# Development and Evaluation of Nano-Silica Sustainable Concrete
## 3 Results
### Compressive Strength  
The compressive strength test was performed after one day, three days, seven days, and 28 days of curing. The compressive strength test results, which are average of three specimens for each mixture, are shown in Table 3.  
For the 70% GGBS concrete mixtures, the incorporation of 1% of nano-silica at one day almost did not affect the compressive strength, while the incorporation of 2% of nano-silica at one day reduced the compressive strength by 17.3% for the control mixture (M1). After three days, adding 1% and 2% dosages of nano-silica to the 70% GGBS concrete mixtures reduced the compressive strength by 7.8% and 11.1%, respectively, when compared to the compressive strength of M1. Unlike the one-day and the three-day compressive strength results, the seven-day compressive strength of M3 was higher than M2 by 1.4 MPa. However, both seven-day compressive strength results of M2 and M3 were lower than the control mixture M1, as shown in Figure 3. At 28 days, the percentage decrease in compressive strengths of M2 and M3 compared to the control mixture M1 increased remarkably to 19.4% and 17.7%, respectively. The reduction in compressive strength that took place in the mixtures that incorporated nano-silica (M2 and M3) can be attributed to the consumption of calcium hydroxide, which was released by the Portland cement during the hydration process, by the high dosage of GGBS. Therefore, this would leave almost no chemical hydration between nano-silica and calcium hydroxide. In terms of compressive strength, it is not recommended to use nano-silica with the 70% GGBS concrete mixtures. For the 30% GGBS concrete mixtures, the addition of 1% and 2% dosages of nano-silica increased the one-day compressive strength by 2.8 MPa and 4.7 MPa, respectively, compared to the control mixture (M4). Unlike the one-day compressive strength, the combination of 30% GGBS and 1% nano-silica (M5) improved the three-day compressive strength more than the combination of 30% GGBS and 2% nano-silica (M6), as demonstrated by Figure 4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mix.**} & **1 Day–Compressive** & **3 Day–Compressive** & **7 Day–Compressive** & **28 Day–Compressive** \\  & **Strength (MPa)** & **Strength (MPa)** & **Strength (MPa)** \\ \hline M1 & 25.5 & 48.7 & 61.7 & 71.8 \\ M2 & 25.6 & 44.9 & 52.8 & 57.9 \\ M3 & 21.1 & 43.3 & 54.2 & 59.1 \\ M4 & 20.8 & 39.9 & 55.6 & 63.1 \\ M5 & 23.6 & 43.0 & 60.1 & 64.5 \\ M6 & 25.5 & 41.3 & 55.9 & 64.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Compressive strength test results. Figure 2: The unit weight of all the mixtures. At seven days, the addition of 2% nano-silica to the 30% GGBS concrete mixture (M6) almost had an equal compressive strength to the control mixture (M4) with a minor percentage increase of (0.5%). On the other hand, the addition of 1% nano-silica to the 30% GGBS mixture (M5) improved the seven-day compressive strength by (8.1%) in comparison to the control mixture (M4). At 28 days, M5 had a compressive strength of 64.5 MPa, which is 2.2% higher than the control mixture (M4), while M6 had a compressive strength of 61.4 MPa, which is 2.7% lower than the control mixture (M4). In general, the improvement in the compressive strength for the 30% GGBS concrete mixtures that incorporated nano-silica can be ascribed to the high pozzolanic activity and the nano-filler effect of nano-silica, which makes the concrete's microstructure denser, compact, and homogeneous. Most of the compressive strength's improvement took place within the first seven days. Furthermore, the 1% dosage of nano-silica had a greater impact on the compressive strength development of the 30% GGBS concrete mixtures compared to the 2% dosage of nano-silica. One possible exception is the 2% change in the compressive strength of the 30% GGBS concrete mixtures. Figure 4: The compressive strength results of the 30% GGBS concrete mixtures. Figure 3: The compressive strength results of the 70% GGBS concrete mixtures.