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 degrees C) was studied by X-ray powder diffraction. X-ray diffraction data indicate that up to 90 degrees C, the main zeolites formed are X-type zeolite and sodalite; at higher incubation temperature (95-105 degrees C), gmelinite is formed together with sodalite. Rietveld quantitative analysis shows that the amount of amorphous material (geopolymer) does not change up to 90 degrees C, and instead it strongly increases after 95 degrees 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.