Synergistic Modification of Static Chemical Agent with Fly Ash and Hemihydrate Gypsum for Mechanically Compatible Gas Drainage Borehole Sealing

Abstract Borehole sealing quality plays a critical role in coal seam gas extraction efficiency, while the mechanical mismatch between conventional cement-based sealing materials and surrounding coal masses often leads to interface damage and sealing failure under mining-induced stress conditions. To address this issue, a modified static chemical agent (MSCA) was developed by incorporating fly ash (FA) and hemihydrate gypsum (HG) into a static chemical agent (SCA) system. The hydration behavior, expansion characteristics, mechanical properties, and engineering applicability of MSCA were systematically investigated through laboratory experiments, microscopic analyses, numerical simulations, and field industrial tests. The results indicate that HG significantly accelerates the hydration reaction and promotes the early development of expansion pressure of MSCA. The maximum expansion pressure of MSCA increases linearly with SCA content and quadratically with HG content. XRD and SEM analyses reveal that the hydration products mainly consist of Ca(OH)2, calcium silicate hydrate (C–S–H), calcium aluminate hydrate (C–A–H), dihydrate gypsum (DG), and ettringite (AFt). Increasing FA and HG contents facilitates the formation of C–S–H gel and interwoven AFt crystal networks, resulting in a denser microstructure and improved mechanical performance. The splitting strength of MSCA increases significantly with decreasing SCA content and increasing HG content, whereas the expansion pressure exhibits an opposite trend. Among all tested formulations, the D1MSCA achieves the optimal balance between expansion pressure and mechanical strength, with a splitting strength of 1018 kPa and an expansion pressure of 5.7 MPa, which are close to the mechanical properties of coal seams and the field grouting pressure conditions. Numerical simulation results demonstrate that the mechanical compatibility between the sealing material and the coal seam is a key factor controlling interface damage under mining-induced stress. A sealing material with an elastic modulus close to that of the coal seam can effectively alleviate stress concentration around the borehole and suppress crack propagation. Field industrial tests further confirm that MSCA-sealed boreholes maintain significantly higher gas extraction concentrations than conventional cement-sealed boreholes during the entire mining process. Under strong mining-induced stress conditions, the gas extraction concentration of MSCA-sealed boreholes remains 31%–50% higher than that of conventional cement-sealed boreholes. The proposed MSCA provides an effective approach for improving borehole sealing quality and gas extraction performance in underground coal mines under mining disturbance conditions.