Experimental and computer simulation studies have provided a clearer understanding of the role of silica fume in reducing the diffusivity of cement-based materials. The results are consistent with an inherent reduction in the diffusivity of the pozzolanic C-S-H relative to that of the conventional C-S-H. The reduction appears to be on the order of a factor of five for the diffusion of tritiated water and a factor of 25 for the diffusion of chloride ions. The microstructural models suggest that in systems containing silica fume, at high (> 20 %) capillary porosities, the diffusivity is regulated by the percolated capillary pore network, while at low (< 20 %) capillary porosities, it is controlled by the volume fractions and percolation characteristics of the two types of C-S-H.
Considering the reduced capillary porosity present at equal degrees of cement hydration in the systems containing silica fume, along with the change in the nanostructure and diffusivity of the C-S-H, after equal hydration times, a 10 % addition of silica fume could result in a chloride ion diffusivity more than fifteen times less than that in a comparable concrete made without silica fume. While the tendency of silica fume to detrimentally reduce the chloride threshold necessary to initiate corrosion must also be considered, in certain circumstances, this reduction in diffusivity should translate directly into a fifteen-fold increase in the service life of the exposed concrete, resulting in substantial life cycle cost savings for concrete construction.