# Development and Evaluation of Nano-Silica Sustainable Concrete
###### Abstract
The addition of nano-silica showed a modification to the inherently slower rate of gaining strength of concrete that contains class F fly ash. The physical penetration depth was decreased, which consequently decreased the conductivity. The specimens showed a significant reduction in porosity and threshold pore diameter. [PERSON] et al. [8] studied the performance of nano-silica modified high strength concrete at elevated temperatures. The main focus of this experiment was on the effect of elevated temperature on the compressive strength, tensile strength, spalling, and mass loss of high strength concrete modified with nano-silica. Six samples with different percentage dosages of nano-silica were considered in this experiment along with two samples without nano-silica. The performance of the nano-silica modified high strength concrete was measured by using (150 \(\times\) 100 mm) cylinders that were heated to 400, 600, and 800 \({}^{\circ}\)C at a rate of 20 \({}^{\circ}\)C/min. In general, the results of this experiment demonstrated that the mass loss is decreased as the dosage of nano-silica is increased, and that is due to the improvement in tensile strength, which helped in preventing spalling. Moreover, the nano-silica also increased the residual compressive strength of the heated specimens. [PERSON] et al. [9] investigated the effect of nano-silica and nano-alumina on frost resistance and the mechanical properties of normal concrete. Seven different mixtures were used in this experiment. NSC3, NSC5, and NSC7 denoted the concrete containing 3 wt%, 5 wt%, and 7 wt% nano-silica, by the weight of cement, respectively [9]. NAC1, NAC2, and NAC3 denoted the concrete containing 1 wt%, 2 wt%, and 3 wt% nano-alumina, by the weight of cement, respectively [9]. The specimens were subjected to numerous tests. The compressive strength was determined at seven, 28, and 120 days. The percentage of water absorption was obtained after 28 days of moisture-curing. Furthermore, the seven mixtures were subjected to cycles of freezing and thawing. The loss of mass, change in length, increase in water absorption, and reduction in compressive strength of specimens was measured after a specified number of freeze and thawing cycles [9]. The experiment results demonstrated that the addition of nanoparticles increased the compressive strength whether they were nano-silica or nano-alumina. However, the nano-silica addition showed a remarkable increase in the compressive strength compared to the addition of nano-alumina. For example, at 28 days, the NSC5 mixture showed an increase of 31.13% in the compressive strength, while the NAC3 mixture showed an increase of 8.00% in the compressive strength. Moreover, the mixtures that incorporated nanoparticles showed a remarkable decrease in the water absorption compared to the control specimen due to the nano-filler effect, which improved the pore structure of the concrete. The experimental results also showed that the addition of nanoparticles improved the frost resistance considerably. However, the frost resistance of concrete that incorporates nano-alumina was better than the concrete that incorporates the same amount of nano-silica. [PERSON] and [PERSON] [10] studied the effect of nano-silica on the properties of concrete. The partial replacement of Portland cement with nano-silica accelerated the hydration process and reduced the setting time due to the high pozzolanic activity of nano-silica. Nano-silica has a large specific surface due to its small particle size. In the process of concrete mixing, a large number of unsaturated bonds promoted the nano-SiO\({}_{2}\) to absorb more water molecules, which led to a decrease of a concrete slump [10]. Chemically, due to the pozzolanic activity of nano-silica, more C-S-H was produced in the concrete mixing process, which reduced the number of pores. Physically, due to the nano-filler effect of nano-silica, the microstructure of nano-silica-based concrete was more compact and less porous.