## 3 Results and Discussion
### Microstructural Characteristics
#### 3.3.1 SEM/EDS Analyses
Some of the nanotubes are partially pulled out of the matrix during the breaking/fracture process, even though they are well bonded to the aluminosilicate matrix via hydrogen bonds of carboxymethyl cellulose [40]. MWCNTs can be distinguished in some areas in crack openings of the fractured surface in Figure 16e,f. A good bonding between the 0.3% MWCNTs and the surrounding AAEC is shown obviously in the SEM images of the fractured surface of B1C3 mixes. The images indicate the pull-out of MWCNTs and show that many MWCNTs are bridging the micro crack of the AAECs. The compressive strength increased with an increasing MWCNT concentration up to 0.3 wt%, and then subsequently dropped to a strength close to that of the cube specimens at 0.6 wt%. This indicates that a percolation threshold was achieved at 0.3 wt%, where there was no significant increase in the strength at MWCNT contents above 0.5 wt% due to agglomeration of nanotubes. The increase in strength was achieved by the cube specimens containing 0.3 wt% and 0.6% MWCNTs, with a 9.2% increase and 10.8% decrease for the B2C3 and B2C6 mixes, respectively. The presence of the reaction compounds, as mentioned earlier, can be confirmed from the elements (Ca = 22.6%, Si = 11.1%, Al = 5.0%, Mg = 2.0%, C = 8.6%, and O = 41.8%) noted in the EDS graph (Figure 16f) for the rGO-incorporated AAEC mixes (such as B2R3). rGO reduces the porosity of the matrix by accelerating the growth of geopolymer gel and the filling of nanoscale pores with self-adsorbed zeolites (Na\({}_{2}\)Al\({}_{2}\)Si\({}_{3}\)O\({}_{8}\cdot\)2H\({}_{2}\)O) and other materials, as shown in Figure 16f. The process of forming zeolites within the geopolymer involves the absorption of energy [88], and factors including the activation energy and mass transfer rate influence its rate. Graphene oxide (GO) is a material with high electron activity that attracts free ions in the gel and speeds up the mass transfer rate during crystal growth [16]. When GO comes in contact with the crystal nucleus, it is likely to aid in the growth of the zeolite-like phase and promote the formation of a monocrystalline layer [89]. This is also a contributing factor for the compressive strength increase of rGO-incorporated mixes, as observed in this study. Overall, as evident from the EDS analyses, the carbon content varied from 1% to 23%, showing especially higher values in the MWCNT- and rGO-incorporated AAECs, as expected. In general, the mixes with reagent 2 exhibited a denser microstructure, as evident from the formation of crystalline C-A-S-H with additional C-S-H gels, than their counterparts with reagent 1 (developing C-A-S-H and comparatively amorphous N-C-A-S-H)d due to the higher calcium content in the mix compositions. This resulted in enhanced mechanical and durability characteristics in terms of the higher compressive strength, lower shrinkage/expansion, and better resistance to freeze-thaw cycles of mixes with reagent 2 compared to their counterparts with reagent 1 [1]. The formation of reaction products on the PVA fibers can be observed in Figure 15a-d. The primary reaction products/acute phases developed on the fibers consisted of C-A-S-H with si-al solid linkages. Similar observations were made in previous studies where the adhesion or chemical bonding of the reaction products with the fibers was improved by incorporating fly ash and slag in geopolymer composites [4].