## 3 Results and Discussion
### Hardened Characteristics: Compressive Strength and UPV
#### 3.2.1 Influence of MgO, MWCNT and rGO on Compressive Strength Development
The geopolymers solidify quickly and generate C-S-H and C-A-S-H gels, leading to considerable chemical, autogenous, and drying shrinkage at the early stage. The highly reactive MgO accelerates the early age hydration of slag as a result of fast heat release during the dissolution process of MgO, which accelerates the hydration reaction, leading to the formation of more hydration products [57]. The addition of fine reactive MgO generated a large number of well-dispersed, fine, worm-like Mg(OH)\({}_{2}\) crystals in a high-alkalinity liquid phase environment, which did not overgrow, resulting in uniform-volume micro expansion, effectively compensating for volume shrinkage in the hardening process, matching the shrinkage process, refining the matrix pore size, and increasing the compressive strength, as observed in this study [53]. Figures 8-10 show the evolution of the compressive strength of the MWCNT-, rGO-, MgO-MWCNT-, and MgO-rGO-incorporated AAECs with ages up to 180 days compared to their control counterpart. The strength increased with the increase in age and the trend of the rate of strength development was similar to those of the control and MgO-incorporated AAECs. Figure 6: Evolution of compressive strength of AAECs (without MgO/MWCNTs/rGO) with age. For the MWCNT-incorporated mixes, the compressive strength was increased (+ve value)/decreased (\(-\)ve value), respectively, with the use of 0.3% and 0.6% MWCNTs by 15.1-23.4% and 6.4-7.0% at 7 days, 7.0-15.6% and 3.2-16.9% at 14 days, 7.8-9.2% and 7.8 --14.8% at 28 days, 7.8-9.3% and \(-\)3.5-\(-\)5.8% at 56 days, and 7.1-8.3% and \(-\)3.8-\(-\)5.5% at 180 days (Table 4). The compressive strength decreased at a higher 0.6% MWCNT content at 28 days and beyond. For the rGO-incorporated mixes, the compressive strength was increased, respectively, with the use of 0.3% and 0.6% rGO by 28.8-31.5%  
Figure 8: Evolution of compressive strength with age for MWCNT-incorporated (**a**) binary and (**b**) ternary AAEC mixes with reagents 1 and 2. Figure 7: Evolution of compressive strength with age for MgO-incorporated (**a**) binary and (**b**) ternary AAEC mixes with reagents 1 and 2. Figure 9: Evolution of compressive strength with age for rGO-incorporated (**a**) binary and (**b**) ternary AAEC mixes with reagents 1 and 2.  
and 12.8-14.0% at 7 days, 14.6-20% and 6.5-8.9% at 14 days, 1-14.8% and 4.9-6.6% at 28 days, 9.4-11.9% and 4.2-5.3% at 56 days, and 8.6-10.6% and 3.8-4.7% at 180 days. The rGO incorporated mixes exhibited higher compressive development than their MWCNT counterparts at all ages, showing no decrease. For the MgO-MWCNT-incorporated mixes, the compressive strength was increased with the use of 0.3% and 0.6% MWCNT, respectively, by 52.4-54.3% and 38.6-40.4% at 7 days; 30.5-36.8% and 23.5-28.6% at 14 days; 24.8-30.1% and 20-23.8% at 28 days; 19.1-22.5% and 14.6-17.3% at 56 days; 16.9-19.6% and 12.8-15.0% at 180 days (Table 4). For the MgO-rGO-incorporated mixes, the compressive strength was increased with the use of 0.3% and 0.6% rGO, respectively, by 24.2-25.2% and 21.7-22.2% at 7 days; 35.1-42.8% and 25.3-31.5% at 14 days; 28.2-34.6% and 20.8-26% at 28 days; 22-26.2% and 15.8-19.2% at 56 days; 19.6-22.9% and 13.9-16.6% at 180 days. The MgO-MWCNT/rGO combination significantly increased the compressive strength development, showing a higher % increase in strength at all ages for the AAECs. Moreover, the negative influence of higher dosages (>0.3%) of MWCNTs was overcome by using a 5% MgO-0.3%/0.6% MWCNT combination, changing the decrease in compressive strength to an increase. Figure 11a-c shows the influence of the MWCNT, rGO, and MgO content on the 28-day compressive strength of AAECs. The increase in MgO content from 0 to 5% increased the 28-day compressive strength of the AAECs (Figure 11a). On the other hand, the 28-day compressive strength increased with the increase in MWCNT or rGO content to 0.3% then showed a decreasing trend up to 0.6% (Figure 11b,c).