# Hydration of Calcium Sulfoaluminate-Based Binder Incorporating Red Mud and Silica Fume
## Materials and Methods
### Mixture Proportions, Sample Preparation, and Curing  
The mix design of the samples is presented in Table 2, in which each sample is denoted with a specific letter, \"C\", \"R\", and \"S\" representing CSA, RM, and SF, respectively. The number following the specific letter indicates the weight percent of each material. The weight ratio of the water to the ternary compound material (CSA + RM + SF) was 0.5. Five different ratios of 10:90, 20:80, 30:70, 40:60, and 50:50 were selected as the weight ratios of the raw materials between CSA and RM. RM was replaced with 0%, 5%, and 10% SF by weight. Fifteen mixtures were made to produce sample pastes.  
The samples were prepared in a laboratory according to ASTM C305 [23]. After mixing, samples of \(50\times 50\times 50\) mm\({}^{3}\) and \(40\times 40\times 160\) mm\({}^{3}\) were cast for the determination of compressive strength and drying shrinkage, respectively. The samples for the MIP and SEM tests were cast in cubic molds with dimensions of \(25\times 25\times 25\) mm\({}^{3}\). All samples were placed in a humidity chamber at \(95\pm 5\%\) relative humidity and \(20\pm 2^{\circ}\)C temperature for 24 hrs. After demolding, the samples were stored in the same chamber until the testing age.  
Figure 1: XRD patterns of raw materials (CSA: calcium sulfoaluminate expansive agent; RM: red mud; SF: silica fume). E: ettringite (98-002-7039), G: gypsum (98-040-9581), P: portlandite (01-084-1265), A: anhydrite (01-080-0787), Y: ye’elimite (98-002-8480), C: calcite (00-003-0596), K: carcinrite (98-015-6526), S: sodalite (98-041-0158), B: boehmite (00-001-1283), H: hematite (00-006-0502), V: vaterite (98-016-2480), L: larnite (C\({}_{2}\)S, 98-002-4640), R: perovskite (98-007-4212), T: C\({}_{3}\)A (98-015-1369). The numbers in parentheses are ICDD (International Center for Diffraction Data) PDF-2 or ICSD (Inorganic Crystal Structure Database) data numbers for identified phases.  
The compressive strength of the samples was determined at 1, 3, 7, and 28 days in accordance with ASTM C109 [24]. Each strength value was the average of three samples. The XRD, MIP, and SEM tests were carried out on the samples cured for 28 days. Fractured specimens after strength testing were ground finely, and the particles that passed through sieve No. 200 were used for XRD analysis. The XRD analysis was carried on an X-ray diffractometer (Empyrean, PANalytical, Netherlands) using Cu K\(\alpha\) radiation with 20 scan from 5\({}^{\circ}\) to 60\({}^{\circ}\). For the MIP test, \(5\times 5\times 5\) mm\({}^{3}\) cubic pieces were obtained from the central inner part of the samples. MIP measurement was conducted using an AutoPore IV 9500 mercury porosimetry (Micromerities, USA). An FE-SEM (Supra 40 VP, Zeiss, Germany) secondary electron (SE) image was used to observe the surface morphology, and energy dispersive X-ray spectroscopy (EDS) was performed to determine the elemental compositions of a few spots on the samples. Cross sections with a thickness of 5 mm along the length of the samples were prepared using a rock saw. The cross sections were coated with a thin film of gold before the SEM observation.  
The drying shrinkage test was conducted in accordance with ASTM C596 [25]. The samples were stored in a chamber with a controlled temperature of \(23\pm 1\)\({}^{\circ}\)C and a relative humidity of \(50\pm 5\%\) for 91 days.