# Development and Evaluation of Nano-Silica Sustainable Concrete
## 4 Conclusions  
This research presents the influence of nano-silica on the properties of the 70% and 30% GGBS concrete mixtures in terms of strength and durability. Fresh and hardened concrete tests along with the rapid chloride permeability test and scanning electron microscopy were applied to each mixture. Based on the obtained results, the following conclusions were drawn:  
* Adding 1% of nano-silica to the 30% (GGBS) mixture (M5) increased the one-day, three-day, seven-day, and 28-day compressive strength by 13.5%, 7.8%, 8.1%, and 2.2%, respectively, compared to the control mixture (M4), whereas adding 2% of nano-silica to the 30% (GGBS) mixture (M6) had less influence on the strength development compared to the 1%. This is possibly due to either the agglomeration effect, the mixing process, or the released calcium hydroxide from the OPC during the hydration process being consumed more efficiently by the combination of 30% (GGBS) and 1% nano-silica. The majority of the compressive strength's development, which was caused by the addition of nano-silica, was within the first seven days. On the other hand, adding 1% and 2% of nano-silica to the 70% (GGBS) mixtures decreased the 28-day compressive strength by 19.4% and 17.7%, respectively, compared to the control mixture (M1). The reduction in compressive strength associated with the addition of nano-silica could be due to the released calcium hydroxide from the OPC in the hydration process being consumed entirely by the 70% (GGBS). * Nano-silica increased the Young's modulus due to the nano-filler effect, which reduced the concrete's deformability, making it brittle. * The modulus of rupture of the 70% (GGBS) mixtures (M2 and M3) increased by 3.4% and 35.9% due to the incorporation of 1% and 2% of nano-silica, respectively. On the other hand, the flexural strength of the 30% (GGBS) mixtures had similar behavior as the 28-day compressive strength. * The RCP test results illustrated that adding nano-silica to the 70% and 30% (GGBS) mixtures reduced the chloride ingress due to the high pozzolanic activity and the packing effect of nano-silica, which made the microstructure of concrete more homogeneous and less porous. * The SEM images show that adding nano-silica increased the bond strength of the cementitious paste-aggregate interface and made the microstructure more homogeneous due to the nano-filler effect and the accelerated hydration process. The effect of adding nano-silica on the microstructure and the interface structure was more pronounced on the 30% (GGBS) mixtures in comparison to the 70% (GGBS) concrete mixtures. The scanning electron microscopy (SEM) images confirmed the drawn conclusion from the (RCPT) results that nano-silica improves the concrete's durability. * Nano-silica's particles are small, which makes its specific surface large. Consequently, its water demand is large. Comparing the nano-silica mixtures to their respective control mixtures showed that the addition of nano-silica reduced the slump and the flow spread values, which indicated that nano-silica had a negative influence on the workability of the concrete due to its high water demand. Furthermore, as the nano-silica dosage increased, the workability of mixtures decreased. Moreover, comparing the slump and flow results after thirty and sixty minutes to the initial values demonstrated that nano-silica accelerated the hydration process and reduced the setting time. * Due to its high resistance to water penetration and chemical attacks, nano-silica concrete can be used in construction of marine and coastal structures where the reinforced concrete is subjected to harsh environments. Furthermore, it can be used in construction of high-rise buildings, because it allows thereduction in sizes of reinforced concrete members such as columns. [PERSON] was responsible for conducting the laboratory experiments and analyze the re-sults. [PERSON] was responsible to advice and discuss the results. All authors have read and agreed to the published version of the manuscript.