Metakaolin is a calcined product of the clay mineral kaolinite. The particle size of metakaolin is smaller than
cement particles, but not as fine as silica fume.

When kaolinite, a layered silicate mineral with a distance of 7,13 Å between the layer of SiO₂ and Al₂O₃ is
heated, the water contained between the layers is evaporated and the kaolinite is activated for eaction with
cement.

Calcined between 600° and 850°C the kaolin is transformed to an amorphous phase called metakaolin.
This mineral is activated and metastable. Metakaolin can then react with cement and lime.

Heating above 900°C produces mullite, a non pozzolanic material.