## 3 Results and Discussion
### Microstructural Characteristics
#### 3.3.1 SEM/EDS Analyses
However, the compressive strength was higher for the binary mixes (B1, B2) than their ternary counterparts (T1 and T2) due to the higher fly ash C content, as described earlier. Figure 16: _Cont._  
The representative SEM and EDS images of AAECs with varying levels of MgO, MWCNTs, rGO, and reagents, as shown in Figure 16c-f, exhibited a denser microstructure and almost fully reacted matrices with some unreacted chemosphere and pedosphere FA particles. It was reported that the unreacted particles do not act as filler in the mixture but lead to an increase in the strength of the matrix with age [86]. Figure 16: (**a**,**b**) SEM micrographs and EDS analysis: (**a**) B1 and (**b**) B2 [61]. (**c**) SEM micrographs and EDS analysis of the mix B1M5. (**d**) SEM micrographs and EDS analysis of the mix B2M5. (**e**) SEM micrographs and EDS analysis of the mix B2C3. (**f**) SEM micrographs and EDS analysis of the mix B2R3. Similar reaction products and combinations (C-A-S-H, N-C-A-S-H, and C-S-H) are developed in GO-incorporated binary/ternary mixes depending on the reagent types, with the additional formation of Mg(OH)\({}_{2}\) with traces of MgO, which can be identified from the EDS and XRD analyses shown in Figure 16c,d. The presence of these dominant reaction products/compounds can be confirmed from the elements in the binary B1M5 (Ca = 10.3%, Si = 7.6%, Al = 3.3%, Mg = 4.0%, C = 23.4%, and O = 48.4%; Figure 16c) and in binary B2M5 (Ca = 15.3%, Si = 10.9%, Al = 3.6%, Mg = 4.5%, C = 16.8%, and O = 46.6%; Figure 16d) mixes, as noted in the EDS graphs. Partially hydrated round fly ash class C and angular GGBPS particles can also be observed in the SEM micrograph. Compared with the control mixes, AAEC mixes with 5% (wt%) MgO contain almost identical CaO/SiO\({}_{2}\) but a bit higher silica and alumina contents. Moreover, the addition of fine MgO results in a higher surface area, which eventually leads to a higher water absorption on the particle surface, leaving a less porous and denser matrix. These primary factors contributed towards an increase in compressive strength up to 25.2% MPa due to the introduction of MgO as an additive to the matrix. Moreover, the resulting denser matrix with lesser porosity was also validated by the higher UPV values of the MgO-incorporated mixes, as discussed earlier. The SEM results indicated that the hydration of MgO particles, when they are combined with water, forms Mg(OH)\({}_{2}\) crystals, which can produce self-expansive stresses on the walls of the concrete pores. Excessive nano-MgO additives will result in higher expansive stresses that push on the pore walls. Once the pores cannot locally restrain the stresses, macro cracks will occur in the matrix, limiting the dosage of MgO [87]. The presence of reaction compounds, as mentioned earlier, in the MWCNT-incorporated mixes (such as B2C3) above can be confirmed from the elements (Ca = 13.6%, Si = 7.66%, Al = 3.8%, Mg = 2.0%, C = 16.4%, and O = 51.3%) noted in the EDS graph. Figure 16e also shows some embedded MWCNT bridging in the crack area, which means that they did not react with the source materials or act in the polymerization process. The presence of MWCNTs in some areas of crack openings of the fractured surface bridging micro cracks, with the existence of good bonding between the 0.3% MWCNTs and the surrounding matrix, can be noted from the SEM images of the fractured surface of B1C3 mixes with pull-out of the MWCNTs. Such well-dispersed MWCNT bridging with good MWCNT-matrix bonding can also be attributed to the compressive strength increase (in 0.3% MWCNT mix), as well as the reaction products. This argument supports previous research [40]. As the geopolymerization proceeds, fly ash grains are dissolved in an alkaline environment, and the products diffuse into a pore solution with dispersed MWCNTs. The dissolved species then undergo polycondensation reactions to form amorphous geopolymer gel in which the nanotubes remain embedded.