Effect of high-volume substituted nanosilica on the hydration and mechanical properties of Ultra-High-Performance Concrete (UHPC)

The effect of substituting a large amount of silica fume (SF) with nanosilica (NS) on the hydration behavior and mechanical properties of Ultra-High-Performance Concrete (UHPC) was investigated. Derivative thermogravimetric analysis showed that NS had the highest reactivity among the ingredients used in the UHPC mix. As the NS substitution rate increased, the time corresponding to the maximum exothermic peak decreased, and the maximum exothermic peak increased. Nuclear magnetic resonance results verified that the longest mean (silicate) chain length of C-S-H, which is 16.8 that it is improved approximately 68 % than plain specimen, was obtained at 10 % NS replacement rate. In the matrix (i.e., UHPC), the highest compressive strength is 161.5 MPa which obtained 10 % NS replacement rate, and the compressive strength was gradually decreased when NS replacement rate was increased more than 20 %. The highest compressive strength and the best tensile performance of UHP-FRC, i.e., the tensile strength and strain energy density, were also obtained at 10 % NS replacement, and there were improved approximately 2.2 %, 9.3 %, and 42.7 % compared to the plain specimen, respectively. However, the highest pullout strength and lowest porosity were achieved with 20 % NS replacement due to the densified fiber-matrix interfacial transition zone. These results suggest that the optimal rate of substitution of SF with NS in UHPC is 10 %-20 %.