Synthesis of zeolites at low temperatures in fly ash-kaolinite mixtures

•Fly ash, kaolinite and their mixtures were used to synthesise zeolite.•Variable amount of geopolymer and zeolites formed depending of starting material.•Si/Al of solid material, time and chemistry of solution control zeolite formation.•Zeolites amount change during incubation since they amorphized.•Alumina-silicate gel polymerization changes due to metastable zeolite amorphization.

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.

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