Influence of using different surface areas of nano silica on concrete properties

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Abstract
This paper aims to evaluate the influence of surface area and amount of nano silica (NS) on the performance of concrete with different water/binder (w/b) ratios. For this purpose, 63 different mixes were produced using three different types of colloidal NS having various surface areas (52, 250 and 500 m2/g) and water/binder ratios (0.4, 0.5, 0.6). By varying the replacement content of NS between 0.25% and 10% by weight of cement, the optimum ratio for each type was determined. Compressive strengths at 7 and 28 days of concrete mixture with various NS were measured. Moreover, the microstructure of concrete samples was analysed by the scanning electron microscopy and XRD analysis was also carried out for concrete samples. The results indicated that the performance of NS particles in concrete is significantly dependent on its amount and surface area as well as water/binder ratio. The influence of NS on concrete compressive strength was more pronounced at 7 days for all mixes than 28 days due to the ultra-high surface area of NS, accelerating the hydration process. As the w/b ratio increases, a better performance was observed for all types of NS used, whilst NS having 250 m2/g surface area was found to be the most effective. The optimum amount of NS was ranged between 2% and 5%, depending on NS surface area; as both NS particle size and w/b ratio increase, the optimum amount of NS also increases.

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