*3.4. Chemical Composition Analysis*

The chemical composition of control and AAFA specimens placed in different types of water is presented in Table 3. All the AAFA paste showed an increment in SiO2 content due to the reaction of the fly ash with waterglass (Na2SiO3). Meanwhile the content of Al2O3 showed a reduction in AAFA paste relative to the raw fly ash. This is due to the participation of Al2O3 in setting time of the AAFA via acceleration of the condensation of the AAFA product formation [44,45]. The content of Fe2O3 also increased in the AAFA paste, especially those cast in dry condition, hence contributing to the maximum compressive strength. It was suggested that Fe3+ contributed to the formation of AAFA network due

to the similar charge and ionic radius with Al3+ [46,47]. However, the increment of most chemical composition between different specimens is almost similar, which is related to the compressive strength. The compressive strength of the AAFA depends on a few factors such as the formation of reaction products, distribution of Si-Al ratio, calcium content, and the surface reaction between the unreacted Si-Al particles [48,49].


**Table 3.** Comparison of chemical composition for all specimens.

The molar ratio of Si/Al, Ca/Si, and Fe/Si of raw fly ash and the AAFA paste was calculated based on the result from XRF. For the ratio of Si/Al, the compressive strength increased when the Si/Al ratio increased due to the formation of Si-O-Si bonds. The maximum ratio of Si/Al is contributed by the AAFA specimens cast in dry condition. Nevertheless, the ratios between the specimens do not differ much.

The Ca/Si ratio for the source materials rich in Ca content is also linked with the compressive strength of the AAFA. The Ca/Si is responsible for the formation of C-S-H, and according to Timakul et al. [50], the compressive strength increased alongside the Ca/Si ratio. However, in this study, the ratio of Ca/Si is almost similar (~0.64–0.63) between the AAFA specimens cast in dry condition and seawater, which resulted in less of a difference in terms of compressive strength. For the AAFA cast in river water and lake water, the ratio of Ca/Si is similar (Ca/Si~0.62). Additionally, the formation of C-S-H, as AAFA reaction product and/or as OPC hydration product is entirely different. For the formation of C-S-H as hydration of OPC, the ratio of Ca/Si is in the range of 1.2 to 2.3, which is much higher relative to the AAFA [51,52]. The ratio of Fe/Si also plays essential role in forming the reaction product of the AAFA. The specimens' compressive strength increased when the ratio of Fe/Si increased due to the formation of ferro-sialate-siloxo and/or ferro-sialatedisiloxo poly. The XRF result indicated that iron oxide is involved in the forming of the AAFA network and contributed to the AAFA's strength.
