Water‐coal interactions have gained much recent attention, although few studies focus on the strength of coal under wetting‐drying cycles. This study investigates changes in the microstructure and mechanical strength of coal that are induced by water‐coal interactions, at the macroscale and microscale. Fourteen specimens after continuous wetting and fourteen specimens after wetting‐drying cycles were tested, indicating that peak stress and elastic modulus decrease with increasing continuous wetting time and number of wetting‐drying cycles, while peak strain increases. We analyzed changes in the coal microstructure using a scanning electron microscope (SEM) and the Image Pro Plus 6.0 (IPP 6.0) software. The regression model for cyclic wetting‐drying reveals that porosity and pore circularity are the main correlation indicators associated with uniaxial compressive strength. Therefore, different numbers of wetting‐drying cycles could induce different degrees of damage to coal, which increases progressively through gradual changes to its microstructure. These findings indicate that wetting‐drying cycles have a more significant impact on the stability of coal mass than continuous wetting. Our results are of use in determining the size of coal pillars, and expanding the knowledge base related to the mechanical properties of coal mass, water bursting, water recharge channels, and the stress‐crack‐permeability evolution law in mines.

Underground reservoirs in coal mines, consisting of goafs (By goaf, we mean the space that remains underground after the extraction of valuable minerals), are commonly utilized for mine water storage and drainage, with their primary load-bearing structures being the “roof–coal pillar” systems. Consequently, this structure must endure the repeated immersion behavior resulting from fluctuations in the mine water level, resulting in the risk of geological disasters. This paper analyzes the variation in mechanical properties of sandstone–coal composite samples after repeated immersion cycles through axial loading tests. The results indicate that the water content of the sample exhibits a notable and rapid increase with each successive immersion cycle. This corresponds to a decrease in the stress threshold and modulus parameters of the samples. Moreover, the acoustic emission signals serve as indicators of the softening characteristics of the samples. With the increase in immersion cycles, there is an augmentation in both the frequency and extent of shear cracks. The non-linear failure characteristics of the samples become more pronounced. Consequently, water significantly weakens the cementing material between rock grains. Both sandstone and coal display a decrease in deformation resistance capabilities at a macroscopic level. The constitutive model of the composite sample was improved based on the degradation characteristics of mechanical strength and strain energy parameters, which offers enhanced accuracy in analyzing the degradation process caused by water immersion. This paper offers a crucial theoretical foundation for comprehending the deterioration evolution characteristics of the “roof–coal pillar” bearing structure affected by repeated immersion.

The treatment of flue‐gas desulfurization (FGD) gypsum, a common solid waste material that is often affected by chloride ions, typically occupies large amounts of land, causes resource waste, and results in environmental pollution. An emission reduction process involving the dechlorination of FGD gypsum and reducing its emissions has been proposed to solve these problems. Laboratory experiments were conducted to verify the feasibility of the emission reduction process and study any differences from the conventional process. The test results indicated that the average dechlorination rate of the emission reduction process was 80.4%, which is equivalent to 97.5% of the conventional process. The average dechlorination wastewater volume of the emission reduction process was 0.22 L, which is equivalent to 4.9% of the conventional process. The results show that the emission reduction process can effectively reduce the chlorine content of FGD gypsum and its related wastewater discharge. This novel gypsum dechlorination process effectively reduces the chlorine content of FGD gypsum and related wastewater. In addition, the emission reduction process can lead to greater recovery of renewable resources and can result in a cleaner energy conversion process. This process is an alternative method for recovering residues and resources without secondary pollution. Therefore, if this technology is integrated into existing FGD facilities, costly chemical processes could be replaced by those that recover a renewable resource and incorporate a clean energy conversion process.

Flue-gas desulphurisation gypsum—a solid waste from power plants—can be used to prepare paste backfill for reducing costs. Most paste backfills are exposed to dry–wet cycles and chloride salt-rich water in mines. Therefore, the mechanical properties and damage mechanisms of paste backfill with desulphurised gypsum under the coupling action of erosion due to chloride with different concentrations and dry–wet cycles were investigated using methods such as visual observation, mass measurement, uniaxial compression, acoustic emission, Fourier-transform infrared spectroscopy, X-ray diffraction analysis, and field-emission scanning electron microscopy. With an increasing number of dry–wet cycles, the mass, elastic modulus, and strength of the paste backfill exhibited the trend of increasing first and then decreasing. The failure mechanism changed from mainly vertical fractures to the alternating development of vertical and horizontal fractures. The surface denudation effect of the specimens in a solution with a higher concentration was more severe under the same number of dry–wet cycles. In this study, the laws governing the mass change, strength change, degree of surface denudation, and failure pattern of desulphurised gypsum-filled specimens under different concentrations of chloride salt and different numbers of dry–wet cycles were derived.

Flue gas desulfurization gypsum (FGDG), a solid waste material generated by power plants, offers a cost-effective solution for preparing backfill materials. Concurrently, given that most backfill materials are exposed to mine water erosion, an investigation into the mechanical performance degradation and damage mechanisms of FGDG backfill materials subjected to prolonged chloride salt erosion is crucial for ensuring the stability of goaf backfills (by goaf, we mean the area or space that remains underground after the extraction of valuable minerals). This study explores the mechanical properties and damage mechanisms of backfill samples treated with varying concentrations of chloride. The findings indicate that over time, the mass of the sample initially decreases, experiences a slight increase, and then gradually declines. Simultaneously, the strength and structure of the sample deteriorate rapidly, followed by a partial recovery. The rebound in sample mass is attributed to an incomplete hydration reaction that continuously transforms free water into bound water. Microstructure analysis techniques, such as X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), reveals that the degradation in mechanical strength and structure is a consequence of chloride salt erosion. Subsequent reuse is facilitated by the formation of Friedel's salt resulting from the reaction between hydration products and chloride ions, filling voids in the sample and impeding further chloride ion erosion into the deeper regions. Notably, FGDG exhibits outstanding reuse performance in prolonged and high-concentration chloride ion environments. This study serves as a reference for understanding the deterioration mechanism and practical application of backfills in chloride-rich environments.