Strength characteristics, microstructure, and carbon footprint assessment of backfill materials based on multi-source...

The stockpiling of multi-source coal-based solid waste (MCSW) - including coal gangue (CG), fly ash (FA), bottom slag (BS), desulfurization gypsum (DG), and gasification slag (GS) - poses significant environmental risks. Backfilling goafs with MCSW backfill materials improves rock stability, controls surface subsidence, and promotes solid waste recycling and carbon reduction. In this study, a green backfill material was prepared using MCSW, and its strength characteristics, sensitivity to strength variation, microstructure, synergistic reaction mechanisms, and carbon footprint were systematically investigated through multiple analytical methods. Results show that CG particle size primarily influences early unconfined compressive strength (UCS), whereas the proportions of solid wastes and the slurry mass concentration predominantly determine later-stage strength. Under the optimal mixing ratio range (FA/CG = 40-60 %, BS/CG = 20-40 %, DG/CG = 10-20 %, GS/CG = 10-20 %) and a mass concentration of 76-78 %, the 28d UCS exceeds 5 MPa. Under alkaline conditions, the glassy phases of MCSW materials dissolve, releasing reactive Si⁴⁺ and Al³⁺, which react with Ca(OH)₂ to generate C-S(A)-H gels, while Ca²⁺ from DG and BS further promotes AFt formation. However, excessive DG and GS reduce the reactivity of the cementitious system. Life cycle assessment (LCA) indicates that raw material extraction dominates carbon emissions. The lowest-emission MCSW backfill (FA60) reduces emissions by 43 % compared with conventional materials. Overall, MCSW-based backfill materials meet the mechanical strength requirements for goaf filling while reducing carbon emissions and solid waste accumulation, demonstrating strong potential for sustainable and green mining applications.