CO2-Driven Structural Transformation of Steel Slag into Low-Carbon Cementitious Materials for Mine Backfill: Performance Enhancement and Mechanistic Insights

Mine backfilling is a critical geotechnical operation for underground stability and waste manage-ment, while its environmental performance is increasingly constrained by the high carbon footprint of cementitious binders, particularly ordinary Portland cement (OPC). This study investigates the use of CO2-mineralized steel slag as a reactive supplementary binder for mine backfill to develop a car-bon-reducing mine backfill (CRMB), while simultaneously enhancing mechanical performance and reducing carbon emissions. The results show that moderate carbonation (~50% carbonation degree) significantly improves backfill performance, with CRMB2 achieving a 24.5% increase in 28 d com-pressive strength compared with the uncarbonated system and slightly outperforming OPC under iden-tical conditions. Mechanistic analyses demonstrate that CO2 mineralization induces the formation of highly reactive CaCO3 and silica gels, which reprogram hydration pathways by accelerating silicate and aluminate reactions. Furthermore, the availability of carbonate species promotes the formation of stable carboaluminate phases, contributing to sustained strength development at later ages. Nanoinden-tation also confirms that both low- and high-density C–S–H phases exhibit enhanced stiffness at mod-erate carbonation levels. From an environmental perspective, CRMB reduces embodied CO2 emissions by up to ~60% relative to OPC, resulting in superior strength-to-emission efficiency and demonstrating its effectiveness as a carbon-sequestering supplementary binder for mine backfill.