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Since concrete is a composite material, the interfaces between components can be expected to have major effects on physical properties. In ordinary portland cement concrete, the interfacial zone between cement paste and aggregate has been shown to exhibit characteristics greatly differing from those of the bulk paste. The addition of mineral admixtures to the mix has been shown to significantly alter this interfacial zone microstructure and enhance physical properties of the composite. In this paper, a direct comparison is made between results obtained using a three- dimensional microstructural model and those obtained experimentally on a similar set of mixes containing various amounts of silica fume. Quantitative measurements of backscattered electron images of the interfacial zone in the real materials are compared to model results. The model reproduces the experimentally-observed characteristics of the interfacial zone, which are quite different with and without the presence of silica fume. Based on the model and experimental results, it is suggested that the presence of silica fume produces a more homogeneous microstructure by balancing the Ca/Si molar ratio in the interfacial zone relative to that in the bulk paste, unlike ordinary portland cement concrete where this ratio increases dramatically as the aggregate surface is approached.