High volume fly ash cement composite modified with nano silica, hydrated lime and set accelerator

This paper presents the effect of nano silica, used individually and in combination with set accelerator or hydrated lime, on the properties of high volume fly ash (HVFA) cement composites, replacing 70 % of cement. Compressive strength test along with X-ray diffraction and thermogravimetric analysis were undertaken to study the effect of various elements on the physico-chemical behaviour and the pozzolanic activity of the blended samples. The addition of 5 % nano silica improved the 7 day strength of the blended sample by 76 % and by further increasing the nano silica content to 7.5 %, the 7 day strength increased by 94 %. However, at 28 days both 5 and 7.5 % nano silica modified samples showed approximately the similar improvement in strength i.e. 54 and 50 % respectively. With the addition of set accelerator to nano silica modified HVFA cement blend, there was a considerable reduction in both 7 and 28 day strengths. Addition of 5 % hydrated lime to HVFA blend, modified with 5 % nano silica showed no effect on the further improvement of strength. However, when 5 % hydrated lime was added to the 7.5 % nano silica modified HVFA blend, there was a considerable reduction in both 7 and 28 day strengths. This demonstrates that nano silica is highly effective in improving the strength of high volume fly ash cement blends when used alone, but when it is used in combination with either hydrated lime or set accelerator, shows no or negative effect on the development of strength.