Abstract
The thermal dehydration behaviours and x-ray diffraction features of heated kaolinite and halloysite have been investigated, especially for the so-called metakaolin state.
The results on the thermal dehydration studies show that the dehydroxylization of well-crystallized kaolinite is likely to take place with two steps, the first of which starts at about 520°C. Approximately one fourth of hydroxyl groups are expelled gradually between 650°C and 950°C. This is the second step of dehydroxylization. A crystalline-like structure is proposed for the first step of dehydroxylization.
It is also pointed out that the hydroxyl groups of halloysite are easily expelled, but the recombination of the destroyed structure to a new phase needs much more energy than in the case of kaolinite.
