• Incorporation of modified fly ash geopolymer transforms phenolic foam from open-cell to closed-cell morphology, reducing pulverization ratio by 41% and enhancing mechanical properties by 15%.
• The composite exhibits faster gelation dynamics and a significantly lower maximum reaction temperature (40 °C) compared to neat PF, mitigating spontaneous combustion risks.
• PF/MFA composite demonstrates high reliability against gas leakage in laboratory coal mine plugging tests, indicating superior sealing performance.
• Combustion of PF/MFA forms a silica hybrid char layer with higher graphitization degree and continuous closed-cell structure, effectively suppressing release of combustible volatiles and toxic gases.