Water permeability of concrete can be reduced by up to 100 times through the use of silica fume
Test by some independence authorities indicates that so called waterproofing admixtures had no effect on water permeability of concrete. Rather that supervision and improved mix designs associated with such systems can lead to a more impermeable concrete. It should be noted that permeability of concrete is not a simple function of its porosity but depends also on size, distribution and continuity of the pores.
Pores in concrete are reduced significantly with the inclusion of silica fume and illaries reduced through reduced bleeding and production of denser paste. The permeability is lower the higher the cement content of the paste i.e. the lower the water/cement ratio. Values obtained for pastes in which 93% of cement has hydrated. It is the permeability of the paste that has the greatest effect on the permeability of the concrete. Water vapor transmission of concrete is reduced by a decrease in the water cement ratio. A decrease in water cement ratio from 0.8 to 0.4 decreases the vapour permeability by 50-65%.
Vapour permeability, however, decreases as the relative humidity decreases, an increase in relative humidity decreases the airfilled pore space available for diffusion. It follows the that if the moist side is for instance saturated an increase in the relative humidity of the dry side reduces the vapor permeability.
The permeability of concrete is reduced by the use of silica fume in combination with superplasticizers which reduce the water cement ratio up to 30%.