Fabrication and Enhancement of Porous Mullite Ceramics from Solid-Waste-Based Geopolymers

Porous mullite ceramics were synthesized at [Formula: see text] via a polyurethane foam impregnation route using coal gangue (CG) and bauxite-based geopolymer (GP) slurries. The effects of ion exchange on the ceramics were systematically investigated, along with the evolution of phase composition, linear shrinkage, bulk density, porosity and compressive strength as a function of sintering temperature. To address the issues of hollow struts and cracks caused by polyurethane (PU) template burnout during fabrication, a secondary coating strategy was developed to in-situ generate mullite within the defects. The process involved pre-sintering the GP precursor to remove the PU template, followed by vacuum impregnation with a solution of ammonium fluoaluminate and silica sol to supply aluminum and silicon sources for defect filling. Results demonstrated a significant improvement in compressive strength and excellent high-temperature stability. After thermal shock testing, the ceramic retained its macroscopic integrity without structural failure, maintained constant porosity and dimensional stability and exhibited a minimal linear shrinkage of only 1.25% after 25 cycles. Moreover, the material showed a low thermal conductivity of [Formula: see text]. This work presents a green, low-cost and scalable approach for producing high-performance thermal insulation ceramics suitable for high-temperature applications.