## 3 Results and Discussion
### Xrd  
Figure 3 presents the XRD patterns of the hardened pastes at 1, 7, and 28 days. In XRD, regardless of the presence of gypsum or the type of mixing water, all samples still contained a significant portion of anhydrous CSA cement until 28 days of curing. The XRD results of this study illustrate that the use of gypsum likely slightly inhibited dissolution of ye'elimite because the stronger peaks of ye'elimite were observed in the samples with gypsum. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Measured Diameter** & **Flow Value** \\ \hline C & Overflow (i.e., over 253 mm) & \\ SW & 233.0 mm & 133.0\% \\ RO & 191.3 mm & 91.3\% \\ CG & Overflow (i.e., over 253 mm) & \\ SWG & 234.0 mm & 134.0\% \\ ROG & 212.3 mm & 112.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Flow test results using flow table following American Society of Testing and Materials (ASTM) C1437. Figure 2: Compressive strengths of samples. When no gypsum was used (i.e., C, SW, and RO), the use of seawater and RO reject water mainly changed the type of Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)O\({}_{3}\)-mono (AFm) [13] in comparison to the use of purified water. All samples without gypsum displayed monosulfate (Ca\({}_{4}\)Al\({}_{2}\)O\({}_{6}\) (SO\({}_{4}\))\(\cdot\)14H\({}_{2}\)O) and kuzelite (Ca\({}_{4}\)Al\({}_{2}\)(SO\({}_{4}\))(OH)\({}_{12}\)-6H\({}_{2}\)O), which belong to the AFm phase, in XRD at all days. However, when a mixing water had a higher concentration of chloride, these reaction products were largely replaced with [PERSON]'s salt (Ca\({}_{4}\)Al\({}_{2}\)Cl\({}_{2}\)(OH)\({}_{12}\)-4(H\({}_{2}\)O)\({}_{2}\)), which is also a type of AFm phase, mainly due to the substitution of sulfur (S) with chloride (Cl) [34; 35]. Thus, sample C showed the strongest peaks of monosulfate and kuzelite, whereas the RO sample showed the strongest peaks of [PERSON]'s salt in the XRD results, particularly at 28 days. In addition, in all samples without gypsum, despite the small amount of calcium sulfate (\(\sim\)0.7 wt%) in the CSA cement of this study, ettrinigite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{2}\)\(\cdot\)26H\({}_{2}\)O) was also identified, although its XRD peaks were relatively weak, a result similar to those found in earlier studies [36; 37; 38]. When gypsum was used, all samples produced strong ettrinigite peaks because the gypsum supplied sufficient sulfate, whereas the peaks of monosulfate and kuzelite were almost eliminated; however, [PERSON]'s salt still formed when seawater or RO reject water was used. Similar to the gypsum-free samples, the XRD results indicate that the largest amount of [PERSON]'s salt formed when RO reject water was used. Figure 3: X-ray diffraction (XRD) patterns of samples at w/b = 0.5 (**a**) without gypsum, and (**b**) with gypsum. Y: ye’elimite, E: ettrinigite, M: monosulfate, S: stratlingite, K: kuzelite, F: [PERSON]’s salt, G: gypsum. It is worth noting that the use of seawater or RO reject water did not noticeably affect the intensities of the ettringite peaks, although these mixing waters had significantly different concentrations of sulfate ions (SO\({}_{4}\)\({}^{2-}\)) (see Table 3), regardless of the use of gypsum. In this study, the concentration of sulfate ions in the RO reject water (= -4,609 mg/L) was the greatest among mixing waters; however, given that this concentration can be obtained when only 0.415 g of gypsum is dissolved in 50 g of purified mixing water (note that 14.96 g of gypsum was dissolved in 50 g of purified water for sample CG in Table 4), the concentration of sulfate in the RO reject water was likely not high enough to observably increase the formation of ettringite. Thus, although the formation of ettringite has often been reported as a main cause of substantial strength increase in various cementitious binders [39; 40], in this study, the ettringite formation hardly explained why the use of RO reject water significantly increased strength.