Fly ash can be considered a promising secondary raw material for the production of high-strength porous mullite ceramics, since the absence of a need to grind the initial batch materials makes it possible to significantly reduce energy consumption. In this study, promising ceramic samples were produced using fly ash and aluminum ore, specifically low-iron bauxite, as an Al2O3-adjusting component. Thermogravimetric and X-ray diffraction phase analyses were used to determine the sequence and temperature ranges of phase transformations in the system “fly ash – bauxite” and to establish the optimal thermal treatment regime. Several exothermic peaks were observed in the 1200-1300 °C temperature range, reflecting a sequence of phase transformations. Mullite forms through a reaction between quartz and cristobalite, which pass into a transient liquid phase, and γ-Al2O3, formed by the decomposition of boehmite. The total mullite phase content reaches 83-84 %, which is significantly higher than the values reported by other researchers, including those who used various additives to accelerate mullite formation. Scanning electron microscopy was used to trace the evolution of the microstructure of the obtained samples as a function of synthesis temperature. As the treatment temperature increases to 1300 °C, sintering gradually intensifies, resulting in the formation of larger pores. Acicular mullite crystals form around flake-like γ-Al2O3 crystals, some of which subsequently transform into corundum. Laboratory tests of the finished ceramic samples confirmed that their technical characteristics – density, porosity, and compressive strength – meet the requirements for refractory, thermal insulation, and chemically resistant materials. The study presents the technological principles for developing high-strength and porous mullite-based ceramics intended for use in construction and thermal insulation applications.