# Hydration of Calcium Sulfoaluminate-Based Binder Incorporating Red Mud and Silica Fume
## 3 Results and Discussion
### Xrd  
Figure 4 represents the XRD patterns of the 28-day samples with 10, 30, and 50% CSA content for each group (Table 2). In all of the samples, most identified phases were already present in the raw materials (i.e., CSA and RM) such as ettrinigite (PDF 98-002-7039), sodalite (PDF 98-041-0158), cancrinite (PDF 98-015-6526), beohnite (PDF 00-001-1283), hematite (PDF 00-006-0502), anhydrite (PDF 01-080-0787), vaterite (PDF 98-016-2480), calcite (PDF 00-003-0596), larnite (C\({}_{2}\)S, PDF 98-002-4640), C\({}_{3}\)A (PDF 98-015-1369), perovskite (PDF 98-007-4212), and ye'elimite (PDF 98-002-8480). [PERSON] and [PERSON] [30] reported that C-S-H and calcium hydroxide are formed from the hydration of C\({}_{2}\)S in RM. In this study, C-S-H gel and calcium hydroxide were not identified in XRD patterns of the hardened samples. This result suggests that C\({}_{2}\)S in RM was unhydrated, and the pozzolanic activity of SF would not occur. Generally, SF is used as a pozzolanic material, which reacts with calcium hydroxide and forms C-S-H. In this study, it can be seen that SF did not react chemically, but it is expected that SF would associate with a physical filler effect. Note that in each group, the reflection intensity of ettrinite increased with the CSA replacement level. This observation suggests that the main reaction product of the hardened samples is ettrinite. The degrees of reaction could be compared between the samples by comparing the intensities of ettrinite reflections in XRD. It was reported that ettrinite could be formed from C\({}_{3}\)A in RM with gypsum and water [30]. However, it is difficult to assess whether the presence of ettrinite (Figure 4) in this study was formed by the hydration of C\({}_{3}\)A. The residual unhydrated C\({}_{3}\)A (denoted as T) in Figure 4 is not easily identified because the strongest peak of C\({}_{3}\)A (33.210\({}^{\circ}\) 20) overlaps with hematite (denoted as H, 33.280\({}^{\circ}\) 20) and perovskite (denoted as R, 33.114\({}^{\circ}\) 20) peaks. Furthermore, as C\({}_{3}\)A, hematite, and perovskite are contained in the raw RM, their peaks decrease with decreasing RM. From this study, it can be seen that ettrinite forms from the consumption of ye'elimite, anhydrite, gypsum, and Ca(OH)\({}_{2}\) in the curing process, and that the formed ettrinite is expansive. Earlier studies reported that ye'elimite hydrates to form monosulfoaluminate [6; 9], and ye'elimite combined with anhydrite and/or gypsum results in non-expansive ettrinite (C\({}_{6}\)A\(\overline{\text{S}}_{3}\)H\({}_{32}\)) [4]. It was also reported that in the presence of calcium hydroxide, ye'elimite combined with anhydrite and/or gypsum produces expansive ettrinite (3C\({}_{6}\)\(\overline{\text{S}}_{3}\)H\({}_{32}\)) [4; 6]. In this study, gypsum and calcium hydroxide contained in the raw CSA were not observed in the hardened samples with 10, 30, and 50% CSA replacement level, indicating that those were consumed in the curing process. It can be observed from Figure 4 that anhydrite was not fully depleted in the samples. The residual anhydrite in the samples with 0% SF (\(\sim\)S0) was very similar to each other with respect to the CSA content; however, in the case of 5% SF addition (\(\sim\)S5), the peak of anhydrite in the sample with 50% CSA content (C50R45S5) was significantly higher than those with 10 and 30% CSA content (C10R85S5, C30R5555). When 10% SF was added (\(\sim\)S10), the residual anhydrite was increased with the CSA replacement level. For each group (\(\sim\)S0, \(\sim\)S5, and \(\sim\)S10), it seems that ye'elimite is completely converted into ettrinite in the sample with 10% CSA replacement level. This is because the peak of ye'elimite was not detected in the samples. Unhydrated ye'elimite existed in the mixes with 30 and 50% CSA, and the ye'elimite was increased with the CSA replacement level. [PERSON] et al. [31] reported that the complete hydration of binder with 50 wt.% of ye'elimite and 22 wt.% of anhydrite and gypsum did not occur in the water/binder ratio of 0.4.