## 3 Results
### Formation Process of ss-C\({}_{4}\)A\({}_{3}\)$  
The formation process of st-C\({}_{4}\)A\({}_{3}\)$ can be divided into three stages, including the stage for the decomposition of raw materials (mainly referring to the decomposition of CaCO\({}_{3}\) as  
Figure 1: XRD patterns of solid-solution ye’elimite (sintered at 1250 \({}^{\circ}\)C for 4 h) and stoichiometric ye’elimite (sintered at 1300 \({}^{\circ}\)C for 4 h). \begin{table}
\begin{tabular}{c c c c} \hline
**Types of** & **Crystallographic System** & **Lattice Parameters (Å)** & **Sources** \\
**Ye’elimite** & **(Space Group)** & **Lattice Parameters (Å)** & **Sources** \\ \hline \multirow{2}{*}{Stoichiometric} & Orthorhombic & \(a=13.041\), \(b=13.036\), \(c=9172\) & This paper \\  & (Pcc2) & \(a=13.037\), \(b=13.035\), \(c=9168\) & [15] \\ \multirow{2}{*}{Typical} & Cubic & \(a=9191\) & This paper \\  & solid-solution & (I-43 m) & \(a=9197\) & [21] \\ \hline \end{tabular}
\end{table} 
Table 3: Lattice parameters of synthesized solid-solution and stoichiometric ye’elimite. Figure 2: Rietveld plot for XRD pattern of ss-C\({}_{4}\)A\({}_{3}\)$ (sintered at 1250 \({}^{\circ}\)C for 4 h).  
shown in Equation (4)), the stage for the formation of intermediate phases (mainly referring to the formation of CA, as shown in Equation (5)), and the formation of ye'elimite (mainly referring to the reaction between CA and CS, as shown in Equation (6)). To determine the involved solid reactions in the formation process of ss-C\({}_{4}\)A\({}_{3}\)$, the XRD patterns of clinkers sintered at various temperatures for different holding times were compared, as shown in Figure 3.  
\[\text{CaCO}_{3}\rightarrow\text{CaO}+\text{CO}_{2} \tag{4}\]  
\[\text{CaO}+\text{Al}_{2}\text{O}_{3}\rightarrow\text{CaAl}_{2}\text{O}_{4} \tag{5}\]  
\[\text{3 CaAl}_{2}\text{O}_{4}+\text{CaSO}_{4}\rightarrow\text{Ca}_{4}\text{Al}_{6 }\text{SO}_{16} \tag{6}\]  
Comparing the XRD patterns for st-C\({}_{4}\)A\({}_{3}\)$ and ss-C\({}_{4}\)A\({}_{3}\)$ in Section 3.1, it was observed that the additional characteristic peaks of st-C\({}_{4}\)A\({}_{3}\)$ were not in any of the clinkers of ss-C\({}_{4}\)A\({}_{3}\)$ in Figure 3, which means that the formation process of ss-C\({}_{4}\)A\({}_{3}\)$ did not involve the formation of st-C\({}_{4}\)A\({}_{3}\)$. Clinkers sintered at various temperatures without holding times are exhibited in Figure 3a. It can be seen that certain amounts of ss-C\({}_{4}\)A\({}_{3}\)Figure 2:0 steadily increased; meanwhile, the intensity of the peaks of CaAl\({}_{2}\)O\({}_{4}\) and CaSO\({}_{4}\) decreased. Similar variational tendencies for the peaks of ss-C\({}_{4}\)A\({}_{3}\)Figure 2:1 steadily increased with increasing holding time; meanwhile, the contents of calcium aluminate phases (in most instances, CA accounted for the overwhelming majority)  
Figure 2:2 XRD patterns of clinkers for ss-C\({}_{4}\)A\({}_{3}\)$, (**a**) sintered at various temperatures without holding times, (**b**) sintered in conditions of different holding times at 1150 \({}^{\circ}\)C, (**c**) sintered at 1300 \({}^{\circ}\)C for different holding times. and anhydrite continually decreased. The variation tendency illustrates that ss-C\({}_{4}\)A\({}_{3}\)5 was formed through the reaction between the calcium aluminate phases and anhydrite in the forming stage, which was similar to the reaction in Equation (6). After the forming stage, the stable stage was observed at 1250 \({}^{\circ}\)C (in the range of 120-240 min) and 1300 \({}^{\circ}\)C (in the range of 30-120 min). In the stable stage, the contents of ss-C\({}_{4}\)A\({}_{3}\)5 remained stable and generally remained over 90 wt.% in the clinkers; meanwhile, the calcium aluminate phases and anhydrite accounted for the absolute minority (less than 10 wt.%). It should also be noted that the contents of ss-C\({}_{4}\)A\({}_{3}\)5 tended to decrease and generate calcium aluminate phases when the clinkers were sintered over 120 min at 1300 \({}^{\circ}\)C, which can be identified as the decomposing stage.