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

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)