In this investigation, six groups of cemented coal gangue-fly ash backfill (CGFB) samples with varying amounts of kaolin (0, 10, 20, 30, 40, and 50%) instead of cement are prepared, and their mechanical properties are analyzed using uniaxial compression, acoustic emission, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The uniaxial compressive strength, peak strain, and elastic modulus of CGFB samples decreased with the kaolin content. The average uniaxial compressive strength, elastic modulus, and peak strain of CGFB samples with 10% amount of kaolin are close to that of CGFB samples with no kaolin. The contribution of kaolin hydration to the strength of CGFB sample is lower than that of cement hydration, and the hydration products such as ettringite and calcium-silicate-hydrate gel decrease, thereby reducing strength, which mainly plays a role in filling pores. The contents of kaolin affect the failure characteristics of CGFB samples, which show tensile failure accompanied by local shear failure, and the failure degree increases with the kaolin content. The porosity of the fracture surface shows a decreasing trend as a whole. When the amount of kaolin instead of cement is 10%, the mechanical properties of CGFB samples are slightly different from those of CGFB samples without kaolin, and CGFB can meet the demand of filling strength. The research results provide a theoretical basis for the application of kaolin admixture in fill mining.

Based on the acoustic emission (AE) system and the digital scattered-spot deformation monitoring system, uniaxial compression tests were conducted on composite samples consisting of sandstone and cemented coal gangue–fly ash backfill (CGFB) to investigate their deformation and failure characteristics. The results showed that the average uniaxial compressive strength of the composite samples was 83.09% higher than that of the pure CGFB samples and 92.28% lower than that of the pure sandstone samples. In the composite samples, damage occurred in the CGFB part, and they showed obvious plastic damage characteristics. On both sides of the intersection, the sandstone and the CGFB deformed synergistically in the absence of a macroscopic failure. After a macroscopic failure, the interface effect promoted sandstone deformation and restrained CGFB deformation, transforming the sandstone and the CGFB on both sides of the intersection into a nonsynergistically deformed state. The interface effect had the most obvious influence on the horizontal deformation of the sandstone and CGFB monitoring points near the intersection. The failure of the CGFB samples induced sandstone springback deformation with a springback capacity of 0.0089 mm in the vertical direction and 0.0055 mm in the horizontal direction, which led to the further rupture and failure of the CGFB.

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.