*3.3. Compressive Strength*

The compressive strength of the AAFA specimens through underwater placement was evaluated on 3rd, 7th, and 28th days. All specimens displayed increment in strength with respect to aging days as per Figure 4. The AAFA specimens cast in seawater displayed the highest compressive strength for all the aging days; for example, on 3rd days, the AAFA specimens reported a strength of 36.8 MPa. Meanwhile, the lowest compressive strength was found in the specimens cast in river water with strength 34.6 MPa on 3rd days. For control specimens (dry condition), the compressive strength on 3rd days is 79.1 MPa. The specimens cast in seawater recorded a decrease in their compressive strength by 54% compared to the specimens cast in dry condition for 3rd days of testing.

The compressive strength of the AAFA specimens cast in seawater was reported to be 46.0MPa on the 7th days, whereas its dry counterpart has a reported strength of 79.9 MPa. This translates to a 20% strength increment in the AAFA specimens cast in seawater from day 3 to day 7. However, the strength increment from day 7 to day 28 was even higher, which is 55%. In contrast, the 28 day's strength of the specimens cast in dry condition was found to be 80.9 MPa, indicating that it has a lower strength increment. According to Kumar et al. [43], the increment of strength with respect to time can be attributed to calcium silicate hydrate (C-S-H) formation. Further discussion about the reaction product will be provided in Section 3.4.

The AAFA cast in dry condition exhibited almost complete strength development within 3 days, as the strength increment from 3rd days to 28th days testing was only 2%. For the AAFA cast in water, the strength slowly increased from 3rd days to 28th days for all water types. In the case of the 28-day strength, the AAFA specimens cast in seawater recorded a strength decrease by 12% relative to the specimen cast in dry condition, implying that the AAFA can be used for constructing a structure in water due to its impressive strength. Normally, conventional concrete (ordinary Portland cement) requires anti-washout admixture (AWA) and high range water reducer admixture (HRWR) before it can be used as binder materials in construction, especially for underwater structures. However, using AAFA only requires materials rich in silica and alumina, as well as alkaline activator. Additionally, the raw materials used in this case (silica and alumina sources) are waste materials, which falls in line with green technology promotion.
