Abstract
The formation process of kaolinite from the amorphous mixture of silica and alumina by hydrothermal reaction was studied by XRD, IR, DTA-TG, TEM, and high-resolution solid state 29Si-and 27Al-MAS/NMR. The amorphous mixture of the starting material prepared with commercially available silica-sol and alumina-sol was hydrothermally treated with distilled water in a Teflon pressure vessel at 220°C for a reaction time varying from 1 to 144 hours. The Q4 state component of the amorphous starting material changed into an intermediate amorphous Q3 state. The intermediate amorphous phase converted into crystalline kaolinite with spherical shape (Q3 state). The yield and crystallinity of kaolinite increased with an increase in the reaction time. On the other hand, the coordination number of aluminum converted from a mixture of six-and four-coordinations to six-coordination. In the earlier stages of crystallization of kaolinite, spherical kaolinite is more abundant than platy kaolinite, and the spherical kaolinite changes into platy one with increasing hydrothermal reaction time.
