## 3 Results and Discussion
### Hardened Characteristics: Compressive Strength and UPV
#### 3.2.2 Influence of MgO, MWCNT, and rGO on 28-Day UPV  
A UPV test was conducted to determine the quality of concrete, including detecting the presence of cavities, joints, and cracks in addition to other properties over time. A high UPV indicates a lower number of pores, that the particles are closer to each other, a higher matrix homogeneity, and a lower wave travel time. Table 3 summarizes the 28-day UPV values, which ranged between 3067 m/s and 3717 m/s, between 3762 m/s and 3954 m/s, between 3436 m/s and 4068 m/s, and between 3122 m/s and 3943 m/s for control and the MgO-, MWCNT-, and rGO-incorporated AAECs, respectively. Higher UPV values were obtained for the AAEC mixes containing MgO, MWCNTs, and rGO than their control counterparts (Table 3 and Figure 12a-c).  
Figure 12a shows that, by adding 5% MgO in the AAECs, the UPV values were increased from 6.4% (T2M5) to 27% (B2M5). The reason for this improvement was due to additional reactions and extra C-A-S-H formation, which densified the matrix and reduced the time for wave transmission. In addition, the formation of Mg(OH)\({}_{2}\) crystals produced uniform expansion and refined the pore size and pore structure, which also contributed to the higher UPV and compressive strength, as discussed earlier. These results supported the findings of previous studies by other researchers [22,82].  
Figure 12b,c shows that adding 0.3% MWCNT/rGO increased the maximum UPV values up to 12% (for B2C3). This indicated CNTs'/rGO's filling capability, whichreduces the porosity to produce a denser matrix (as discussed earlier), which in turn led to stronger UPV and conductive properties. However, the addition of 0.6% MWCNTs/rGO did not produce a linear increase in UPV; rather, it tended to produce lower UPV values, indicating that 0.3% MWCNT/rGO is the optimum concentration (as observed in the case of compressive strength), with the exception of B2C6 (showing an increase of 19% in UPV). This can happen due to the improper dispersion of MWCNTs/rGO at higher concentrations beyond the optimum doses in the AAECs. This finding supports the results of a previous study by [PERSON] and [PERSON] [83].  
Figure 13 shows a linear correlation (dotted line) between the UPV and compressive strength of AAEC mixes at 28 days. The observed trend reveals an increase in compressive strength with an increase in UPV, as expected. The increase in compressive strength often  
Figure 11: (**a–c**) Influence of (**a**) MgO, (**b**) MWCNT, and (**c**) rGO on 28-day compressive strength of AAEC mixes with reagents 1 and 2.  
corresponds to a denser and more homogeneous concrete matrix. A denser matrix transmits ultrasonic waves more efficiently, resulting in higher UPV values [84,85].  
It is important to note that, while Figure 13 provides a positive correlation, additional analysis considering influencing factors related to the mix design, MWCNT/rGO dispersion, and curing conditions needs to be conducted for a comprehensive understanding of the relationship between the UPV and compressive strength of AAECs. Nevertheless, the derived correlation suggests the potential use of UPV as a reliable non-destructive testing method for assessing the compressive strength of developed AAECs.