Around 450°C, the chemically combined water in clays is released, resulting in the formation of metakaolin. As with the loss of physically adsorbed water, the loss of the chemical water is an endothermic process that is accompanied by weight loss [33]. The magnitude of the weight loss depends on the amount of chemically combined water in the clay. For kaolinite, Al2O^2SiO2^2H2O, the weight loss due to chemically combined water should be 13.9 wt%, which is similar to reported water contents for high-purity secondary kaolins (Table 4). After dehydration, metakaolin appears amor­phous on X-ray diffraction, but the short range ordering of the cations within the sheets that make up the kaolinite structure is retained [33,34]. Brindley has speculated that disruption of the order perpendicular to the sheets causes the change in the X-ray diffraction pattern [33]. Thus, metakaolin is a homogeneous molecular-level mixture of noncrystalline alumina and silica. Metakaolin does not spontaneously rehydrate when it is exposed to water and it remains stable up to approximately 980°C.