Metakaolin (MK) is anhydrous aluminium silicate (Al2O3.2SiO2.2H2O, AS2H2) which is dehydrated from kaolin (Al2O3.2SiO2.2H2O, AS2H2) at a suitable temperature (600-900 C). Kaolin belongs to layered silicate structure, and the layers are bound by Van der Waals bond, in which OH-ions are firmly bound. When the kaolin is heated in the air, several structural changes will occur. When the kaolin is heated to about 600 C, the layered structure of the kaolin is destroyed by dehydration and the transition phase - metakaolin with poor crystallinity is formed. Because the molecular arrangement of metakaolin is irregular, it is thermodynamically metastable and cementitious under appropriate excitation.