*3.5. Microstructure and Elemental Composition of Reaction Products*

The morphology image of fly ash shown spheres particles shapes with smooth surface and various sizes of particles as in Figure 5. Figures 6–9 show the microstructure and EDS of the specimens at three selected spots (represented by the spectrum numbers) in the matrix. Elements such as Si, Na, Fe, Al, Ca, and O were identified in the AAFA matrix for each specimen. The selected spot for each specimen is often different, which means that the EDS elemental composition is incomparable between each specimen.

**Figure 5.** Morphology of fly ash.

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**Figure 6.** (**a**) Morphology of AAFA specimens cast in dry condition. (**b**) EDS for AAFA specimens cast in dry condition.

**Figure 7.** (**a**) Morphology of AAFA specimens cast in seawater. (**b**) EDS for AAFA specimens cast in seawater.

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**Figure 8.** *Cont.*

**Figure 8.** (**a**) Morphology of AAFA specimens cast in river water. (**b**) EDS for AAFA cast in river water.

For the AAFA specimens casted in the dry condition as in Figure 6a, the present of unreacted fly ash still detected on the specimen. For the elemental composition of spectrum 15 is occupied by Si, Al, and Fe, with Ca and Na less than 5 wt.%. Referring to the FESEM images, spectrum 15 showed the particle shapes of fly ash. It can therefore be surmised that the unreacted fly ash contributed to the strength increment with respect to the aging period due to the complex reaction between the surfaces of the particles via bonding strength [53–57]. Meanwhile, the elemental composition in spectrum 16 majorly consists of Si, but for spectrum 17 is dominated by Si, Ca, Na, and Al as in Figure 6b. It can be hypothesised that these elemental compositions represent the reaction product of C-A-S-H and C-S-H due to the high content of Ca in the source material (fly ash).

Figure 7a shows the specimen cast in seawater where unreacted and partially reacted fly ash were detected. Through EDS analysis spectrum 1 is dominated mostly by Si with Na, Al, Ca, and Fe less than 5 wt.%. Spectrum 3 is dominated by Ca, Na, and Si, which indicate the formation of C-S-H. Additionally, spectrum 4 show high concentration of Si and Al as in Figure 7b which represent unreacted fly ash.

The unreacted fly ash remains present between AAFA matrix as confirmed by the FESEM image in Figure 8a. The AAFA specimen's elemental compositions cast in river water (Figure 8b) are represented by spectrum 27, 28, and 29. From the three different spots, the Ca and Si are predominant indicating the existing of calcium silicate hydrate (C-S-H).

The microstructure of specimens cast in lake water (Figure 9a) showed micro-crack and it is believed to be due to sample preparation for FESEM. For spectrum 54, it is dominated by Ca, Na, and Si, which signifying the formation of C-S-H. However, spectrum 55 is mostly dominated by Ca, Si, Al, Fe and by referring to Figure 9b, the location of this spectrum is on spherical shape of fly ash. The elemental composition of spectrum 56 is predominated by Si, Al, and Fe.

**Figure 9.** (**a**) Morphology of AAFA specimens cast in lake water. (**b**) EDS for AAFA specimens cast in lake water.

In Section 3.4, C-S-H presence is confirmed via XRF analysis due to the increment in Ca and Si content. Additionally, the same finding also noted in EDS analysis where the C-S-H supported the compressive strength by acting as a micro-aggregate in the AAFA which produced denser AAFA matrix. The formation of C-S-H started from dissolution of Ca from the source material where some of the Ca will precipitate in the form of calcium hydroxide (Ca(OH)2) and C-S-H. Likewise, Si species also favourably to react with dissolved Ca rather than polymerise with soluble Al [58,59]. Hence, the presence of excessive Al will force out from Ca-rich area into the AAFA network. Past research also found that C-S-H gel contribute to the strength development at later age such as at 28th days [57,58].

Through EDS analysis, the existence of Fe was noticeable from all specimens and was reconfirmed by the XRF result. The high percentage of Fe in AAFA network due to involvement as substitution for Al, which leads to formation ferro-sialate-siloxo and ferro-sialate-disiloxo poly binders where Ca2+ and Na<sup>+</sup> act as charge-balancing cations [57].
