.9 MPa) and markedly higher than untreated steel slag (33.5 MPa). This research provides a theoretical foundation and technical reference for the synergistic performance regulation of steel slag via CO2 treatment and promotes its efficient resource utilization.