## 5 Conclusions  
Overall, compared to the use of purified water, the use of RO reject water as a mixing water for CSA cement was beneficial in improving strength and satisfied the Korean Standards (KS) F 4004 for brick production. In addition, no problem was found in the TCLP results when a brick sample was made. Therefore, this study recommends using RO reject water as a mixing water to produce CSA cement bricks for construction, although it is not appropriate to use for concrete with steel embedment due to its high chloride content. The detailed conclusions are as follows:  
1. The use of RO reject water showed the smallest flow values in each group of samples compared to the other types of mixing waters, implying the increased viscosity. 2. The influence of the use of RO reject water on the strength of CSA cement paste depended on the presence of gypsum in the binder system. When no gypsum was present, the use of RO reject water yielded greater strength than purified water; however, when gypsum was used, the strength difference between samples was notably reduced, although the sample with RO reject water still produced slightly greater strength than the sample with purified water. 3. Ettringite formed in all sample mixtures, and significantly more formed when gypsum was present. However, the use of RO reject water very slightly affected the quantity of ettringite formation, regardless of the presence of gypsum. Thus, the ettringite formation was not likely the main cause of the strength differences due to the use of different mixing waters. 4. In the absence of gypsum, the use of RO reject water significantly reduced the formation of monosulfate, kuzelite, and AH\({}_{3}\), but it produced more Friedel's salt; this change of reaction \begin{table}
\begin{tabular}{c c c} \hline
**Metal** & **Leaching Concentrations (mg/L)** & **Criteria for TCLP (mg/L)** \\ \hline As & 0.004 & 5 \\ Pb & - & 5 \\ Cr & 0.171 & 5 \\ Cd & - & 1 \\ Ba & 0.043 & 100 \\ \hline \end{tabular}
\end{table} 
Table 7: Toxicity characteristic leaching procedure (TCLP) results of targeted metals and criteria. products possibly increased strength. However, when gypsum was present, all samples showed very similar types and quantities of reaction products in XRD and DTG, regardless of the type of mixing water; accordingly, the sample strengths were also quite similar. 5. The use of gypsum tended to reduce the chloride binding capacity of the hardened samples in this study, but as the ionic concentrations of mixing water became higher, the degree of reduction in the binding capacity was significantly reduced. 6. Although the formation of monosulfate, kuzelite, and Friedel's salt from the hydration of CSA cement was advantageous in increasing the chloride binding capacity, the free chloride contents were still very large in the hardened paste of CSA cement when RO reject water was used. 7. No significant differences in microstructure were found between the samples with purified water and RO reject water in SEM BSE images. 8. The mortar brick sample made of CSA cement, gypsum, and RO reject water produced 30.3 MPa in compressive strength and 7.55 wt% in water absorption, which satisfied the standard KS F 4004 for brick production. The brick sample also showed very low leaching concentrations of the toxic metals AS, Pb, Cr, Cd, and Ba in TCLP. Conceptualization, J.E.O. and H.L. ; methodology, J.E.O. and H.L. ; investigation, H.L., D.J., H.S., D.K., J.Y. and S.W.S. ; resources, H.L., D.J., H.S., D.K., and J.Y. ; validation, H.L., K.H.C., and J.E.O. ; writing--original draft preparation, H.L. ; writing--review and editing, K.H.C., and J.E.O. ; supervision, J.E.O. ; project administration, J.E.O. ; funding acquisition, J.E.O. This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade Industry and Energy (MOTIE) (No.