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.

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.

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.

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The acoustic emission (AE) monitoring technique, scanning electron microscope (SEM) system and uniaxial compression test were carried out to study the effect of pyrite-bearing filling joints on the mechanical properties of coal samples. The results show that the pyrite-bearing filling joints have adverse effect on strength, elastic modulus and peak strain of coal samples. Compared with the relatively complete samples, the post peak failure was ductile of samples with pyrite-bearing filling joints, the stress-strain curve was ladder-shaped after the peak and the AE energy of each drop appeared a peak. Many inclined and slender filling joints on the coal surface had a significant effect on the mechanical properties of the samples. Under the action of axial compression stress, the internal cracks of coal-rock with pyrite-bearing filling joints had enough time to expand and connect, and induced failure of surrounding structures, thus reducing the strengths of the samples. When fine pyrite-bearing filling joints on the coal surface, the distribution of macro-cracks were less, and SEM image showed that the fracture surface was relatively flat. While many inclined and slender filling joints on the coal surface, the sample failure accompanied by more macro-cracks, and SEM image showed that there were more holes and cracks on the fracture surface.