## 3 Results and Discussion
### Free and Bound Chloride Contents  
The results of the chloride binding test are given in Figure 5. Chloride generally exists either free or bound in hardened cementitious pastes. Whereas, free chloride ions travel relatively easily through a pore solution in a hardened paste, bound chloride ions are immobile; thus, the content of the free or bound chloride is more important in estimating the risk of chloride-induced corrosion of steel reinforcements in concrete than the total chloride content [8; 35].  
AFm phases from the hydration of CSA cement (i.e., monosulfate, kuzelite, and Friedel's salt in this study) generally have chloride binding abilities [41; 52]. In this study, when the concentration of chloride ions was very low (i.e., as in the case of purified water) and no gypsum was present, the chloride was largely bound by monosulfate and kuzelite (e.g., 85.9% in sample C), as shown in the inset figure in Figure 5. However, Figure 5 illustrates that the chloride binding capacity of the CSA cement was evidently insufficient for the use of seawater or RO reject water because the percentage of bound chloride (bound%) was notably reduced when seawater or RO reject water was used, and the bound percentage was the lowest when RO reject water was used (only 39.7% in the sample RO).  
Figure 4: Thermogravimetric analysis (TG) and differential thermogravimetry (DTG) results of the samples (**a**) at 1 day without gypsum, (**b**) at 28 days without gypsum, (**c**) at 1 day with gypsum, and (**d**) at 28 days with gypsum. E: ettringite, M: monosulfate, K: kuzelite, F: Friedel’s salt, G: gypsum, AH\({}_{3}\): aluminum hydroxide.  
In addition, the use of gypsum clearly reduced the bound percentage, particularly in the samples with purified water and seawater. [PERSON] et al. [PERSON] et al. (2019) similarly observed that the lower ratio of ye'elimite to sulfate in CSA cement resulted in reduced binding chloride capacity through chemical reaction because more ettringite formed than monosulfate. [PERSON] et al. [PERSON] et al. (2019) also reported that the use of gypsum inhibited the formation of monosulfate by forming ettringite. These could be possible explanations for the lowered binding capacity of the gypsum-added samples.  
However, although using gypsum tended to reduce the bound percentage, the bound percentage was less reduced as the ionic concentrations of mixing water became higher; for instance, when RO reject water was used, the bound percentage was barely reduced by the use of gypsum. This lessening tendency should be related to the reduced amount of monosulfate and kuzelite in the SW and RO samples (see Figure 4b). In the case of the RO sample, the binding capacity of RO was already greatly reduced because monosulfate and kuzelite were largely removed; thus, the addition of gypsum did not produce any significant difference between the RO and ROG samples. Rather, greater amounts of Friedel's salt were formed in the ROG sample than in the RO sample, resulting in a slight increase in the bound percentage.  
In this study, although the presence of monosulfate, kuzelite, and Friedel's salt was beneficial in binding chloride ([PERSON], 1999; [PERSON] et al., 2019), the concentrations of free chloride were still significant when RO reject water or seawater was used; thus, RO reject water is not appropriate for use as a mixing water for producing reinforced concrete, nor is seawater ([PERSON] et al., 2018). However, if the RO reject water is used only for producing bricks (i.e., no steel embedment), the free chloride content is hardly essential to the durability of bricks; it is likely more important whether severe leaching occurs on the brick surfaces due to the high ionic concentrations of the RO reject water.