Experimental investigation on CO₂-carbonated fly ash backfill: enhancing underground mining stability and carbon sequestration

Coal remains a critical energy resource in China, driving rapid urbanization and industrial growth. However, extensive underground mining operations have resulted in severe environmental and structural challenges, including ground subsidence, coal-based solid waste generation, and significant CO₂ emissions. This study explores the potential of CO₂-carbonated fly ash-based backfill materials (CFBM) as a sustainable solution that integrates waste management and carbon sequestration while enhancing underground mining stability. A series of laboratory experiments evaluated the mechanical performance, microstructural evolution, and transport properties of backfill materials with varying coal fly ash (CFA) content (50 %, 60 %, 70 %, 80 %, 90 %). Results demonstrated that CO₂ carbonation significantly improved compressive strength, with CFA-rich samples exhibiting up to a 20 % increase and a CO₂ uptake capacity of 1.39 mg/g. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses confirmed the formation of calcium carbonate (CaCO₃) and calcium-silicate-hydrate (C-S-H) gel, which densified the material by filling voids and reducing porosity. Rheological analysis showed that the CFBM exhibited shear-thinning behavior, aligning with the Herschel-Bulkley model, making it suitable for pipeline transport. Numerical simulations using Flac3D validated the effectiveness of CFBM in controlling surface subsidence and improving rock strata stability. At a 60 % backfill rate, surface subsidence was reduced by 63.7 % compared to conventional caving methods. The findings highlight CO₂-carbonated backfill as a viable approach for sustainable mining, offering enhanced stability, reduced environmental impact, and contributing to the broader adoption of carbon sequestration technologies in the mining industry.