The spontaneous combustion of residual coal in coal mine gob has long been a problem and poses a threat to the safe production of coal. Therefore, it is of great significance to conduct an in-depth study of the oxidation and self-heating progress of residual coal in the gob. Considering that the geometric dimensions and physical characteristics of the gob will change during the advance of the working face, the purpose of the present paper is to determine how the coal self-heating develops during and after coal mining. A fully coupled transient model including gas flow, gas species transport, and heat transfer in the gob and the butt entries, as well as heat transfer in the surrounding strata, is developed to quantify the evolution of coal self-heating in gob during and after mining. The model was solved by COMSOL Multiphysics package and then verified by comparing the field data with the simulated data. On this basis, parametric studies including the influences of the surrounding strata temperature, airflow temperature, coal-rock particle size, and advance rate of the working face on coal oxidation and self-heating in the gob were conducted. The results show that a tailing phenomenon of the high-temperature area is formed on the air inlet side of the gob during mining, and the temperature in the high-temperature zone decreases gradually due to the accumulation and compaction of the gob and heat dissipation to the surrounding strata. Also, although the temperature in gob increases gradually after the stopping of mining, the high-temperature area migrates towards the working face. Moreover, when the temperature of the surrounding strata is consistent, different ventilation temperatures have no obvious effect on the maximum temperature of the gob at the initial mining stage, whereas the higher ventilation temperature results in the higher self-heating temperature after several days of mining. Finally, the smaller average particle size or faster advance rate results in a lower maximum temperature of gob.

The current investigation focuses on the stability of the magnesium oxide-based cementitious system under the action of sulfate attack and the dry-wet cycle. The phase change in the magnesium oxide-based cementitious system was quantitatively analyzed by X-ray diffraction, combined with thermogravimetry/derivative thermogravimetry and scanning electron microscope, to explore its erosion behavior under an erosion environment. The results revealed that, in the fully reactive magnesium oxide-based cementitious system under the environment of high concentration sulfate erosion, there was only magnesium silicate hydrate gel formation and no other phase; however, the reaction process of the incomplete magnesium oxide-based cementitious system was delayed, but not inhibited, by the environment of high-concentration sulfate, and it tended to turn completely into a magnesium silicate hydrate gel. The magnesium silicate hydrate sample outperformed the cement sample, in terms of stability in a high-concentration sulfate erosion environment, but it tended to degrade considerably more rapidly, and to a greater extent, than Portland cement, in both dry and wet sulfate cycle environments.