Due to the enhanced porosity of surfaces given by the particle structure of cement, the water soluble Portlandite has a tendency to migrate to the surfaces and deposit there reducing the contact zone between the CSH* binder and the surface. Water absorption by mortars therefore will reduce adhesions between binder and substrate as Portlandite will tend to dissolve.
Transforming the Portlandite into CSH* binder will increase the contact zone and the adhesion on the surface where the mortar is applied. This is especially true when water storage conditions are used.
* CSH - calcium silicate hydrate