## 4 Results and Discussion
### Flexural Strength  
The Design of Experiments and statistical analysis with Minitab 19 software at 95% confidence level and \(\alpha=5\%\) significance level were used. Variance analysis of flexural strength gave results as shown in Table 8 and Figure 9 of the residuals for the plots of the regression model. In Table 8, the lack of fit with \(p=1\) is much more significant than the significance \(\alpha=0.05\), meaning that the model displayed suitable experimental data. In addition, Figure 9 also provided a typical residual plot quite close to line zero. _Regression equation:_  
As with building the regression equation for compressive strength, the flexural strength (Y2) also has a relationship with the variables time, GO, and fly ash. The regression equation for the compressive strength of mortar was expressed as shown in Equation (2) below with the statistically significant variables, \(p\) value \(<0.05\), R-sq = 98.86%, and R-sq(adj) = 98.79%:  
\[ \begin{split}\text{Y2}=0.7038+52.34\text{ GO}+0.06898\text{ FA}+0.6836\text{ t}-788.1\text{ GO}^{2}-0.002917\text{ FA}^{2}-0.016618\text{ t}^{2}\\ -0.1296\text{ GO}\times\text{ FA}+0.3299\text{ GO}\times\text{ t}-0.000276\text{ FA}\times\text{t}\end{split}  \tag{2}\]  
where Y2 = flexural strength (MPa); t = time (in days); FA = fly ash content (in %); GO = Graphene oxide content (in %). Regression Equation (2) revealed the following observations:  
* The factor with the most significant positive impact on flexural strength was also the percentage of GO with a considerable influence weight of (+52.34), which means that \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **DF** & **Adj SS** & **Adj MS** & **F-Value** & \(p\)**-Value** \\ \hline Model & 33 & 554.492 & 16.803 & 588.22 & 0.000 \\ Linear & 9 & 551.706 & 61.301 & 2145.98 & 0.000 \\ Days & 2 & 502.641 & 251.320 & 8798.09 & 0.000 \\ GO (\%) & 5 & 18.028 & 3.606 & 126.22 & 0.000 \\ FA (\%) & 2 & 31.037 & 15.519 & 543.27 & 0.000 \\
2-Way Interactions & 24 & 2.786 & 0.116 & 4.06 & 0.000 \\ Days \(\times\) GO (\%) & 10 & 1.081 & 0.108 & 3.79 & 0.000 \\ Days \(\times\) FA (\%) & 4 & 1.057 & 0.264 & 9.25 & 0.000 \\ GO (\%) \(\times\) FA (\%) & 10 & 0.648 & 0.065 & 2.27 & 0.018 \\ Error & 128 & 3.656 & 0.029 & & \\ Lack-of-Fit & 20 & 0.066 & 0.003 & 0.10 & 1.000 \\ Pure Error & 108 & 3.590 & 0.033 & & \\ Total & 161 & 558.149 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Analysis of variance of flexural strength. Figure 9: DoE on flexural strength of hardened cement mortar. under the condition that other variables did not change when considered with 0.01% GO, then the flexural strength of the mortar will be increased by 0.5234 MPa. * Additionally, the flexural strength of mortar was significantly affected by time and fly ash content with an influence weight of (+0.6836) and (+0.06898), respectively. For every additional day, the flexural strength increases by 0.6836 MPa, while for every 1% increase in fly ash content, the flexural strength increases by 0.06898 MPa. Furthermore, the results in Table 5 also show that the rate of increase in flexural strength of the samples at 3 and 7 days compared to 28 days was different depending on the component content in the mixture. Specifically, at 3 days, the flexural tensile strength reached approximately 38.31% to 43.87%, while at 7 days, it reached 66.71% to 71.49%. For mortar samples containing GO, the change in flexural strength was also clear depending on the GO content and fly ash content used. The flexural strength for mortar samples with a GO content also changed remarkably from the usage level of 0.01 wt% up to 0.03 wt%, reaching the highest level. It is worth noting that when increasing the GO content to higher than 0.04 wt% and 0.05 wt%, the flexural strength tended to decrease, similar to the results of the compressive strength. The flexural strength within the mortar is directly correlated with its overall strength, but the parameters of the interfacial transition zone influence the compressive strength.