Phase transformation and characteristics of hydrothermal zeolites formed under alkaline conditions

Zeolites form in various geological environments; among all, volcanic materials are the most important zeolite precursors. In this study, zeolites were synthesized by hydrothermally treating the volcanic material. The fine powdered pumice sample, used in this study, is one of the pyroclastic products of the Central Anatolian Volcanic Province (CAVP). This study mainly focused on the phase-transition and characteristics of zeolites formed under different reaction agent contents in a hydrothermal synthesis reactor. The products of the hydrothermal treatment were characterized using various analytical techniques, such as X-ray diffraction, scanning electron microscopy, and FTIR measurements. The hydrothermal treatment was performed with NaOH (1M, 2M, and 3M) and 1M KOH reaction agents at 150 ºC for 8 to 16 h. Meanwhile, diatomite is specifically utilized in this study to observe its impact on the reaction and to increase the silica content of the starting material. The crystallinity and characteristics of the reaction products were affected by the molar concentration and type of the reaction agents. The products mainly consist of analcime, chabazite, Na-phillipsite, and Na-P1. The alumina-silicate gel produced by the reaction of 2M NaOH with natural volcanic material has the potential to transform into chabazite and Na-P1. With the increase in molar concentration of NaOH, more sodium participates in the reaction to form zeolite phases of analcime, Na-P1, and Na-phillipsite. Following the addition of diatomite to the reaction with 3M NaOH, it has been noted that Na-Al-Si-rich gel has been produced beside analcime, Na-P1, and Na-phillipsite. The results obtained in this study showed that natural volcanic material led to the formation of gels under different alkaline activation conditions, which seemed to control the generation of zeolites.