A hydrothermal processing strategy of MSWI fly ash is presented for obtaining stable minerals with low toxic potential. Different hydrothermal conditions were tested to obtain high yields of new stable minerals. Experimental parameters including temperature, nature and molarity of alkali reagents, and reaction time were evaluated. The chemical stability of hydrothermal products was examined by the toxicity characteristic leaching procedure (TCLP) test and subsequent XRD for the leached residue. The significant amounts of Al-substituted 11 angstrom tobermorite and katoite in addition to minor amounts of zeolites were formed under experimental conditions at 0.5 M NaOH, 180 degrees C for 48 h, however KOH treatment in a similar regime resulted in smaller amounts of Al-substituted 11 angstrom tobermorite and katoite. Similarly, a product of mixed Al-substituted 11 angstrom tobermorite and katoite could be formed from the washed fly ash treated in 0.5 M NaOH at 180 degrees C for 48 h. Under the acidic condition, the treated fly ash exhibited an excellent stability of the mineral assemblage and less release of heavy metals relative to the untreated parent materials. (C) 2008 Elsevier B.V. All rights reserved.