Effect of gas–liquid medium adsorption on mechanical properties and crushing characteristics of coal

Coal and gas outburst is a major disaster problem to be solved urgently in coal mining. It is of great significance to reveal the mechanical response mechanism of coal under the adsorption of gas–liquid medium for the optimization of outburst prevention technology. In this paper, drop hammer impact experiments were carried out on CO2-containing coal (CDC) and water-soaked coal (SC). Combined with energy conservation analysis and fractal theory, the influence of gas–liquid medium adsorption on the mechanical properties and crushing characteristics of coal was systematically explored. The self-developed closed drop hammer impact system was used in the experiment. The impact stress time–history curve was divided into four stages: compaction, elasticity, instability, and post-peak failure. After CO2 was adsorbed by the coal, the peak value of the impact force decreased by 31.67% to 44.72%, and the pulse time is prolonged by 15.87% to 66.88%. This indicates that the plasticity of the coal body has been enhanced. After that, by discussing the change in the impact time–history curve of the SC and the change in the characteristic index of the drop hammer impact force, it is found that the peak value of the impact force is reduced by 1.78%–13.25%. The pulse time is first fast and then slow. Finally, based on the fractal theory and the mass distribution of broken particle size of CDC and SC after drop hammer impact, it is found that CO2 and water reduce the fractal dimension of broken coal particles by 6.59% and 8.78%, respectively. The latter is more significant due to pore compression and chemical reconstruction effects. It further explains the mechanism that CO2 weakens the effective stress by adsorbing expansion stress, and water reconstructs the pore network through the dual paths of physical filling and hydration reaction. The research results of this paper provide theoretical support for gas outburst prevention and control.