The deep mining of coal resources can lead to surface subsidence, thereby damaging the stability of buildings on the surface; however, this issue can be effectively mitigated through goaf backfilling with coal gangue and other solid wastes. In this study, the technical principles of goaf solid waste backfill mining are illustrated, and the surface movement and deformation patterns under different burial depths and compression rates are examined. Additionally, the monitoring and analysis of the roof subsidence and surface building conditions after goaf solid waste backfill mining are described. The results show that as the burial depth and compression rate increase, the maximum values of the surface subsidence and horizontal deformation gradually decrease. When the burial depth is between 800 and 1200 m, the design value of the compression rate for the goaf should be higher than 79%. The actual measurement results show that the maximum subsidence value of the roof after goaf backfilling is 635 mm, and the compression rate is about 80.8%, which is consistent with the theoretical design value. Hence, surface buildings are little affected by mining activities and are within the safe range for use.

When coal gasification slag is used as a substitute for cement, the prepared cementitious materials may exhibit inadequate properties due to the slag’s limited hydration reactivity, which limits its effectiveness in applications of backfill materials. In this study, alkali activation was used to improve the hydration activity of coal gasification slag. The effect of alkali equivalent on the setting time, rheological properties, and uniaxial compressive strength of the alkali-activated coal gasification slag-based backfill material (ACBM) sample was systematically investigated, and the optimal alkali equivalent was identified. The mineral composition, pore structure, and micromorphology of ACBM samples were characterized using the X-ray diffractometer (XRD), nitrogen adsorption–desorption analyzer (BET), and scanning electron microscope–energy dispersion spectrum (SEM-EDS). The results show that when the alkali equivalent is 4%, the comprehensive performance of ACBM samples is optimal. At this time, the initial setting time and final setting time of ACBM samples are 125 min and 172 min, and the rheological properties are in accordance with the Herschel–Bulkley model. The yield stress, plastic viscosity, and hysteresis loop area are 9.22 Pa, 0.74 Pa·s, and 1014 Pa/s, respectively, and the compressive strength of the ACBM sample at the curing age of 28 days is 2.18 MPa. When the alkali equivalent is further increased to 6%, the initial hydration reaction becomes more intense due to the excessive alkali level, leading to a rapid decline in flowability; the sample cracked at 28 days and its strength decreased considerably. This study provides theoretical guidance for the application of coal gasification slag in the field of backfill mining.

The mechanical durability of gangue-based roadway shotcrete material (GRSM) in aqueous environments was systematically investigated by evaluating the effects of immersion duration, dry–wet cycles, and pH variations on its uniaxial compressive strength (UCS). The results indicate that prolonged immersion significantly degrades the mechanical performance of GRSM. After 28 days of immersion, the UCS decreased by 8.68 MPa (22%) compared with specimens under standard curing conditions. In contrast, limited dry–wet cycling (up to two cycles) enhanced the UCS to 36.05 MPa by promoting continued hydration and pore refinement, whereas additional cycling led to progressive deterioration. GRSM exhibited pronounced pH sensitivity: acidic environments induced the most severe strength loss, followed by alkaline conditions, whereas neutral to weakly alkaline environments (pH 8–12) resulted in relatively stable mechanical performance. Mercury intrusion porosimetry (MIP) confirmed that pore structure evolution governed strength variation, with acidic exposure and dry–wet cycles producing the greatest increases in porosity. Mechanically activated gangue (MA-gangue) was prepared by ball milling and partially substituted for cement. Although MA-GRSM exhibited lower UCS than conventional GRSM under all conditions, both materials demonstrated similar environmental response patterns. These findings elucidate the coupled physicochemical mechanisms governing the durability of gangue-based shotcrete materials in underground water-bearing environments.

In the coal pillar dam of underground water reservoirs, groundwater exerts a certain degree of dissolution and erosion on the coal body, inducing the development of internal cracks and the deterioration of its mechanical properties. To this end, coal samples with varying moisture contents were prepared through a water-absorption experiment; the changes in the mechanical strength of coal samples with five moisture contents (0%, 3.62%, 4.93%, 5.52%, and 6.11%) were tested via uniaxial compression tests, uniaxial tension tests, and variable-angle shear tests; and the degradation in mechanical performance in water-immersed coal samples and their sensitivity to moisture content were evaluated. The experiment yielded the following results: (1) The moisture content of coal samples increases with the increase in immersion time, and the water-absorption rate first rises, then decelerates and gradually becomes stable. When the immersion time is about 72 h, the coal sample reaches a saturated state. (2) As the samples transition from a dried state to full saturation, the uniaxial compressive strength of coal samples decreases from 29.17 MPa to 7.38 MPa, and the uniaxial tensile strength decreases from 0.78 MPa to 0.33 MPa. The peak shear strength also decreases with an increase in immersion time and the increase in shear angle, while the deterioration degree gradually increases with the increase in immersion time and tends to be stable. (3) Based on a sensitivity analysis, the mechanical performance evolution of water-immersed coal samples can be divided into four stages based on the moisture content: tensile-dominated stage, shear-dominated stage, compression catching-up stage, and compression-dominated stage.

In traditional gangue backfilling mining, the use of gangue as backfill material often leads to issues such as supply–demand imbalance, low crushing efficiency, and insufficient early strength. To address these challenges, a novel gangue-cemented high-porosity backfill material (GCBM) was developed, using coal gangue (CG, 15–20 mm) as aggregates bonded with an alkali-activated modified coal gangue powder and slag-based fast-setting cementitious material (FCM). The effects of alkali equivalent on the workability and mechanical properties of FCM were investigated to determine the optimal composition. The influence of FCM proportion on the porosity and strength of GCBM was analyzed, and a strength–porosity correlation model was established. The microstructure and bonding mechanism of the interfacial transition zone (ITZ) between FCM-CG aggregates were revealed through multiple microscopic techniques. Results indicate an optimal alkali equivalent of FCM is 6%. The FCM proportion is positively correlated with the compressive and flexural strengths of GCBM, with a critical proportion of 24% at which flexural strength increased by 172% compared with 22%, reflecting enhanced interfacial bonding. Microscopic observations further show that increasing the FCM proportion optimizes the ITZ structure and promotes secondary hydration products, thereby improving interfacial bonding density and the overall mechanical performance of the backfill material.