There are two main types of efflorescence*. The primary efflorescence is due to the development of calcium hydroxide during the hydration of cement and its migration to the surface due to the evaporation of access water.
The secondary efflorescence is given by the wetting of a mortar or concrete and the dissolution of the Portlandite present in the pores.

Due to the rather slow reactivity of metakaolin with Portlandite, the primary efflorescence can partly be reduced
but the secondary ones can be avoided by the use of sufficient metakaolin to convert the Portlanditeinto binder
(about 20% cement replacement).
Effect of Metakaolin on Calcium Hydroxide Content of Portland Cement Concrete as it Cures
*efflorescence = transport of calcium hydroxide to the surface and formation of calcium carbonate with
                             carbon oxide of the air

more info: Dr. Denis Bezard - are-questions-xya34[at] - phone: +43 (0)2252 45970