# Development and Evaluation of Nano-Silica Sustainable Concrete
## 3 Results
### Scanning Electron Microscopy (SEM)  
SEM Images Analysis  
Figure 6A-C shows the secondary electron and backscattered electrons images of the 70% GGBS concrete mixtures (M1, M2, and M3), respectively, while Figure 7A-C shows the secondary electron and backscattered electrons images of the 30% GGBS concrete mixtures (M4, M5, and M6), respectively. The secondary electron images show the cementitious paste-aggregate interface and the microstructure of the concrete mixture. On the other hand, the backscattered electron images show the micro-cracks that took place in the cementitious paste-aggregate interface and other areas of the specimen. Generally, the 70% GGBS mixtures, due to incorporating 70% dosage of GGBS, have homogeneous microstructures and great interface structure, which is shown clearly in the SE and BSE image of M1 (Figure 6A). However, using 1% and 2% dosages of nano-silica further refined the microstructure and increased the bond strength of the cementitious paste-aggregate interface by nano-filling the micro-cracks and the pores, as shown in (Figure 6B,C). The 2%  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mix.** & **Charge Passing (Coulombs)** & **Pentrability Class** \\ \hline M1 & 775.5 & Very low (100 to 1000 Coulombs) \\ M2 & 505.5 & Very low (100 to 1000 Coulombs) \\ M3 & 367.0 & Very Low (100 to 1000 Coulombs) \\ M4 & 1458.3 & Low (1000 to 2000 Coulombs) \\ M5 & 614.7 & Very low (100 to 1000 Coulombs) \\ M6 & 967.0 & Very low (100 to 1000 Coulombs) \\ \hline \hline \end{tabular}
\end{table}
Table 5: RCPT results of all the mixtures.  
dosage of nano-silica had a more pronounced effect compared to 1% dosage of nano-silica in refining the microstructure and the interface structure of the 70% GGBS mixtures.  
Figure 6: _Cont._  
Figure 6: SE and BSE images of (**A**) M1, (**B**) M2, and (**C**) M3.  
Figure 7: _Cont._  
Therefore, as the nano-silica's dosage increased, the microstructure's homogeneity and bond strength of cementitious paste-aggregate interface increased. Furthermore, the SE and BSE images of the 70% GGBS concrete mixtures reinforced the drawn conclusion from the rapid chloride permeability test results that the incorporation of nano-silica improved the pore structure of the concrete, which results in the reduction of the ingress rate of water and chloride ions due to the packing ability of nano-silica. Thus, nano-silica incorporation improved the durability of the concrete. The SE and BSE images of the control mixture of the 30% GGBS concrete mixtures (M4) illustrate that the cementitious paste-aggregate  
Figure 7: SE and BSE images of (**A**) M4, (**B**) M5, and (**C**) M6.  
bond is as weak as the micro-crack in the interface is wide as shown in Figure 7A. Moreover, the microstructure contains micro-cracks and voids. The addition of 1% and 2% dosages of nano-silica improved the cementitious paste-aggregate interface's bond significantly, as there are almost no visible micro-cracks shown at 2.00 KX and 500 X SEM magnification in Figure 7B,C. Furthermore, the microstructure of M5 and M6 became more homogeneous, dense, and compact due to the nano-filler effect of nano-silica, which refined the pore structure of the 30% GGBS concrete mixtures. Additionally, the SE and BSE images of the 30% GGBS concrete mixtures show greater improvement in terms of cementitious paste-aggregate interface's bond due to the incorporation of nano-silica when compared to the 70% GGBS concrete mixtures. All in all, nano-silica addition increases the sustainability of the concrete by enhancing its durability.