## 2 Materials and Experimental Program  
A commercial CSA cement (Grade 72.5, Oreworld Trade, Tangshan, China), which contained a high content of ye'elimite, was used in this study. X-ray fluorescence (XRF) (S8 Tiger wavelength dispersive spectrometer; Bruker, Billerica, MA, USA) was used to examine the oxide composition of the CSA cement, as shown in Table 1. The mineralogical composition of CSA cement is given in Table 2, measured using XRD (D/MAX 2500V/PC, Rigaku, Tokyo, Japan) with Cu-K\(\alpha\) radiation (\(\lambda\) = 1.5418 A) from 5\({}^{\circ}\) to 60\({}^{\circ}\) in 20 degree, and Rietveld analysis. Table 2 shows that the CSA cement mostly consisted of ye'elimite and \(\beta\)-dicalcium silicate (C\({}_{2}\)S). The particle size distribution of CSA cement was tested with a laser diffraction particle size analyzer (HELOS (HI199) and RODOS, Sympatec, Clausthal-Zellerfeld, Germany), as shown in Figure 1. The CSA cement exhibited a particle size range of \(\sim\)0.5-100 \(\upmu\)m, with a median particle size of 8.34 \(\upmu\)m.  
The RO reject water was obtained from the RO desalination plant at Yeosu in South Korea. The seawater was collected from the sea close to the RO plant; thus, it had a similar ionic composition to that of the feed water of the RO plant. The purified water used in this study was a deionized (DI) water. Cations (Ca\({}^{2+}\), K\({}^{+}\), Mg\({}^{2+}\), and Na\({}^{+}\)) and anions (Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\)) in the water were quantified using ion chromatography (IC), as shown in Table 3. RO reject water contains about twice as many ions as seawater. Chloride ions were the most abundant ions in RO reject water and seawater. The mixture proportions of prepared paste samples are given in Table 4. Analytical grades of gypsum (CaSO\({}_{4}\)-2H\({}_{2}\)O, Sigma Aldrich, US) were used in this study. An earlier study [22] reported that CSA cement is classified by its \(m\)-value, which is defined as the molar ratio of calcium sulfate (gypsum in this study) to ye'elimite; (1) when \(m\) = 0-1.5, the CSA is categorized as a high-strength or a rapid hardening cement; (2) when \(m\) = 1.5-2.5, it is an expansive cement; (3) when \(m\) = 2.5-6, it is regarded as a self-stressing cement. It is worth noting that the weight \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**CaO** & **Al2O3** & **SO3** & **SiO2** & **Fe2O3** & **TiO2** & **MgO** & **K2O** & **P2O5** & **SrO** & **XrO2** & **Cl** \\ \hline
43.66 & 33.1 & 9.48 & 7.93 & 2.04 & 1.47 & 1.38 & 0.44 & 0.13 & 0.11 & 0.08 & 0.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of calcium sulfoaluminate (CSA) cement by X-ray fluorescence (XRF) (wt%). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Minerals** & **Ye’elimite (C\({}_{4}\)A\({}_{3}\)\(\overline{\bar{5}}\))** & \(\beta\)-C\({}_{2}\)\(\underline{S}\) & **CT** & \(\overline{\overline{C}}\) \\ \hline wt\% & 70.7 & 27.0 & 1.6 & 0.7 \\ \hline \hline \end{tabular} In cement notation, C = CaO, A = Al\({}_{2}\)O\({}_{3}\), S = SiO\({}_{2}\), \(\overline{\bar{5}}\) = SO\({}_{3}\), T = TiO\({}_{2}\)[13].  
\end{table} 
Table 2: Mineralogical composition of calcium sulfoaluminate (CSA) cement (wt%). Figure 1: Density and cumulative distributions of the particle sizes of calcium sulfoaluminate (CSA) cement. proportion of CS was only 0.7 wt% in the CSA of this study. Thus, in this study, the weight of added gypsum was calculated to generate an \(m\)-value of 1 because rapid hardening is more beneficial than expansion or self-stressing in producing bricks commercially. Purified water, seawater, and RO reject water were used as mixing water. The weight ratio of water-to-binder (w/b) was set as 0.5 for all mixture samples, given that a higher w/b is generally necessary for CSA cement than PC to gain a proper mixing of paste [23; 24]. For compressive strength testing, 3 identical paste samples were prepared for each mixture proportion. The fresh pastes were cast in 5 cm \(\times\) 5 cm \(\times\) 5 cm cubic molds.