## 3 Results and Discussion
### Thermal Property Evaluation  
The initial strength of CAC cement is the formation of hexagonal hydrates (CAH\({}_{10}\) and C\({}_{2}\)AH\({}_{8}\)) as the main hydration products [27]. However, CAH\({}_{10}\) and C\({}_{2}\)AH\({}_{8}\) are metastable and are known to get converted to the more stable C\({}_{3}\)AH\({}_{6}\) and AH\({}_{3}\) species. In Figure 5a, every sample shows an AH\({}_{3}\) conversion endothermic signal at 240 \({}^{\circ}\)C [28; 29]. Additionally, all samples exhibit an inflection point with varying strengths near 800 \({}^{\circ}\)C. The DSC profile of the S + C sample showed that siliceous sand sintering occurs at approximately 1250 \({}^{\circ}\)C, resulting in a strong bond between the sand, but the strength of the actual mold body does not significantly increase. Therefore, the cristobalite formation apparently dominates the crystallization of sand [30; 31]. The M + C and S + C samples have similar peaks owing to sintering above 1200 \({}^{\circ}\)C [25; 32]. However, in the A + C sample, the peak is sintered near 1350 \({}^{\circ}\)C [32]. These results support the increase in strength observed at heat treatment above 1000 \({}^{\circ}\)C.  
Figure 5b compares the reduction in mass according to temperature change. Because the CAC content was different in volume ratio for each prepared sample, the amount of hydration was expected to be different. The CAC content was measured by hydrating the sand, in which 20 wt% was added in mass ratios in the same manner as the S + C, M + C, and A + C samples. Assuming that the rapid gas generation is caused by the molten metal during the casting process, the speed of generation of the initial gas and the amount of generated gas affects the casting defects [27]. The S + C sample exhibits a relatively small mass decrease of 0.9%. In contrast, in the A + C sample, the mass decrease is 1.6% up to 1200 \({}^{\circ}\)C, and the M + C sample shows a mass decrease of 2.4%. For the S + C and A + C  
Figure 5: (a) DSC profiles of molded body, (b) TG profiles of molded body. samples, the mass loss upon heating the mold using molten cast iron is \(\leq\)1.6%, the level that is encountered in commercial foundry sand [33]. This reduction in mass is proportional to the amount of gas generated during casting. Therefore, the smaller the generation of gas, the more suitable the mold material is for casting. In addition, the mass reduction velocity can also predict the slowest gas generation process in the case of the S + C sample. To confirm that the strength trends for each type of sand by high-temperature heat treatment are different, we attempted to identify the phase changes on the surface of molds as the temperature dropped to 1400 \({}^{\circ}\)C or less, assuming that the steel was cooled after initial molding for each sample and the temperature of molten metal is approximately 1400 \({}^{\circ}\)C. Therefore, the phases after 30 min of heat treatment at 1400 \({}^{\circ}\)C were compared. Figure 6 shows the XRD pattern of the S + C sample. The C\({}_{2}\)AH\({}_{8}\) and C\({}_{4}\)AH\({}_{19}\) phases, which are hydrated by the CAC observed at 200 \({}^{\circ}\)C, confirm that the peaks weaken owing to dehydration at 1400 \({}^{\circ}\)C. The existing amorphous silica crystallizes at 1400 \({}^{\circ}\)C, which promotes the cristobalite phase formation [34; 35]. Cristobalite reduces the hardening strength of the mold, thereby affecting the low sintering strength shown in Figure 4b. Figure 7 shows the XRD pattern of the M + C sample. The molded body dried at 200 \({}^{\circ}\)C is sufficiently hydrated and shows no CA, CA\({}_{2}\), and CA\({}_{6}\) component peaks, which are the CAC phases.