Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

It is well recognized that the use of mineral admixtures such as silica fume enhances the strength and durability of concrete. This research compares the effects of adding silica fume and nanosilica to concrete and provides a better understanding of the changes in the concrete nanostructure. Nanoindentation with scanning probe microscopy imaging was used to measure the local mechanical properties of cement pastes with 0% and 15% replacement of cement with silica fume. A reduction in the volume fraction of calcium hydroxide in a sample with silica fume provides evidence of pozzolanic reaction. Furthermore, replacing 15% cement by silica fume increased the volume fraction of the high-stiffness calcium silicate hydrate (C-S-H) by a small percentage that was comparable with the decrease in the volume fraction of calcium hydroxide. A parallel study of cement pastes with nanosilica showed that nanosilica significantly improves durability of concrete. This research provides insight into the effects of nanosilica on cement paste nanostructure and explains its effect on durability of concrete. The nanoindentation study showed that the volume fraction of the high-stiffness C-S-H gel increased significantly with addition of nanosilica. Nanoindentation results of cement paste samples with similar percentages of silica fume and nanosilica were compared. Samples with nanosilica had almost twice the amount of high-stiffness C-S-H as the samples with silica fume.