AbstractOBJECTIVEThe purpose of this study was the conversion of calcareous lignite fly ash (LFA) into synthetic zeolite by hydrothermal, fusion‐hydrothermal, and hydrothermal‐sonochemical processes.METHODExperiments were carried out using raw and washed (5 N HCl) fly ash. Initially, the fractional factorial experimental design was used to plan the hydrothermal experiments and assess the process parameters, such as solid to liquid ratio (S/L), silica to aluminum oxide ratio (SiO2/Al2O3), calcium oxide content (% CaO), time, and temperature of the hydrothermal process. The optimized conditions were then applied to a two‐stage fusion‐hydrothermal process, while the influence of sonication on the conversion efficiency was tested before and after the hydrothermal process. Mineralogical analysis was performed by x‐ray diffractometer (XRD) to detect zeolite formation. Further characterization was conducted by x‐ray fluorescence (XRF), specific surface area, and laser particle analysis to assess process efficiency.RESULTS AND DISCUSIONThe results indicated the formation of four types of zeolite crystals. The washed LFA (wLFA) yielded the maximum zeolite yield when it was hydrothermally treated for 6 h at 150 °C, S/L = 1:2, and SiO2/Al2O3 = 6.3. In such conditions, phillipsite was synthesized at 27%. The yield of phillipsite zeolite was improved to 32% when the hydrothermal process was followed by a 30 min sonication process. At the same processing conditions, the use of raw LFA produced 72% crystalline tobermorite. The hydrothermal‐late sonication process can effectively convert waste fly ash into valuable products. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).