## 3 Results and Discussion
### Hardened Characteristics: Compressive Strength and UPV
#### 3.2.1 Influence of MgO, MWCNT and rGO on Compressive Strength Development  
All AAECs achieved a 28-day compressive strength ranging from 26.0 MPa to 48.5 MPa \(>18\) MPa (as per Tables 3 and 4), which is the range specified for structural concrete as per ACI 318 [81], and which is also comparable to other conventional two-part FA and GGBFS-based geopolymers [4; 61]. The binary AAECs (FA-C + GGBFS designated as B) that had higher CaO/SiO\({}_{2}\) ratios (as presented in Table 2) obtained higher compressive strengths than their ternary (FA-C + FA-F + GGBFS designated as T) counterparts (Table 3) due to the formation of additional CSH binding phases/gels with C-A-S-H/N-C-A-S-H, as is evident from the microstructural analysis (discussed later). All the mixes exhibited a ratio of Na\({}_{2}\)O/Al\({}_{2}\)O\({}_{3}\) of less than 1 (Table 2), which prevented efflorescence, as observed in previous studies where Na\({}_{2}\)O/Al\({}_{2}\)O\({}_{3}\)\(<1\) resulted in sodium consumption in the reaction process and, in turn, prevented efflorescence. The evolution of the compressive strength with the age (0, 7,14, 28, 56, and 180 days) of control AAEC mixes (B1, B2, T1, and T2) without any MWCNTs/rGO or MgO are presented in Figure 6 and Table 4. The compressive strength increased with age, and the rate of increase was higher than that of controls up to 56 days (due to a higher rate of polymerization) and then slowed down, showing a steady increase up to 180 days (Figure 6). The compressive strength was higher for the binary mixes (B1, B2) than their ternary counterparts (T1 and T2) and, also, the reagent 1 mixes developed a lower strength than their reagent 2 counterparts throughout the aging process. Binary mix B2 with reagent 2 exhibited the highest compressive strength (with 40.8 MPa, 48.1 MPa, and 52.6 MPa at 28 days, 56 days, and 180 days, respectively) throughout the aging followed by B1, T2, and T1. Reagent 2 (Ca(OH)\({}_{2}\):Na\({}_{2}\)SO\({}_{4}\) = 2.5:1) performed better than reagent 1 (Ca(OH)\({}_{2}\):Na\({}_{2}\)SiO\({}_{3}\):5H\({}_{2}\)O = 1:2.5) in terms of its compressive strength development. The reagent 2 component ratio of Ca(OH)\({}_{2}\):Na\({}_{2}\)SO\({}_{4}\) = 2.5:1 was judged to be the optimal composition based on the strength results (52.6 MPa at 180 days) of binder system B2. The high alkalinity of sodium metasilicate (pH = 14) enhanced the dissolution of Si and Al ions from the FA-F, which otherwise showed low reactivity. The high-calcium precursors (GGBFS and FA-C) also formed C-A-S-H acute phases, which compacted the amorphous N-A-S-H, resulting in a dense microstructure and high compressive strengths. However, for reagent 2, a Ca(OH)\({}_{2}\)/Na\({}_{2}\)SO\({}_{4}\) = 2.5:1 component ratio was determined to be superior, and also outperformed reagent 1, as depicted in Figure 6. The additional C-S-H gel formation from the composition with reagent 2 led to higher compressive strengths due to the high calcium levels in the system. B2 exhibited a higher strength (28% higher at 28-days) than its counterpart T2 due to its 30% higher fly ash C content. This can also be attributed to the high Ca\({}^{2+}\) content in the matrix, coming from the FA-C, the GGBFS, and the activator Ca(OH)\({}_{2}\) component, which formed a denser microstructure. The MgO-incorporated AAEC mixes (B1M5, B2M5, T1M5, T2M5) exhibited higher strength development with age (Figure 7) compared to their control counterparts (B1, B2, T1, and T2). Adding 5% MgO in the AAEC mixes increased the compressive strength (for example, 19-20% and 23.1-25.2% at 28 days for the binary and ternary mixes, respectively, Table 4). Overall, compared to the control mixes, strength increases of 37.1-40.4%, 22.7-31.1%, 18.9-25.25%, 14.15-19.05%, and 12.45-15.4% were observed at 7, 14, 28, 56, and 180 days, respectively (Table 4), showing strength increases with increases in the age and that the % of strength increase decreased with increases in the age.