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The influence of temperature of alkaline hydrothermal treatment on the conversion in zeolite of Spanish coal low calcium-fly ash (ASTM class F) is presented in this work. Zeolite Na-P1 gismondine type (Na6Al6Si10O32.12H2O) was formed at the temperature of 100ºC, which transformed in zeolite; analcime-C type (Na(Si2Al)O6H2O) and sodalite (1.08 Na2O.Al2O3.1.68SiO2.1.8H2O) at 200ºC together with traces of tobermorite-11Å (Ca5(OH)2Si6O16.4H2O). At this temperature the 100% of the fly ash reaction was allowed. An equivalent study was carried out in water as reference. The zeolite conversion of the fly ash was characterized by X ray diffraction (XRD), FT infrared (FTIR) spectroscopy, surface area (BET-N2) and thermal analyses.