# Development and Evaluation of Nano-Silica Sustainable Concrete
###### Abstract
One possible reason is poor dispersion and agglomeration of small particles of 12 and 20 nm-SiO\({}_{2}\)[13]. The optimum replacement content of cement mortars with NS particle size of 12, 20, and 40 nm, as well as cement mortar with SF, was obtained with NS 9% by weight of cement, independent of NS particle size [13]. SEM photographs showed that the microstructure of the cement pastes was improved by the incorporation of nano-silica making the paste more compact, homogeneous, and denser. [PERSON] et al. [14] investigated the influence of nano-silica on the properties of recycled aggregate concrete. The properties of colloidal nano-silica used in this study are 1.12 specific gravity, 39% solid content, 8-20 nm particle size, 99.1% SiO\({}_{2}\) content, and 10.11 pH value. Eight different mixtures were cast, in which four of them contained natural coarse aggregate (NCA) and the other four contained recycled coarse aggregate (RCA). Multiple conclusions were obtained. As the percentage of nano-silica increased, the slump values decreased due to the high surface area of colloidal nano-silica, which causes absorption of mixing water by the nanoparticles. Furthermore, replacement of natural coarse aggregates with recycled coarse aggregates reduced the workability of the concrete mixture due to the high water absorption capacity of RCA, and a further decrease in workability was observed due to the addition of NS to RAC mixes [14]. Moreover, the addition of nano-silica enhanced the compressive strength results in early days because of the nano-silica's high pozzolanic activity at initial periods. A decrease of 14% of compressive strength was observed when replacement of NCA was done with 100% RCA [14]. However, the addition of NS enhanced the compressive strength of RAC, and with the incorporation of 3% NS, the 28 days compressive strength equalized with control concrete [14]. Compared to the natural coarse aggregate mixes, the recycled coarse aggregate mixes had weaker tensile strength. However, the decrease in tensile strength caused by using recycled coarse aggregate can be compensated by incorporating nano-silica. [PERSON] et al. [15] studied the influence of a nano-silica addition on the durability of ultra-high performance concrete (UHPC). The properties of nano-silica (NS) used in this experiment are (160 \(\pm\) 20 m\({}^{2}\)/g) specific surface area, (<99.9%) purity, amorphous crystal phase, (15 \(\pm\) 5 nm) diameter, (<0.15 g/cm\({}^{3}\)) density, and spherical morphology. Three different sets of mixtures were considered for this test, consisting of UHPC containing NS, UHPC without NS, and high-performance concrete (HPC) [15]. Based on the obtained results, multiple conclusions were drawn. First, UHPC-NS presented the best corrosion resistance performance, as the time to crack effectively increased with the NS addition [15]. Second, incorporating nano-silica contributed to extending the service life of concrete structures by delaying corrosion in steel rebars. Corrosion rate measurements, based on LPR and Tafel techniques, pointed out that the UHPC specimens containing NS addition had the lowest corrosion rate when compared with HPC and UHPC specimens [15]. [PERSON] et al. [16] conducted a study about the self-consolidating concrete (SCC) modification by the use of amorphous nano-silica. Three different SCC mixes were studied in the experiment, in which the first mix did not contain nano-silica, the second mix contained colloidal nano-silica, and the third mix contained powder nano-silica. Under the laboratory conditions, the compressive and tensile splitting strength of the reference SCC was improved by the addition of both types of nano-silica [16]. The colloidal nano-silica SCC mix had higher compressive strength and lower splitting tensile strength compared to the powder nano-silica SCC mix.