Multiscale properties and molecular fracture mechanism of alkali-activated cemented gangue backfill materials

Replacing conventional coal pillars with cemented gangue backfill material (CGBM) enables co-optimized mine waste reduction at source and goaf stability control. However, its industrial-scale application is constrained by the multi-objective optimization trade-offs involving binder cost, carbon emission intensity, and mechanical performance. This study synthesizes CGBM via in-situ alkali activation of coal gangue. Utilizing a combination of uniaxial compression, acoustic emission (AE), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), Thermogravimetric (TG/DTG), and X-ray diffraction (XRD), the multi-scale regulatory mechanisms of the activator dosage on the strength evolution, micro-defect distribution, and phase composition of the resulting CGBM were elucidated. Molecular dynamics simulations further elucidate fracture propagation and energy dissipation patterns in calcium silicate hydrate (C-S-H) molecular models under tensile loading. The results demonstrate that properly controlling activator dosage significantly enhances the uniaxial compressive strength (UCS) of CGBM and its load-bearing capacity in the dilatancy deformation phase, while concurrently reducing both the AE counts and distribution density during the initial compression and linear elasticity deformation phases; the optimal activator content for maximizing UCS ranges from 12.87% to 16.02%. Regulating activator dosage effectively reduces micro-defect populations (e.g., micropores and microcracks) while promoting secondary hydration of gangue to generate abundant hydration products that repair localized damage. Alkali-induced variation in calcium-to-silica (C/S) ratio governs C-S-H molecular mechanics: an optimal ratio enhances stress transfer via compacted silicate chain layers and stabilizes water-rich layers through calcium-mediated bridging bonds, while homogenizing energy distribution to suppress mesoscale fracture initiation from localized high-potential-energy concentrations.