Synergistic effect of nano-silica and silica fume on hydration properties of cement-based materials

Synergistic effect of nano-silica and silica fume on hydration properties was studied in cement-based materials. In the present study, various tests were used to characterize hydration properties including hydration rate, non-evaporable water content, crystal phase analysis, pore size distribution, micrographs of hydration products and compressive strength. The results showed that nano-silica can significantly increase the non-evaporable water content and decrease calcium hydroxide (CH) content at 3 days, while the silica fume takes equal effect at 28 days. The addition of 0.8 mass% nano-silica and 12.5 mass% silica fume can increase the total hydration heat of cement paste by 48.49% compared with the control sample. Besides, the total porosity of hardened cement sample reduces by 6.14%, which finally results in the more compact matrix of hardened paste. In addition, the microstructure of hardened paste changes from porous to dense matrix at 3 days after nano-silica and silica fume modification. Silica fume concrete incorporated with 1.2 mass% nano-silica significantly increased the compressive strength for 3 days. Synergistic effect mechanism of nano-silica and silica fume on the filling effect enriches particle size distribution by combination of nanoscale fineness of nano-silica and micron scale fineness of silica fume. In the meantime, nano-silica can effectively remedy the insufficient activity of the silica fume in the early age.