## 4 Results and Discussion
### Evaluation Increment Strength of Hardened Mortar  
Figure 10 provides the results obtained from the percentage of increment strength of hardened cement mortar at 28 days. The inclusion of graphene oxide (GO) in the mixture resulted in a noticeable rise in both compressive and flexural strength. Furthermore, GO had a more pronounced impact on flexural strength compared to compressive strength.  
The chart shows that the level of change in compressive and flexural strength of the mortar samples was quite similar at 28 days of age. Additionally, the chart also shows that the flexural strength had a higher increase in intensity than the compressive strength compared to the standard sample (M-GOFAO); specifically, the highest increase in flexural and compressive strength at the M-G3 FA15 sample was 30.09% and 26.81%, respectively, for each type. This result was also quite close compared to recent studies [21; 24].  
At sample locations with GO components in the mixture, the level of strength increase was obvious, and was highest when the GO content used was 0.03 wt%. The intensity tended to increase when the GO content changed from 0.01 wt% to 0.03 wt%. Surprisingly, the intensity tended to decrease when increasing to 0.04 wt% and 0.05 wt%.  
Considering the aspect of fly ash in the mixture, the most reasonable content to use was 15 wt%, but if too much was used, about 30 wt%, the intensity tends to decrease clearly. In the sample with 0.03 wt% GO content and 30 wt% fly ash, the increase was only about 5.69% for flexural strength and 3.21% for compressive strength. This growth rate was not significant, and the change in intensity was not high compared to the control sample. Even with samples without GO or 0.01 wt% GO and 30 wt% fly ash in the mixture, there is an apparent decrease in strength, as in samples M-GOFA30 and M-G1 FA30. Most other samples, when the GO content increased, had a minimal difference in intensity compared to the control sample. Thus, the above experimental results partly prove the influence of GO and fly ash. The findings align with the pertinent study findings in the literature [20,22,24], which suggest that the inclusion of GO and FA leads to a significant increase in both compressive and flexural strength compared to the control sample. Additionally, study [24] notes that the compressive strength and the flexural strength decreased when the GO content was 0.05 wt%.  
This is not unexpected compared to previous studies on mortar with fly ash, where the amount of FA used was high (precisely 30%, in this study). At 28 days, the fly ash displayed its properties, and the compressive and flexural strengths of mortar M-GOFA30 (0 wt% GO and 30 wt% FA) were also much lower than those of the control sample. Meanwhile, the mortar sample M-GOFA15 (0 wt% GO and 15 wt% FA) had a slight increase in strength compared to the control sample, namely 6.59% and 5.19% for flexural strength  
Figure 10: Percentage of increment strength at 28 days.  
and compressive strength, respectively. This may be due to the reasonable amount of fly ash used. The result was also partly explained by the fact that fly ash participated in and promoted its role at a late stage. The increase in the long-term strength over time of concrete with fly ash is due to the pozzolanic reaction between Ca(OH)\({}_{2}\) produced from the hydration reaction of Portland cement continuing to react with the silica oxide (SiO\({}_{2}\)) in the ash. Flying to form C-S-H gel contributes to the increase in intensity.