In re-inforced concretes the steel bars are protected by high alcalinity of hydrated cement. Clorides attack the steel in a first step of the destruction of concrete.

Chloride ion diffusion is dependent on water being present in mortars and concrete. The water content present is dependent on the pore volume and the environment of the concrete.

Due to the fact that with metakaolin the pore diameteris reduced, the migration or diffusen of water is hindered and the chlorine diffusion is reduced.

The use of metakaolin is therefore of major importance to concretes in sea water environment and sea side constructions.