Hydro-Mechanical Degradation and Progressive Failure of Sandstone–CGFB Composite Structures Under Cyclic Water Immersion: Implications for Water-Induced Instability Risk in Backfilled Goafs

Sandstone and cemented coal gangue-fly ash backfill (CGFB) form composite load-bearing structures in goafs, where repeated water ingress and drainage may weaken stability. This study investigated sandstone–CGFB composite specimens subjected to 0, 3, 6, and 10 water immersion–drying cycles using uniaxial compression, energy analysis, acoustic emission (AE) monitoring, scanning electron microscopy (SEM), and digital image analysis. The results show that increasing the cycle number prolonged initial compaction, reduced the pre-peak slope and peak stress, and promoted gradual post-peak deformation. From 0 to 10 cycles, the mean uniaxial compressive strength decreased from 4.78 to 1.70 MPa, while the 50% secant modulus E50 decreased from 579.63 to 125.08 MPa, representing reductions of 64.44% and 78.42%, respectively. Meanwhile, the peak strain increased from 0.0098 to 0.0146. The peak total input energy decreased from 23.93 to 13.01 kJ·m−3, and dissipated energy accounted for 97.9% of the total input energy at test termination after 10 cycles. AE counts and energy shifted from concentrated release near and after the peak to earlier, more dispersed activation. Tensile-type AE events remained dominant, reaching 95.1% at 6 cycles, whereas the shear-type proportion increased to 21.2% at 10 cycles. The dynamic b-value showed stage-dependent rather than monotonic variation, reflecting a transition from fracture localization near the peak to early defect activation and subsequent small-scale damage. In the selected post-failure CGFB-side SEM fields, the apparent fracture-surface defect fraction increased from 21.23% to 35.89%. Together with the mechanical and AE results, the fracture-surface observations are consistent with a progressive weakening of the cemented structure and a shift toward earlier, more dispersed damage during loading. These laboratory findings provide indicators for assessing the water-related deterioration of sandstone–CGFB composite structures in backfilled goafs.