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Summary

Experimental studies have been applied to verifying a microstructural model for simulating the development of microstructure in the interfacial zone region in concrete. Good agreement between experimental and model results has been exhibited. Based on the simulation and experimental results, silica fume has been shown to be effective in improving at least two aspects of interfacial zone microstructure. The ability of the small silica fume particles to pack efficiently against the aggregate surface followed by the subsequent pozzolanic reaction results in a much more homogeneous distribution of total (C-S-H and cement) phases. This increased homogeneity of the primary load-bearing matrix phase should increase strength by improving the weak link present in ordinary portland cement concrete. Additionally, the presence of silica fume reduces the porosity gradient normally present in the interfacial region. This may result in increased durability of concrete containing silica fume, due to decreases in transport coefficients like ionic diffusivity and fluid permeability. The increased homogeneity of microstructure is exemplified by the homogeneity of the Ca/Si molar ratio as a function of distance from the aggregate surface. The large increase observed in this ratio as the aggregate surface is approached in ordinary portland cement concrete is practically eliminated by the addition of silica fume at either of the levels examined in this study (10 or 20% by weight of cement replaced).