# The Role of Brownmillerite in Preparation of High-Belite Sulfoaluminate Cement Clinker
## 3 Results and Discussion
### Sintering of HBSC Clinker Containing Brownmillerite  
#### 3.2.1 Burnability  
The observed physical appearance of HBSC clinker is summarized in Table 7. According to the TG-DTA results, sintering of HBSC clinker containing brownmillerite at 1300 \({}^{\circ}\)C, 1320 \({}^{\circ}\)C, 1330 \({}^{\circ}\)C and 1350 \({}^{\circ}\)C for 30 min was proposed, which was also used to evaluate the burnability of HBSC raw materials. The sintering temperature of clinker with proper appearance quality decreased with the increase in designed brownmillerite content. Normal and dense clinker was obtained at 1300 \({}^{\circ}\)C when the designed brownmillerite content in HBSC clinker exceeded 20%. Under these conditions, the clinker was hard and fully calcined. Higher sintering temperatures above 1300 \({}^{\circ}\)C led to over-burnt or molten HBSC clinker with higher designed brownmillerite content, causing the clinker to stick to the platinum sheet. According to the appearance of HBSC clinkers sintering at different temperatures, the shrinkage of clinkers GF-1, GF-2 and GF-3 was normal and dense at 1330 \({}^{\circ}\)C, while clinkers GF-4 and GF-5 were slightly over-calcined, leading to slight sticking, which could be removed from the platinum sheet. Most clinkers were seriously over-calcined and molten and adhered to the platinum sheet and proved hard to remove when the sintering temperature exceeded 1330 \({}^{\circ}\)C. In additiona sintering temperature below 1330 \({}^{\circ}\)C led to the under-burning of HBSC clinker. It can be seen from Figure 2 that the color of the clinker gradually changed from dark brown to light black with the increase in designed brownmillerite content, and the clinker became denser and glossier in this case. HBSC clinkers also became easier to melt with the increase in designed brownmillerite content.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Stage I** & **Stage II** & **Stage III** \\ \hline GF-1 & 4.65 & 25.63 & 0.89 \\ GF-2 & 4.51 & 26.63 & 1.08 \\ GF-3 & 3.70 & 26.84 & 0.89 \\ GF-4 & 3.26 & 27.30 & 0.65 \\ GF-5 & 2.87 & 28.48 & 0.58 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Weight loss of HBSC raw materials in TG curves (%).  
Figure 1: (**a**) DTA (1-Peak 1, 2-Peak2, 3-Peak3, 4-Peak4) and (**b**) TG (1-Stage I, 2-Stage II, 3-Stage III) curves of HBSC raw materials.  
\(f\)-CaO of HBSC linker containing brownmillerite sintering at different temperatures was detected. The results are shown in Table 8. \(f\)-CaO of the HBSC linker sintering at 1300 \({}^{\circ}\)C was lower than 1.0% due to the formation of a large number of clinker minerals. The content of \(f\)-CaO in HBSC clinker decreased gradually with the increase in sintering temperature. Less than 0.30% of \(f\)-CaO was detected when the sintering temperature was elevated to 1330 \({}^{\circ}\)C, which indicated that the clinker had been completely sintered. As observed in Figure 1b, sulfoaluminate phases would decompose above 1300 \({}^{\circ}\)C, which increased the \(f\)-CaO in the HBSC clinker. However, \(f\)-CaO in HBSC clinker sintering at 1350 \({}^{\circ}\)C, except for GF-1 and GF-2, was further decreased compared with HBSC clinker sintering at 1330 \({}^{\circ}\)C. This finding demonstrated that brownmillerite had a particular stabilizing effect on anhydrous calcium sulfoaluminate when the designed brownmillerite content in HBSC linker was greater than 20%. Based on the above, we concluded that HBSC raw materials designed in this study had better burnability, especially the samples sintering at 1330 \({}^{\circ}\)C.