The present work was accomplished to investigate silica fume (SF) and nano silica (NS) effects on the compressive strength (f cu28) of concrete made by cement contents (CC) of 300, 400, 500 and 600 kg/m 3. Two NS types having purity of 89% (Type I) and 99% (Type II) were partially replaced CC by the percentages of 1.5% and 3%. The influence of replacing cement by 5% and 10% SF was also studied. Nano silica was mixed by two techniques, mechanically and by ultrasonic device. The influence of SF, NS and CC on f cu28 and water absorption (WA) was found. The obtained results indicated that f cu28 increased and WA% reduced by replacing part of CC by either NS or SF. For both of NS types, the enhancement ratio in f cu28 was higher when replacing cement by 1.5% NS compared to replacement ratio of 3% and both ratios recorded greater values of compressive strength when compared to that without NS. At 1.5% NS ratio, f cu28 increased by about 14.81% for type I and by 41.33 for type II while at 3% NS, the enhancement ratios were 5.86% for type I and 35.46 for type II respectively. Mixing Type I NS by ultrasonic mixing method recorded higher values for f cu28 as compared to those recorded by the mechanical method.

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This paper presents an experimental investigation on the effect of Nano Silica (NS) on the compressive strength of concrete made with different cement contents. Two percentages of NS, 1.5% and 3%, as partial replacement of cement were investigated at cement contents of 300, 400, 500 and 600 kg/m 3. The effect of NS and cement contents on the compressive strength of concrete at different testing ages of 7, 28 and 56 days was evaluated. The effect of NS and cement contents on the water absorption percentage (WA%) was also investigated. The effect of cement content on the compressive strength ratios at 7 and 56 days to that at 28 days was found and correlated. The effects of cement content on the enhancement ratios of compressive strength and WA% were also evaluated and correlated. NS addition enhanced the compressive strength and reduced the WA%. The maximum enhancement in the compressive strength was recorded at 1.5 % NS. As this ratio increased to 3%, the compressive strength decreased but to a values not less than that of the concrete mix without NS.