The Influence of the Ageing Process on the Mechanical Properties of Cement Mortars with Nano-SiO2 Admixture Initially Subjected to Thermal Treatment †

: The research aimed to evaluate the effect of nano-silica (NS) on the strength recovery of the previously heated cement mortars. One hundred twenty cement mortar specimens were prepared with 1% to 5% of NS. The specimens were heated at temperatures ranging from 200 ◦ C to 800 ◦ C. Half of the samples, after heating and cooling, were mechanically tested. The other half of the specimens were stored for 24 months in 90% relative humidity. The presence of NS fostered partial recovery of the compressive strength of the heated mortars. The cement mortars regained more than 40% of their compressive strength after heating at 400 ◦ C and about 25% after heating at 600 ◦ C. The strength recovery phenomenon faded away at 800 ◦ C.