*3.4. SEM Observation*

The SEM photographs of each group of specimens are shown in Figure 1a–e. Many needle-like products (C-A-S-H or ettringite) could be seen on the surface of the microstructure of the specimen maintained in water. The needle-like hydration was more pronounced on the surface of the G1 specimen with lower strength, while hexagonal calcium hydroxide crystals were also observed on the surface of the G1 and G5 air-cured specimen. It was also indirectly stated that water curing was favorable for the continuous hydration of calcium hydroxide to produce C-S-H or C-A-S-H colloids. Figure 1e is an SEM photograph of the specimen with ultrafine fly ash, and transparent unreacted fly ash particles can be observed on its surface. It was clear that the reaction of ultrafine fly ash was favorable to the generation of C-A-S-H and ettringite. Figure 1f shows the experimental spectrum of SEM combined with energy dispersive X-ray (EDX) analysis for G5 water-cured specimen. It was found that elements with higher percentages were Si, Ca, Al, etc. It was evident that needle-like hydrates would contain C-S-H, C-A-S-H colloid, and calcium alumina, which was also the source of strength in the ternary cementless specimens.

**Figure 1.** SEM photos (x3000).
