The microstructural and thermal characteristics of silica nanoparticle-modified cement mortars after exposure to high temperatures. Part I

This study presents an investigation of the effects of high temperature on the thermal and microstructural properties of cement mortars modified with nanosilica. In the first stage of the research, the effects of nanosilica (NS) and silica fume (SF) on the hydration and compressive strength of cementitious composites were compared. In the second stage, four different types of cement mortars, containing an optimal dosage of NS, were produced. Two of them contained a normal weight aggregate (quartz or limestone), whilst two contained a heavy weight aggregate (barite or magnetite). Specimens without NS were produced for control purposes. The specimens were exposed to 300, 450, 600 and 800°C, with their post-heating properties – including thermal conductivity, specific heat, solvent absorption and cracking behavior – analyzed. The results show that NS exhibits significantly higher reactivity with cement than SF. NS accelerates the cement hydration process and contributes more significantly to the 28 and 365 day compressive strength of mortar, as compared to SF. The incorporation of NS in a composite substantially decreases the amount of CH in the mixture and leads to the production of additional C–S–H gel phase, which improves microstructure. The study also shows that NS contributes to a decrement in the thermal conductivity and density of mortar, both prior to and after heating. The incorporation of NS has a beneficial effect on decreasing the deterioration rate of mortars after heating, by decreasing absorption rate and the amount of cracks in in them.