The compressive strength of silica fume concretes was investigated at low water-cementitious materials ratios with a naphthalene sulphonate superplasticizer. The results show that partial cement replacement up to 20% produce, higher compressive strengths than control concretes, nevertheless the strength gain is less than 15%. In this paper we propose a model to evaluate the compressive strength of silica fume concrete at any time. The model is related to the water-cementitious materials and silica-cement ratios. Taking into account the author's and other researchers’ experimental data, the accuracy of the proposed model is better than 5%. 1. Introduction The use of silica fume in combination with a superplasticizer is now a usual way to obtain high-strength concretes. The improvement of mechanical properties of concretes with silica fume accounts for the increasing consumption of this admixture in concrete. Furthermore, apart from mechanical properties, the durability of high-performance concretes concerning the most common harmful ions (sulfate, chloride, and seawater) is also improved; indeed the reduction of permeability which is due to the more compact microstructure of concrete slows down the diffusion of ions. Nevertheless various authors point out some drawbacks regarding the use of silica fume in concretes. Among these, the loss of plasticity during the production of concrete and the great sensitivity to plastic shrinkage during the initial curing are the most important. However, researchers seem to disagree about the interpretation of the exact role silica fume plays in the increase of mechanical strengths. Some authors claim that silica fume improves the strength of the bond between the aggregates and the cement matrix [1–5]. The partial replacement of cement by silica fume increases the strength of mortar and concrete; yet it does not seem to have an important impact on the strength of pure cement paste. To other researchers, however, the positive result due to the admixture of silica fume stems from the increase in strength of the cement matrix [6, 7]. Researchers also disagree about the definition of the optimal content of silica fume which enables to obtain the highest strengths. To some researchers [8, 9], the content is about 15% whereas to others [5, 10] the increase in compressive strength may be reached at 30% to 40% of replacement of cement by silica fume. In this study we aim at defining the influence of the content of silica fume on the compressive strength of concrete. Moreover, we introduce a prediction model of the compressive
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