## 3 Results
### Micro-Morphological Analysis  
C8F120 cured for 7 and 28 days and C8F280 and C8F20 cured for 28 days were selected for SEM analysis, which allows the observation of morphological changes throughout  
Figure 10: TG-DTG results of **(a)** C8F120 after 7, 14, and 28 days of curing and **(b)** C8F120 and C8F280 after 28 days of curing.  
the hydration process. Morphological differences between different IFCs could also be observed. Figure 11a,b presents the SEM analysis on the morphology of samples with an IFC of 120 mg/L after 7 and 28 days of curing, respectively. Amorphous C-S-H gel and ettringite were found in many of the hydrated structures. In Figure 11a, the PCPB sample is in the early stage of hydration, and the distribution of hydration products is relatively loose. The presence of more pores of a larger size on the surface of the sample responds to its incomplete hydration reaction. In contrast, Figure 11b reveals the presence of a relatively dense network of hydration products found in conjunction with other hydration products to form a complex structure, which highlights the change of the structure of the hydration products of PCPB. However, the C-S-H gel will continue to develop and form a denser structure over time, increasing the overall strength of the sample. The main difference between Figure 11a,b is the structural complexity of the sample. A relatively dense structure of hydration products after 28 curing days, comprising \"cloudy\" C-S-H gels as well as clustered rods or chains of AFt crystals, combine to provide a more homogeneous distribution of the overall structure observed in Figure 11b. However, differences in the distribution of hydration products (e.g., build-up in certain areas) result from the inhibitory effect of higher IFC on hydration reactions, which may lead to localized volume changes and cracking. This may explain the poor strength of some samples with high IFC as well as the cracking that was observed in some samples with high IFC during the UCS test.  
Figure 11c,d presents the SEM images of the samples with the most and least fluorine content, respectively, to compare the effects of fluorine on the microstructure of the samples. The structure formed by the denser coverage of hydration products is shown in Figure 11d. Compared to Figure 11c, the PCPB samples with low IFC have fewer and shallower pores, and the hydration products of the samples are more tightly distributed. Combined with the results of the XRD part of the analysis, it is clear that the high fluorine content affects the progression of the C\({}_{3}\)S and C\({}_{2}\)S reactions as well as the production of C-S-H. The  
Figure 11: SEM images of (**a**) CSF120 after 7 days of curing, (**b**) CSF120 after 28 days of curing, (**c**) CSF280 after 28 days of curing, and (**d**) CSF20 after 28 days of curing.  
effects of this are clearly illustrated at the microscopic level, where fluorine hinders the hydration of the cement minerals resulting in slower hydration in samples with higher IFCs [22; 61]. Figure 11d also highlights the presence of some granular crystals on the surface of the hydration product structure, which may be CaF\({}_{2}\) crystals [56]. The immobilized fluorine may be well dispersed within the cement matrix and the dense hydration product structure. In particular, CaF\({}_{2}\) may be the predominant form of immobilized fluorine in the samples and is expressed either in the form of complexes embedded in gypsum crystals or as adsorbed materials [62]. For both mechanisms, the formation of a denser hydrated structure through the promotion of hydration can better enhance the immobilization of fluoride.