metakaolin (referred to as MK) is based on kaolin (Al2O3·2SiO2·2H2O, referred to as AS2H2) as raw materials, at the appropriate temperature (600 ~ 900 ℃) through dehydration formed anhydrous aluminum silicate (Al2O3·2SiO2, referred to as AS2). Kaolin belongs to the layered silicate structure, and the layers are bound by van der Waals bonds, in which OH- ions are firmly bound. When kaolin is heated in the air, it will undergo several structural changes, and when it is heated to about 600 ° C, the layered structure of kaolin is destroyed by dehydration, forming a transition phase with poor crystallinity – metakaolin. Because the molecular arrangement of metakaolin is irregular, it shows thermodynamic metastable state and has gelling property under proper excitation.
Metakaolin is a kind of highly active mineral admixture, which is amorphous aluminum silicate formed by low temperature calcination of ultra-fine kaolin. It has high pozolanic activity and is mainly used as concrete admixture and can also make high-performance geopolymers.