## 3 Results and Discussion
### Microstructures  
Figure 6 shows the SEM images of the 10% (Figure 6a), 20% (Figure 6b), and 30% CSAe samples (Figure 6c) after 28 d of curing. Long, needle-shaped C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) was observed in all samples. C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) is a typical hydration reaction product found in previous studies involving CSA [33, 42, 52, 53]. C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) was observed in all samples, as shown in the XRD results in Figure 3. As illustrated in Figure 6, the length of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) varied according to the substitution ratio of CSAe. According to Figure 6a, the ettrinigite of 10% CSAe was about 1.5-\(\upmu\)m long. In the case of the 20% and 30% CSAe samples in Figure 6c,e, the C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) length was increased to 2 and 3 \(\upmu\)m, respectively. Therefore, increasing the substitution rate of CSAe increased the amount and length of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\). This led to an increase in the amount of C\({}_{4}\)A\({}_{3}\)\(\overline{\text{S}}\) as the substitution rate of CSAe increased, which promoted the formation and growth of fast C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\)[33, 53]. The increase in C\({}_{4}\)A\({}_{3}\)\(\overline{\text{S}}\) as the substitution rate of CSAe increases is supported by the results of the XRD analysis in Figure 3.  
Figure 6: _Cont._  
EDS analysis was performed on the C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) in each SEM image, and the results are shown in Figure 6b,d,f. The EDS analysis results are consistent with the C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) seen in previous studies on CSA [33]. The EDS analysis showed Si elements, which are not constituents of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\). In samples not substituted with SF, this phenomenon probably resulted from the intermixed ettrinite crystals and amorphous C-S-H gel or from the appearance of C-S-H gel in the detecting range of the electron beam during EDS testing [33]. In the samples where SF is substituted, it is judged to be due to the influence of SF as well as the aforementioned cause.  
Increasing the replacement rate of CSA increased the amount of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) produced. This affected the mechanical properties of the specimen through pore filling and expansion. This is also confirmed by the increase of the ettrinite peak in Figure 3 mentioned above and the thermal analysis of Figure 8, which will be mentioned later.  
Therefore, the sample whose amount of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) increased with the substitution rate of CSAe showed a relatively high compressive strength. This is clearly indicated in the strength characteristics in Figure 4. Moreover, Figure 3 shows that the pore size and number of the CSA-substituted sample with increasing amounts of C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\) decreased.  
Figure 6: SEM images at 28 d, (**a**) C80F10C10, (**b**) EDS result of ettrinite (EDS point 1), (**c**) G70F10C20, (**d**) EDS result of ettrinite (EDS point 3), (**e**) G60F10C30, and (**f**) EDS result of ettrinite (EDS point 3).  
These strength enhancement and void reduction effects show synergistic effects as the C\({}_{6}\)A\(\overline{5}_{3}\)H\({}_{32}\) produced by the hydration of CSAe increased with the SF substitution ratio. However, the experimental factors considered in this study were a low \(w/b\) = 0.5 and high CSAe replacement rates. This can lead to excessive expansion and thus increased internal pores and cracks.