Examining the Effects of Using Nano-silica on the Mechanical Properties of Sand-cement Mortar Under the Effects of Heat, Considering Cementitious Nanostructure Changes

The present research investigates the effects of nano-silica on strength parameters of sand-cement mortar at high temperatures. For this, sand cement mortar replacing 5, 10 and 15 wt% of cement with nano-silica was prepared. The mortar, having been processed at 3, 28 and 90-day ages, was subjected to 25, 100, 200, 400, 600 and 800 ºC, respectively. Effects of high-temperature rates on the physical and mechanical properties of the sand-cement mortar were examined by macrostructural experiments of compressive strength, weight loss and water uptake, as well as microstructural experiments using XRD and SEM. The research found that the macrostructural behavior of sand-cement mortar was highly dependent on microstructures and nanostructure cementitious changes when subjected to heat. At 600 ºC, the initial portlandite was fully degraded, which caused the CaO to form as water exited. At 800 ºC, in addition to alite (C3S) and Belite (C2S), β- Wollastonite was formed from the degradation of the C-S-H nanostructure. The addition of nano-silica improved the strength properties of the sand cement mortar against heat, with the compressive strength of the 28-day samples without nano-silica experiencing a 57% weight loss as the temperature rose to 800 ºC, decreasing from 31.1 MPa to 13.3 MPa. On the other hand, the compressive strength of the sand cement mortar samples containing 15% nano-silica experienced lesser strength loss (52%) at 800 ºC, decreasing from 40.2 MPa to 19.2 MPa.