Hydrothermal reaction between kaolinite and Ca(OH)(2) with starting CaO/kaolinite ratios from 0.2 to 1.2 g/g was carried out at 150-300 degrees C for up to 5 h. The effect of digestion conditions on the conversion of kaolinite was investigated. Kaolinite was completely converted to calcium-bearing compounds after Ca(OH)(2) digestion at 300 degrees C for 2 h. Reaction products were identified by X-ray diffraction, thermogravimetric, and differential thermal analyses. Different digestion conditions resulted in varying reaction products, including hibschites, omisteinbergite, tobermorite or poorly-crystalline tobermorite, and jaffeite. The dependence of reaction sequence on digestion conditions has been proposed from the results obtained. After digestion, the dissolution of reaction products in dilute HCl solution was investigated. Leaching with 6% HCl at room temperature under stirring could entirely extract the aluminum, silicon, and calcium from digested samples so long as kaolinite was completely converted.