HYDROTHERMAL REHYDROXYLATION OF KAOLINITE STUDIED BY THERMAL ANALYSIS

The effect of hydrothermal conditions on the process of kaolinite rehydroxylation at 180 C and 230 C and for soaking times ranging from 1 to 200 h was studied by thermal analysis (DTA, DSC, TG). It is shown that the kaolinite dehydroxylation of rehydroxylated samples shifts to lower temperatures compared to the original kaolinite. The content of kaolinite in rehydroxylated samples was calculated from TG curves. The curve of the kaolinite weight gain plotted against the time of hydrothermal treatment indicates a two stage process with a different weight gain rate representing short and long-time measurements. Multiple dehydroxylation was studied by repeated calcination (2 h at 600 C) and hydrothermal treatment (10 h at 230 C). It was proved that values of activation energy of multiple dehydroxylations decreased with an increasing number of repeated treatments. The findings were used to investigate the origin of kaolinite in medieval pots from Roztoky, near Prague.