The effect of thermal activation of kaolinite on its dissolution and re-precipitation as zeolites in alkaline aluminate solution

In this study, kaolinite was heated to temperatures in the range of 400-700 degrees C to investigate the relationship between thermal activation of kaolinite and its dissolution and re-precipitation as zeolites. Heat treatment with temperature above 600 degrees C converted the kaolinite to amorphous metakaolin. This treatment substantially increases its dissolution rate and also changes the phase composition of its re-precipitation products in alkaline aluminate solution. The re-precipitate of kaolinite was found to be only sodalite when the heat treatment was below 600 degrees C. With temperatures above 600 degrees C, dissolution of metakaolin is followed by rapid precipitation of an amorphous phase, then zeolite LTA which finally transforms to sodalite. UV-Raman spectroscopy can clearly discern between kaolinite and metakaolin despite the presence of small amounts of anatase by focussing on the hydroxyl region.