Thermoresponsive mineral grout for autonomous oxygen isolation in underground coal pillars

Abstract

A thermo-responsive mineral-based grout composite (GC) was developed for rapid sealing of fracture networks in underground coal pillars, aiming to eliminate oxygen ingress and prevent spontaneous combustion in goaf areas. Coal spontaneous combustion triggered by air leakage poses severe threats to mine safety. Traditional sealants such as rapid-hardening calcium sulfoaluminate (CSA)-sodium silicate systems and polyurethane materials suffer from thermal-runaway risks, poor fire resistance, and complex parameter control requirements. The GC slurry is composed of Mg(OH) 2 , Al(OH) 3 , ultra-fine cement and NIPAM-grafted bentonite. It maintains 200 mm flowability at 20 °C for deep penetration, yet triggers abrupt thickening within 3 min when ≥ 40 °C is encountered, forming a dense oxygen barrier. Adiabatic calorimetry revealed a maximum temperature rise of only 18.4 °C (ΔTmax = 18.4 °C, t max = 7.8 h), 13% and 52% of those measured for polyurethane and cement–sodium-silicate systems, respectively, thereby minimising the thermal-runaway risk during bulk injection. Laboratory micro-CT confirmed 85% reduction in connected porosity (> 50 μm) and crack-free coal–grout interfaces. Field trials in the 102 auxiliary transport roadway (No. 3 coal seam, Hongqinghe Mine, Inner Mongolia, China) reduced O 2 concentration from 9 to 17% to 1–5% and decreased CO to 0 ppm within 30 d under mining-induced dynamic loading. The 28-d compressive strength reached 30.1 MPa (w/c = 0.7) while zero bleeding or shrinkage cracks were observed. Raw-material cost analysis shows GC (USD 185 t − 1 ) achieves 35% lower life-cycle cost than polyurethane over a 5-year horizon. With ΔTmax well below the 60 °C thermal-safety limit adopted in Chinese coal-mine grouting practice (AQ/T 1089–2020, a standard issued for polymeric materials and used here as a conservative benchmark) and goaf O 2 maintained below the 7% critical oxygen concentration specified in GB 51,078 − 2015, GC offers a low-exotherm, fire-resistant and environmentally benign alternative to organic sealants for coal-mine fire prevention.