## 3 Results and Discussions
### Flexural Strength  
The flexural strength tests were performed on 100 \(\times\) 100 \(\times\) 400 mm prisms after 28 days of curing following the ASTM C78/78M-16. The flexural strength values of GPC along with the compressive strength values are presented in Table 7. The effect of NaOH solution concentration, slag content, alkaline activator content, SS/SH ratio, and inclusion of extra water on flexural strength were studied. The influence of these mix design variables on 28 days flexural strength of FS-GPC is shown in Figure 10. It can be observed that the results of flexural strengths for all mixes follow the trends similar to that of the compressive and split tensile strength results. The comparison between groups A and C (Figure 10) showed that the flexural strength of GPC mixes increased by increasing the slag replacement levels (10% to 50%) and decreasing the amount of AAS content (40% to 35%). The specimens of group B prepared with 12 M and 14 M NaOH solution showed higher flexural strength in comparison to the corresponding specimens of group A (10 M NaOH solution). It can be observed from the results of groups A and D that the GPC mix showed increase in flexural strength by increasing SS/SH ratio from 2.0 to 2.5, while a decrease in strength was observed by increasing this ratio from 2.5 to 3.0. The flexural strength of group E specimens prepared with reduced AAS content (35%) and additional water content (12 kg/m\({}^{3}\)) was decreased as compared to the group C mixes with similar AAS content and without extra water. Figure 13 shows the compressive strength variations of group A mixes with different slag content. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Mix ID**} & \multirow{2}{*}{ \begin{tabular}{c} **Compressive** \\ **Strength,\(f_{cm}\) (MPa)** \\ \end{tabular}  } & \multicolumn{5}{c}{**Flexural Strength,\(f_{cf}\)(MPa)**} \\ \cline{3-7}  & & **Test** & \begin{tabular}{c} **[PERSON]** \\ **et al.** \\ \end{tabular}  & \begin{tabular}{c} **[PERSON] and** \\ **[PERSON]** \\ \end{tabular}  & \ \begin{tabular}{c} **Test/[PERSON]** \\ **et al.** \\ \end{tabular}  & \begin{tabular}{c} **Test/[PERSON] and** \\ **[PERSON]** \\ \end{tabular}  \\ \hline A1-S10 & 20.0 & 2.2 & 3.1 & 4.2 & 0.71 & 0.53 \\ A2-S20 & 40.0 & 3.1 & 4.4 & 5.9 & 0.71 & 0.53 \\ A3-S30 & 54.0 & 3.7 & 5.1 & 6.8 & 0.73 & 0.54 \\ A4-S50 & 65.0 & 4.1  
Generally, a strong relationship between flexural and compressive strengths of conventional concrete exists. Therefore, the flexural strength of conventional concrete can be predicted from the correlations suggested by the design codes and studies. ACI 318-14 and AS 3600, 2009 provide simple equations (Equations (7) and (8) respectively) to estimate the flexural strength of the OPC mixes. However, there is a need to validate the use of these equations for estimating flexural strength of GPC mixes. \[f_{ct,f}=0.62\ \sqrt{f_{c}^{\prime}} \tag{7}\]  
\[f_{ct,f}=0.60\ \sqrt{f_{c}^{\prime}} \tag{8}\]  
where, \(f_{ct,f}=\) characteristic flexural tensile strength of concrete. Figure 14 shows the influence of mix design variables on flexural strength of FS-GPC specimens at the age of 28 days. Figure 14: The influence of mix design variables on flexural strength of FS-GPC specimens at the age of 28 days. Figure 13: The compressive strength variations of group A mixes with different slag content. There are empirical relationships for GPC as well which were suggested by previous research studies to estimate the flexural strength. [PERSON] et al. (2011) used ACI 318-14 (Equation (7)) and AS 3600 (Equation (8)) to estimate the flexural strength of heat cured fly ash-based GPC and found that estimated values were lower as compared to their test results [47]. Based on their experimental results, a relationship (Equation (9)) was proposed by [PERSON] et al. (2011) to predict the flexural strength of fly ash-based GPC mixes. \[f_{ct,f}=0.69\ \sqrt{f_{c}} \tag{9}\]  
[PERSON] and [PERSON] (2017) used ACI 318-14 (Equation (7)), AS 3600 (Equation (8)), and [PERSON] et al.