Coal gangue-based geopolymer (CGGP) is one of the hot spots existing in the recycling of coal gangue resources due to its good comprehensive mechanical properties. However, the coal gangue structure is stable and reactivity is poor, so the coal gangue needs to be activated before utilization. In this paper, the microstructure changes of activated coal gangue by different mechanical and thermal activation methods, as well as the mechanical properties and microstructure changes of the CGGP specimens were studied by experimental investigation. The results indicated that mechanical activation and thermal activation were two effective methods to change the reactivity of coal gangue, which consisted of destroying the stable kaolinite structure and improving the activity of coal gangue. Conversely, part of the amorphous structure in coal gangue was destroyed when the activation temperature reached 900 °C, which was not conducive to the further enhancement of coal gangue activity. For the CGGP prepared by thermally activated coal gangue and modified sodium silicate alkali solution, the uniaxial compressive strength of the CGGP specimens decreased with thermal activation temperatures of the raw coal gangue materials at 700 °C, 800 °C, and 900 °C. The main reason for this was the lower amount of the active metakaolin structure in coal gangue at 900 °C, which was not conducive to the geopolymerization process.

Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.

Induced by coal mining, the fractures constantly occur in geologic strata until failure occurs, which provide channels for water flow. Therefore, it is essential to investigate the permeability evolution of rocks under load. Borehole sampling was conducted in a bedrock layer beneath an aquifer, and the permeability evolution of sandstone specimens under different confining pressures was tested in rock mechanics testing laboratories. The results indicated that the permeability gradually decreases with the increasing confining pressures, while the peak strength increases with the increase of confining pressures. The minimum and maximum permeabilities occurred in the sandstone specimens that were subjected to elastic deformation and strain-softening stages, respectively. The failure, and maximum permeability, of these sandstone specimens did not occur simultaneously. To prevent the flow channel being formed due to the development and failure of rock fractures, a method of backfill gob was proposed and also the influence of backfill on fracture development was discussed.

The wettability of room temperature ionic liquids (ILs) on coal structure was studied by measuring contact angle, water-retaining property, and FTIR spectra. The eight ILs utilized were [EMIm][BF₄], [EMIm][NTf₂], [PMIm][BF₄], [BMIm][BF₄], [HOEtMIm][BF₄], [HOEtMIm][NTf₂], [P_4,4,4,1][MeSO₄] and [P_4,4,4,2][DEP]. The contact angle results showed that compared with distilled water, the contact angles of ILs decreased, indicating good wetting effect. However, different ILs showed different wetting effects. Among of them, the [HOEtMIm][NTf₂] had the best wetting effect, the [P_4,4,4,2][DEP] and [P_4,4,4,1][MeSO₄] were in the second place, and the worst one was [HOEtMIm][BF₄]. The water-retaining experiments showed that the evaporation rate of coal containing ILs was slower and the water-retaining time was longer, indicating that the coal containing ILs had stronger resistance to moisture evaporation. The water-retaining property of [EMIm][BF₄] was the best. The FTIR experiments showed that the main structure of coal was not changed but some structures changed by ILs treatment. The significant changes were the aliphatic hydrogen groups (1435 and 1371 cm⁻¹), the C-O bond (1300–1000 cm⁻¹) and the mineral structures (1032 and 472 cm⁻¹). This study will provide a new development platform for the ILs used for coal dust prevention.