Zeolite from Fly Ash: An Investigation on Metastable Behavior of the Newly Formed Minerals in a Medium-High-Temperature Range

In this paper, the behavior of newly formed phases synthesized from fly ash in a medium-high-temperature range (60–105 °C) was studied by X-ray powder diffraction. X-ray diffraction data indicate that up to 90 °C, the main zeolites formed are X-type zeolite and sodalite; at higher incubation temperature (95–105 °C), gmelinite is formed together with sodalite. Rietveld quantitative analysis shows that the amount of amorphous material (geopolymer) does not change up to 90 °C, and instead it strongly increases after 95 °C. An opposite trend characterizes the behavior of X-type zeolite and sodalite. Beyond the confirmation of a complex competitive system, this study highlights the metastable behavior of newly formed zeolites in a very short range of temperature at fixed NaOH concentration, and it shows the role played by the amorphous phase.