Conversion of three different types of incineration ash, coal fly ash (CFA), paper sludge fly ash (PSFA) and industrial waste incineration fly ash (IWIFA) into useful materials using the alkali fusion method is attempted. The chemical and mineralogical compositions of these ashes differed. The order of Ca content in the ash was IWIFA>PSFA>CFA, while the Si and Al contents in the ashes were CFA>PSFA>IWIFA. CFA was mainly composed of quartz [SiO₂] and mullite [3Al₂O₃·2SiO₂], PSFA mainly of gehlenite [Ca₂Al₂SiO₇], anorthite [CaAl₂Si₂O₈] and talc [Mg₃Si₄O₁₀(OH)₂], and IWIFA mainly of portlandite [Ca(OH)₂], halite [NaCl] and sylvine [KCl]. Zeolites (zeolite-X, -A, -P and hydroxysodalite), hydrogrossular (katoite), and calcite were synthesized from the ashes. With an increasing Ca content of the ash, the Si concentration in the solution decreased, and the main product phases changed from zeolite (aluminosilicate) to hydrogrossular (calcium aluminosilicate), and finally to calcite (calcium carbonate). The observed concentrations of Si, Al, and Na in the solution during the reaction explain the crystallization of these phases.