Optimization and Field Validation of an Ultrafine-Cement-Based Grout for Coal Mine Borehole Sealing

Conventional borehole sealing materials used in coal mines often suffer from slow setting times, insufficient early strength, and poor fluidity, resulting in unsatisfactory sealing performance and reduced gas drainage efficiency. In this study, an ultrafine-cement-based grout was optimized using sodium chloride, LSN, and MZSG as the early-strength agent, accelerator, and water reducer, respectively. Laboratory experiments, including tests of setting time, fluidity, compressive strength, and permeability, were conducted to determine the recommended formulation, followed by field verification in a coal mine in Shanxi Province, China. The optimized grout achieved an initial setting time of 41 min, a fluidity of 348 mm, a compressive strength of 18.1 MPa, and a permeability of 0.0808 mD, indicating a favorable balance between rapid setting, injectability, mechanical integrity, and gas-tightness. The improved performance was attributed to the synergistic effects of the additives: Sodium chloride promoted early hydration, LSN accelerated setting and contributed to early matrix formation, and MZSG improved slurry dispersion and injectability. The field performance further confirmed that the optimized material provided more stable gas drainage behavior under the tested mine conditions. These results demonstrate that the proposed ultrafine-cement-based grout offers a practical sealing material for gas-drainage boreholes in fractured coal seams and provides an effective material-design strategy for coupling early strength development, slurry transportability, and sealing compactness.