Development and properties characterization of fly ash-based inorganic grouting materials for preventing coal spontaneous combustion

Air leakage in goafs is a major cause of spontaneous coal combustion, posing serious risks to mine safety. Traditional grouting materials often suffer from slow setting and insufficient bonding with fractured coal, limiting their effectiveness in emergency sealing applications. To address this challenge, this study aims to develop and optimize a novel fly ash-based inorganic grouting material with rapid setting, high early strength, and strong coal interfacial adhesion. A three-factor, three-level Box–Behnken design (BBD) within the framework of response surface methodology was employed to evaluate the effects of fly ash content, sodium silicate modulus, and water–cement ratio on three performance indicators: initial setting time, and 7-day and 28-day compressive strength. Regression modelling and ANOVA revealed that fly ash content had the most significant influence, followed by water-cement ratio and sodium silicate modulus. Interaction analysis further demonstrated that the combined effect of fly ash content and water-cement ratio played a critical role in governing early hydration kinetics and strength development. Scanning electron microscopy (SEM) confirmed that these interactions facilitated the formation of dense gel phases, resulting in refined pore structure and improved microstructural integrity. The optimal mix, 50% fly ash, 0.80 sodium silicate modulus, and 0.50 water-cement ratio, achieved an initial setting time of 2.17 min and a 7-day compressive strength of 4.18 MPa. This study provides a quantitative optimization strategy and microstructural understanding for the design of fast-setting, high-performance sealing materials, offering a practical solution for air leakage control and spontaneous combustion prevention in underground coal mines.