Abstract: High calcium-fly ash (HCFA) collected from the Mae Moh electricity generating plant in Thailand was utilized as a raw material for ceramic production. The main compositions of HCFA characterized by X-ray fluorescence mainly consisted of 28.55wt% SiO₂, 16.06wt% Al₂O₃, 23.40wt% CaO, and 17.03wt% Fe₂O₃. Due to high proportion of calcareous and ferruginous contents, HCFA was used for replacing the potash feldspar in amounts of 10wt%–40wt%. The influence of substituting high-calcium fly ash (0–40wt%) and sintering temperatures (1000–1200°C) on physical, mechanical, and thermal properties of ceramic-based materials was investigated. The results showed that the incorporation of HCFA in appropriate amounts could enhance the densification and the strength as well as reduce the thermal conductivity of ceramic samples. High proportion of calcareous and ferruginous constituents in fly ash promoted the vitrification behavior of ceramic samples. As a result, the densification was enhanced by liquid phase formation at optimum fly ash content and sintering temperature. In addition, these components also facilitated a more abundant mullite formation and consequently improved flexural strength of the ceramic samples. The optimum ceramic properties were achieved with adding fly ash content between 10wt%–30wt% sintered at 1150–1200°C. At 1200°C, the maximum flexural strength of ceramic-FA samples with adding fly ash 10wt%–30wt% (PSW-FA(10)–(30)) was obtained in the range of 92.25–94.71 MPa when the water absorption reached almost zero (0.03%). In terms of thermal insulation materials, the increase in fly ash addition had a positively effect on the thermal conductivity, due to the higher levels of porosity created by gas evolving from the inorganic decomposition reactions inside the ceramic-FA samples. The addition of 20wt%–40wt% high-calcium fly ash in ceramic samples sintered at 1150°C reduced the thermal conductivity to 14.78%–49.25%, while maintaining acceptable flexural strength values (~45.67–87.62 MPa). Based on these promising mechanical and thermal characteristics, it is feasible to utilize this high-calcium fly ash as an alternative raw material in clay compositions for manufacturing of ceramic tiles.