Influence of hydrothermal treatment parameters on the phase composition of zeolites

Analcime was prepared by hydrothermal transformation of metakaolin-based geopolymer activated by means of sodium water glass. Gradual transformation of geopolymer through primary and unstable zeolitic phases up to final analcime was studied as a function of hydrothermal treatment duration (6–48 h) and access of water vapour under the selected conditions (165 °C, 0.5 MPa). Composition, microstructure, and thermal stability of the prepared samples were assessed using simultaneous thermogravimetry and differential scanning calorimetry, X-ray diffraction and high-temperature XRD analyses, and scanning electron microscopy. In the case of shorter autoclaving and access of water vapour, the mix of different zeolitic phases was formed: analcime, zeolite P2 gmelinite-Na, and chabazite-Na. From the time of 24 h and autoclaving in the closed moulds, pure cubic analcime was detected. Uniform microstructure of these samples comprised of trapezoid particles with the diameter between 50 and 60 µm. Prolongation of autoclaving time did not lead to the significant change of particle size and their composition. Dehydration of detected zeolites took place through the formation of defect or unknown zeolitic structures before the structural collapse happened.