High-Temperature Resistant Sealing Materials from Chitosan-Modified Alkali-Activated Coal Gangue for Mine Fire Prevention

Coal spontaneous combustion remains a persistent hazard in underground mining, demanding sealing materials that retain structural integrity and low gas permeability under sustained thermal loading. Therefore, this study prepared a cementitious firefighting material by alkali activation of coal gangue and modified with low-dosage chitosan for goaf sealing applications. The formulation for the base aluminosilicate matrix was optimized to a 4:6 mass ratio of coal gangue to ground granulated blast furnace slag, a water-to-precursor ratio of 0.40, an activator modulus of 1.0, and a 6% alkali dosage. To overcome the inherent thermal vulnerability of the conventional alkali-activated materials, chitosan was introduced to coordinate with Ca2+ ions released during activation. FTIR, XRD, and SEM characterizations reveal that this coordination forms an interpenetrating organic–inorganic dual-network that bridges the aluminosilicate matrix. Results indicate that at a low chitosan dosage of 0.5%, the material exhibits an optimal 3-day compressive strength of 11.05 MPa. Following thermal exposure, the coordination cross-links effectively dissipate thermal stress and suppress crack propagation, reducing thermally shrinkage by approximately 40% relative to the unmodified matrix at 600 °C. Although this differential narrows at 800 °C, the modification critically preserves macroscopic integrity throughout the 200–600 °C range where crack suppression is most consequential for sealing performance. Permeability and porosity analyses demonstrate that chitosan modification preserves the sealing capacity primarily by disrupting the connectivity of the thermally damaged pore network, yielding an SDR-estimated permeability as low as 0.03 mD. The material utilizes coal gangue as the dominant precursor and achieves marked thermal-resistance enhancement at low chitosan dosage, offering a scalable and resource-efficient strategy for preventing coal spontaneous combustion in underground coal mines.