Effect of particle size of nanosilica on microstructure of C-S-H and its impact on mechanical strength

The present work is focused on the effect of particle size of silica on its early age reactivity and its impact on C-S-H microstructure and mechanical strength at the early and later ages of hydration. Two different types of commercially available silica i.e. Elk-microsilica (Elk-Si) (100-300 nm) and colloidal nanosilica (CNS) (8-15 nm) were used for the study. The lime silica reaction was carried out with a C/S ratio 2.0 and the reaction was monitored in the first 24 h with a range of instrumentation techniques. The results showed that the CNS had similar to 60% higher reactivity than the Elk-nSi. Calorimetric results revealed that in the presence of CNS, the hydration rate and the formation of AFm phase were accelerated. Furthermore, FTIR and NMR results revealed that in the presence of the CNS, the structure of C-S-H gel was also affected. This acceleration and modification C-S-H gel led to the formation of a compact microstructure as the capillary porosity of cementitious system measured by MIP was found to reduce significantly. The formation of a compact microstructure at the early age of hydration improved the early age mechanical strength. However, it hindered the later age hydration and thus more unhydrated cement was observed in the semi-quantitative XRD and BSE/IA analysis, especially in the presence of higher dosages of CNS, which was responsible for the lower long term mechanical strength.