Effects of nanosilica addition on increased thermal stability of cement-based composite

Recently, nanosilica has been widely suggested as an excellent supplementary cementitious material due to its superior reactivity in comparison to other types of siliceous materials. Nanosilica is reported to increase the residual compressive strength of mortar after exposure to high temperatures. The addition of nanosilica has also been reported to cause fundamental changes in hydration products, increasing the average chain length of calcium silicate hydrate (C-S-H) and volume fraction of high-density C-S-H in cement paste in addition to decreasing calcium hydroxide content. However, it is not well understood exactly how nanosilica addition increases thermal stability of cement-based composites. In this study, the effects of replacement of a small amount of cement with nanosilica on the degradation of cement paste exposed to various heating and cooling regimes were investigated. Following heat treatment of cement paste samples with and without nanosilica up to 500°C (935°F), two different cooling regimes (cooling down to room temperature of 23± 2°C [73.4 ± 3.6°F] and prolonged heat treatment at 50°C [122°F] for 3 days) were followed. The residual states of each sample were analyzed by compression test, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The experimental results show that replacing 5% cement by weight with nanosilica leads to a 7 to 20% higher residual compressive strength after exposure to elevated temperatures. Furthermore, maintained exposure at above ambient temperature for long periods of time after exposure to high temperatures caused severe damage only to paste samples that did not contain nanosilica. This damage was not seen in samples containing nanosilica. Reduction in calcium hydroxide content due to the pozzolanic activity of nanosilica seems to be the primary reason for the minimized level of damage by reducing the degree of carbonation of hydration products immediately following heat treatment.