Coal fly ash from an Italian coal-fired power plant, kaolinite from the Source Clay Repository of The Clay Minerals Society, and four derived mixtures were used to synthesize zeolite using hydrothermal treatment at 45 °C in distilled water after alkaline fusion. The results documented that geopolymers, A-type and X-type zeolites were formed in different quantities, depending on the starting material and the duration of the experiment. Zeolite-X was the prevailing phase synthesized using pure fly ash, zeolite-A formed in higher amounts from kaolinite, and comparable amounts of A- and X-type zeolites crystallized, thereby adding 20 and 40% kaolinite to the fly ash, respectively. Zeolite-A as main phase was synthesized already adding 60% or even up to 80% kaolinite to the fly ash. Sodalite occasionally formed from the source materials, whereas zeolite ZK-5 was synthesized from only fused fly ash (100FA). The data indicated that, in addition to the Si/Al ratio of solid source materials, zeolite formation was controlled by the time and chemistry of the solution. The polymerization of alumina-silicate gels changed during the experiments, likely due to the amorphization of metastable zeolites.

We wish to thanks Prof. Piero Mastrorilli for the NMR analyses and suggestions. This study was partially financed by IMAA-CNR and the Italian Association for the Study of Clays (AISA).