Development and coagulation mechanism of a high accumulation, highly stable alkali-activated solidified foam for preventing the spontaneous combustion of coal.

To reduce greenhouse gases (CO2, CH4, etc.) released by spontaneous combustion of coal, and addressing the issues of poor stability and weak stacking capacity of traditional inorganic solidified foams, fly ash (FA) is utilized in a resource-efficient and high-value manner, and liquid sodium silicate (LSS) is introduced to develop alkali-activated high-stacking solidified foam (AHSF). First, we selected highly stable water-based foams under alkaline conditions. Second, we studied the influence of LSS modulus and alkali content on the macroscopic properties and microstructure of AHSF. Finally, we elucidated the LSS Setting-acceleration mechanism. The results showed that sodium fatty alcohol polyoxyethylene ether sulfate exhibited the best foaming ability and stability under alkaline conditions, with bubble diameter increasing by only 30.15% in 1 hour. AHSF exhibited the best overall performance at a modulus of 1.9 and an alkali content of 2.5 wt%, with a packing height of 28.13 mm and a 28-day compressive strength of 4.12 MPa, representing a 57.3% improvement over the control group. AHSF reduced the free radical concentration in coal by a maximum of 28.2% at 20 °C, increased the crossover temperature by 13.8 °C, reduced CO release by 80.9% at 135 °C, and reduced CO2 release by 92.2% at 150 °C. Field application results of AHSF showed that after grouting 8760 m3 using the ground borehole curtain grouting process, the highest temperature in all monitoring boreholes remained below 90 °C. Taking the location at a depth of 19 m in monitoring hole 3# as an example, compared with before AHSF injection, the temperature decreased by 29.0%, 74.9%, and 87.7% after 10, 20, and 30 days, respectively. This study provides a scientific basis for the development of rapidly depositing and durable stable high-efficiency leak-sealing foam for coal mines.