## 1 Introduction  
Reverse osmosis (RO) is a desalination process that removes salts and minerals from seawater or brackish water with a salinity of total dissolved solids (TDS) of ~1000-60,000 mg/L to produce clean water with less than 500 mg/L TDS [1]. The RO process currently produces about 50% of the total desalinated water available worldwide [2]; however, it also yields a huge amount of further concentrated brines (or RO reject water) with over ~10,000 mg/L TDS as a waste byproduct. Currently, all desalination plants (e.g., RO, nanofiltration, multi-stage flash) globally produce 51.7 billion m\({}^{3}\) of concentrated brines every year, ~38% of the volume of which is RO reject water (i.e., ~19.6 billion m\({}^{3}\)/year worldwide) [3]. The RO reject water is generally discharged to the sea or local bodies of water, which changes their salinity, alkalinity, and/or water temperature, resulting in significant negative environmental impacts [2]. Thus, various technologies designed to minimize or reuse RO reject water have been developed, such as evaporation and crystallization, forward osmosis, membrane distillation, electrodialysis, and zero discharge desalination; however, all these technologies demand substantial additional costs [2, 4, 5]. Therefore, it is necessary to develop a new way of treating RO reject water, such as recycling RO reject water at a low cost. In the construction industry, fresh water has generally been used as mixing water for the production of Portland cement (PC) concrete, and a few billion tons of fresh water are used worldwide for concrete production every year in the era of fresh water scarcity [6]. Thus, to reduce the shortage of fresh water that is worsening worldwide, many studies have investigated the viability of using natural saline water (e.g., seawater), as a mixing water for cementitious binders (e.g., PC, cementless binders) [7; 8; 9]. However, to the best of the authors' knowledge, there have been very few investigations (likely only three publications [10; 11; 12]) exploring the reuse of reject brine, including RO reject water, as mixing water for cementitious binders. In these studies, PC samples made with reject brine produced lower compressive strengths than those produced with distilled water when PC was used without any supplementary cementitious materials (e.g., slag); this may suggest that PC is not a good candidate for reusing RO reject water. It is worth noting, that calcium sulfoaluminate (CSA) cement was developed to replace PC in concrete production in the 1970s, and its mineralogical composition consists mainly of C\({}_{4}\)A\({}_{3}\)\(\overline{\text{S}}\) (ye'elimite) (in cement chemistry [13], C = CaO, A = Al\({}_{2}\)O\({}_{3}\), S = SiO\({}_{2}\), \(\overline{\text{S}}\) = SO\({}_{3}\)). The hydration of ye'elimite generally produces amorphous aluminum hydroxide (AH\({}_{3}\)), which is not detectable in X-ray diffraction (XRD), and monosulfate (C\({}_{4}\)A\(\overline{\text{S}}\)H\({}_{14\text{ (or 12)}}\)) [14; 15]. However, when sulfate sources such as anhydrite (\(\overline{\text{S}}\)) or gypsum (\(\overline{\text{S}}\)H\({}_{2}\)) are sufficiently present with ye'elimite, ettrinite (C\({}_{6}\)A\(\overline{\text{S}}\)H\({}_{32}\)) dominantly forms rather than monosulfate, and the formation of ettrinite is a main contributor to the strength of CSA cements [16]. CSA cement is an environment-friendly cement because, compared to PC, the production of CSA cements requires less use of limestone and a lower sintering temperature (\(\sim\)1250 \({}^{\circ}\)C; \(\sim\)1450 \({}^{\circ}\)C in the case of PC), resulting in lower CO\({}_{2}\) emissions [17]. Alongside its environmental benefits, CSA cement shows outstanding properties, such as high early strength, shrinkage compensation, rapid setting, and self-stressing, and these properties can be adjusted by changing the amount of calcium sulfates (e.g., CaSO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O (gpsum)) added to the cement [18; 19].