*3.3. SEM Observations*

Scanning electron microscopy photographs and corresponding energy dispersive Xray (EDX) analyses are shown in Figure 8. Figure 8a,b show the surface microstructure of the waste gypsum specimens. It can be found in Figure 8a that hexagonal calcium hydroxide accounted for the majority of hydrates. In contrast, the larger particles of calcium hydroxide produced finer hydrates on the surface, and some areas of hydrates showed needle-like or column-like hydrates. Unhydrated fly ash particles were also observed on the surface of the microstructure. By magnifying the SEM photograph up to 10,000× (Figure 8b), it can be clearly observed that the interstices were filled with needle-shaped hydrides, which were presumed to be C-S-H and ettringite. The long needle-like crystals indicated the formation of large amounts of calcium–alumina hydrates (ettringite) in the waste gypsum specimens. In addition, since gypsum was the main component of the specimens, dispersed fibrous structures of the C-S-H gels were also observed, which is consistent with previous literature results [10,11]. The results of EDX in Figure 8c confirmed that the hydrates were primarily composed of Ca, Si and Al and that the hydrates were C-S-H or ettringite.

Figure 8d,e illustrate SEM images of the specimens after 56 days of curing, followed by 100 days of immersion in tap water and seawater. The specimen in Figure 8d revealed a large area of sodium hydroxide and needle-shaped hydrates (ettringite), and the water immersion facilitated the continued hydration of the specimens. In contrast, the unreacted fly ash particles continued to react to form C-S-H colloids, which filled the interface between the pores and the waste gypsum. Sodium hydroxide was observed in the specimen in Figure 8f; the specimen was immersed in seawater and the sulfide in the seawater was favorable for the growth of needle-like hydrates (ettringite). Dense ettringite could be seen filling the pores and interfaces on the surface of the specimens. From the EDX analysis results (Figure 8f), it can be verified that most of the hydrated reactants were ettringite (Ca-Si-Al crystals).

(**a**) Air-cured specimen at 56 days (×1000) (**b**) Air-cured specimen at 56 days (×10,000)

(**c**) EDX for ai-cured specimen at 56 days (**d**) Immersion in tap water for 100 days

(**e**) Immersion in seawater for 100 days (**f**) EDX for specimen in seawater

**Figure 8.** SEM photos (×3000).
