Influence of silica fume on diffusivity in cement-based materials II. Multi-scale modeling of concrete diffusivity

Based on a set of multi-scale computer models, an equation is developed for predicting the chloride ion diffusivity of concrete as a function of water-to-cement (w/c) ratio, silica fume addition, degree of hydration and aggregate volume fraction. Silica fume influences concrete diffusivity in several ways: (1) densifying the microstructure of the interfacial transition zone (ITZ) regions, (2) reducing the overall (bulk and ITZ) capillary porosity for a fixed degree of cement hydration, and (3) producing a pozzolanic C-S-H gel with a relative diffusivity about 25 times less than that of the C-S-H gel produced from conventional cement hydration. According to the equation and in agreement with results from the literature, silica fume is most efficient for reducing diffusivity in lower w/c ratio concretes (w/c < 0.4). In these systems, for moderate additions of silica fume (e.g., 10%), the reduction in concrete diffusivity may be a factor of 15 or more, which may substantially increase the service life of steel-reinforced concrete exposed in a severe corrosion environment. Published by Elsevier Science Ltd. All rights reserved.