CHARACTERISTICS OF SILICA-FUME CONCRETE

Proper introduction of silica fume in concrete improves both the mechanical and durability characteristics of the concrete. This paper presents the results of research effort conducted at the American University in Cairo using Egyptian silica fume in concrete. The program investigated various characteristics of silica-fume concrete. It emphasized the effect of silica fume on workability level and its maintenance of fresh concrete; strength development, strength optimization and elastic modulus of hardened concrete; and chemical and mechanical durability of mortar. The experimental program comprised six levels of silica-fume contents (as partial replacement of cement by weight) at 0% (control mix), 5%, 10%, 15%, 20%, and 25%, with and without superplasticizer. It also included two mixes with 15% silica fume added to cement in normal concrete. Durability of silica-fume mortar was tested in chemical environments of sulfate compounds, ammonium nitrate, calcium chloride, and various kinds of acids. It was found that there was an optimal value of silica-fume content at which concrete strength improved significantly. Due to the slow development of pozzolanic effect, there was a drop in early strength up to seven days and late significant gains up to 56 days upon introducing silica fume to concrete. Elastic modulus, toughness, and steel-concrete bond increased at the optimum silica-fume content in concrete. Silica-fume mortar exhibited significant improvement in durability against chemical attacks of most salts and acids. The improvement was moderate in the case of sulfate compounds. Mechanical erosion resistance increased moderately in silica-fume concrete.