Study on mechanical behavior and microstructure evolution of saturated coal gangue under CO₂-water-rock interaction

Geological storage of CO₂ in deep coal mine goaf is an important way to alleviate greenhouse gas emissions. In this study, the mechanical behavior and microstructure evolution of saturated broken coal gangue under the interaction of CO₂-water-rock were systematically studied. Based on the simulation experiment platform of CO₂ storage in goaf, the particle crushing characteristics and compaction characteristics of saturated broken coal gangue under different CO₂ reaction pressures (0 MPa, 2 MPa, 4 MPa, 6 MPa, 8 MPa) were systematically studied. The microstructure changes of coal gangue after CO₂-water-rock interaction were analyzed by X-ray fluorescence and scanning electron microscopy, and the microscopic mechanism of coal gangue particle crushing was discussed. The results showed that with the increase of CO₂ reaction pressure, the relative crushing rate of broken coal gangue increased from 36.07% to 45.49%, with an increase of 9.42%. The fractal dimension increased from 2.677 7 to 2.736 3, with an increase of 14.01%. Under the stress of 25 MPa, the strain of the sample increased from 0.309 at 0 MPa to 0.354 at 8 MPa, with an increase of 14.56%. X-ray fluorescence analysis showed that the content of Ca decreased from 65.455% to 15.531%, with a decrease of 76.27%, indicating that the Ca-containing minerals underwent strong dissolution during the reaction. The results of scanning electron microscopy showed that as the CO₂ reaction pressure increased from 0 MPa to 8 MPa, the proportion of pore cracks on the surface of coal gangue increased from 1.446% to 2.641%, and the fractal dimension of surface pores increased from 0.993 2 to 1.143 1. Based on the experimental results, an improved empirical model considering the reaction pressure of CO₂ is proposed, which can accurately describe the mechanical behavior of saturated broken coal gangue under high CO₂ pressure. The research results provide a theoretical basis for evaluating the stability and safety of CO₂ sequestration in coal mine goaf.