*3.7. SEM Observation*

Figure 9 presents SEM images of various specimens under 10K× magnification at 28 days. The non-cement blended specimens presented more pronounced pore formation than did the cement specimens. C-S-H gel is the product of reactions between water and tricalcium silicate or dicalcium silicate. Approximately 50% of the cement gel was C-S-H gel, which served as the primary source of strength in the cement paste. C-S-H gel and ettringite appeared as irregular needle-like and spherical continuums, whereas Ca(OH)2 appears as hexagonal flakes. However, the appearance of fine exterior capillary tube spikes indicates that the C-S-H gel was covered in pores, i.e., it was not a smooth continuum. The formation of high-quality concrete depends on finer materials filling in the pores. As shown in the Figures 9 and 10, most of the pores have been filled with ettringite, which gave the material a dense appearance and reduced the likelihood of infiltration by harmful substances.

**Figure 9.** SEM photos at 28 days: (**a**) P; (**b**) S40; (**c**) S50 and (**d**) S60.

**Figure 10.** Scanning electron microscope (SEM) photos with energy dispersive analysis (EDS) analysis: (**a**) S60 specimens; (**b**) EDS results.

Based on the SEM images, the reaction mechanisms in non-cement blended materials comprising a mix of S and FA can be divided into two phases. In the first phase, the CaO in the FA violently reacted with water to produce Ca(OH)2. It was shown that increasing the proportion of FA accelerated the process of setting. Insufficient FA would slow the hydration setting speed after the S and FA mix. In the second phase, the Ca(OH)2 and SiO2 in the S and FA with reacted with SiO2 and Al2O3 to produce C-S-H and C-A-S-H gel [35] as shown in Figure 10. It can be consistent with previous study [18].
