To understand the effects of water immersion on the spontaneous combustion of low-rank bituminous coal, and the underlying mechanism, the pore size distributions and active functional groups of water-immersed coal samples were studied. Characterization data were acquired by liquid nitrogen adsorption and Fourier transform infrared spectroscopy. The spontaneous combustion behaviors of the experimental coal samples were assessed based on oxidation kinetics obtained using a programmable temperature device. Water immersion promotes pore development and enhances the activity of functional groups, which in turn stimulates spontaneous combustion. A lower metamorphic degree of coal yields a more obvious effect. Water immersion causes existing pores to expand or forms new pores, such that the specific surface area and total pore volume of the coal increase. Pores with a radius value near 20 nm grew to the greatest extent. The activities of hydroxyl, ether and carbonyl functional groups and of multi-substituted aromatic hydrocarbons in the coal molecules increase, whereas the activities of methyl and methylene groups decrease. The expanded pores provide more space for the coal–oxygen reaction and enhance the functional group activities. These changes increase the risk for spontaneous combustion by increasing the oxygen consumption rate, decreasing the associated activation energy and increasing the pre-exponential factor.

Coal gasification coarse slag (CGCS) is a by-product of coal gasification. Despite its abundance, CGCS is mostly used in boiler blending, stacking, and landfill. Large-scale industrial applications of CGCS can be environment-friendly and cost saving. In this study, the application of CGCS as a substitute for river sand (RS) with different replacement ratios in ultra-high performance concrete (UHPC) was investigated. The effects of CGCS replacement ratios on the fluidity and mechanical properties of specimens were examined, and the effect mechanisms were explored on the basis of hydration products and the multi-scale (millimetre-scale and micrometre-scale) microstructure analysis obtained through X-ray diffraction (XRD), scanning electron microscopy, and X-ray energy-dispersive spectroscopy. With an increase in the CGCS replacement ratio, the water–binder ratio (w/b), flexural strength, and compressive strength decreased. Specimens containing CGCS of ≤25% can satisfy the strength requirement of non-structural UHPC, with flexure strength of 29 MPa and compressive strength of 111 MPa at day 28. According to the XRD results and multi-scale microstructure analysis, amorphous glass beads in CGCS positively influenced ettringite generation due to the pozzolanic activity. Porous carbon particles in CGCS showed strong interfacial bonding with cement slurry due to internal hydration; this bonding was conducive to improving the mechanical strength. However, CGCS hindered hydration in the later curing stage, leading to an increase in the unreacted cement and agglomeration of fly ash; in addition, at a CGCS replacement ratio of up to 50%, an apparent interfacial transition zone structure was observed, which was the main contributor to mechanical strength deterioration.

Understanding the development process of coal oxidation reactions can aid in preventing coal spontaneous combustion. Oxidation heating experiments on anthracite, bituminous coal, and lignite at different heating rates were carried out using simultaneous thermal analysis–Fourier transform infrared spectroscopy (STA–FTIR). The experimental results show that combining TG, DSC, CO, CO₂ and H₂O curves in the spontaneous combustion process of coal, the maximum acceleration point T_a and maximum mass loss rate T₄ are important for judging the severity of the coal spontaneous combustion hazard. Meanwhile, T_a is more accurate as the ignition temperature point than the ignition temperature T₃. As the coal’s rank and heating rate increases, the characteristic temperatures moved to higher temperature. The higher degree of metamorphism has a higher ignition activation energy. Additionally, the reaction models for ignition activation energy between anthracite and bituminous coal changed from three-way transport to random nucleation and nuclei growth with the increase of metamorphic rank, but were not affected by the heating rate.