Hydrothermal formation of kaolinite from various metakaolins

Metakaolins formed by total dehydroxylation of kaolinite, dickite and nacri te were hydrothermally treated at 240 degrees C, for 2, 21 or 182 days, at initial pH = 5-6 or 1. Kaolinite was the only mineral of the kaolin subgrou p crystallized in these treatments. Kaolinite is less ordered for pH = 4-6 than for pH = 1. This can be attributed to the influence of specific chemic al, morphological, and/or crystalline properties of the metakaolins. Two di fferent processes of kaolinite formation and crystal growth must be involve d depending on the pH and metakaolin. The first process implies the dissolu tion of the metakaolin and crystallization of thin prismatic or dendritic k aolinite particles which curl, probably due to surface tension. The second process implies the rapid formation of small pseudohexagonal kaolinite part icles which would aggregate and coalesce to form bigger particles.