Case II. Multi-Scale Modeling of the Influence of Condensed Silica Fume Additions on the Diffusivities of Cement Pastes

• Silica fume influences concrete diffusivity in three ways: 1) by densifying the microstructure of the interfacial transition zones, 2) by reducing the overall capillary porosity for a fixed degree of cement hydration, and 3) by producing a pozzolanic C-S-H (formed from the reaction between silica fume and calcium hydroxide) that has an inherent diffusivity to chloride ions that is significantly lower than that of the C-S-H gel formed from conventional cement hydration.
• The influence of silica fume on concrete diffusivity is a strong function of w/c ratio (see Fig. 10.3.8). For low w/c ratio concretes, silica fume replacements on the order of 10% may reduce the concrete diffusivity by a factor of 15.

Figure 10.3.8. Computed multiplicative increase in diffusion resistance due to silica fume addition vs. w/c ratio for an assumed degree of hydration of 0.675. D(csf=0) is the computed diffusivity without silica fume, D is the computed diffusivity with various silica fume replacements. Filled data points are experimental data of Hooton and others (7.7% and 13.8%) and of Alexander and Magee (10%).