# Properties and Mechanism of Hydration of Fly Ash Belite Cement Prepared from Low-Quality Fly Ash
## 2 Materials and Methods  
### Raw Materials  
The FA was stored in the Xingtai Thermal Power Plant of Northern China. NaOH (Analytical Reagent, NaOH \(\geq\) 96.0%, Chron Chemicals, Chengdu, China) was used as an activator in the process of granule-hydrothermal synthesis. The preparation steps of FABC were as follows. First, FA, pure lime with 99 wt % CaO (Analytical Reagent, CaO \(\geq\) 98.0%, Chron Chemicals, Chengdu, China), and Na\({}_{2}\)O (Na\({}_{2}\)O was added as equivalent NaOH) at a ratio of 70:30:1 were mixed uniformly. Second, the mixtures were wound into a ball with the diameter ranging from 8 to 12 mm through a small disk granulator. Third, these granule specimens were hydrothermally treated at 90-100 \({}^{\circ}\)C for at least 12 h. Finally, all dried precursors were burned in a furnace in the following parameter settings: rate of temperature rise, 10 \({}^{\circ}\)C/min; final temperature, 800 \({}^{\circ}\)C; holding time, 1-1.5 h, and rapid cooling. The residual over the 45-\(\upmu\)m sieve in the clinker was less than 5.0%, and the density of the clinker was 2.82 kg/m\({}^{3}\). Then, FABC was prepared by adding designed proportions of gypsum (Analytical Reagent, CaSO4\({}_{4}\)2H\({}_{2}\)O \(\geq\) 99.0%, Chron Chemicals, Chengdu, China). FABC was compared with Chinese P:I 42.5 cement, which was defined as reference cement (RC). ISO679-standard sands (Xiamen ISO Standard Sand Co.,Ltd, Xiamen, China) were used for testing the strength of cement mortars. The chemical composition of FA, clinker, and RC are given in Table 1.  
The phase composition of the clinker under the above preparation conditions is shown in Figure 1. The clinker had \(\alpha^{\prime}\)1-C\({}_{2}\)S (d = 0.2730 and 0.2663 nm), C\({}_{12}\)A\({}_{7}\) (20 = 18.0\({}^{\circ}\) and 34.0\({}^{\circ}\)), quartz (20 = 20.8\({}^{\circ}\) and 26.7\({}^{\circ}\)), and mullite (20 = 16.4\({}^{\circ}\) and 26.1\({}^{\circ}\)). These broader characteristic peaks of 20 from 32\({}^{\circ}\) to 36\({}^{\circ}\) demonstrated the lower crystallinity and smaller grain size of \(\alpha^{\prime}\)1-C\({}_{2}\)S compared with \(\beta\)-C\({}_{2}\)S. It also proved that the hydration activity of \(\alpha^{\prime}\)1-C\({}_{2}\)S was higher than that of \(\beta\)-C\({}_{2}\)S. However, gehlenite (2 CaO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\); C\({}_{2}\)AS, 20 = 18.0\({}^{\circ}\) and 34.0\({}^{\circ}\)) without any hydraulicity was created from CaO, SiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\) when the temperature exceeded 900 \({}^{\circ}\)C. With the increasing calcination temperature, the number of C\({}_{2}\)AS increased gradually, while the content of \(\beta\)-C\({}_{2}\)S and C\({}_{12}\)A\({}_{7}\) decreased, resulting in lower hydration activity. For all these reasons, the proper calcination temperature of clinker ranged from 800 to 900 \({}^{\circ}\)C and effectively avoided the formation of C\({}_{2}\)AS. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Composition/Property** & **Fly Ash** & **Clinker** & **P:I 42.5 Cement (RC)** \\ \hline Chemical composition, & & & \\ wt \% & & & \\ SiO\({}_{2}\) & 49.21 & 32.94 & 22.04 \\ Al\({}_{2}\)O\({}_{3}\) & 29.46 & 19.68 & 4.76 \\ Fe\({}_{2}\)O\({}_{3}\) & 4.36 & 3.80 & 3.10 \\ CaO & 2.26 & 36.87 & 64.5 \\ MgO & 0.80 & 0.82 & 0.92 \\ TiO\({}_{2}\) & 1.21 & 0.83 & N/A \\ Na\({}_{2}\)O & 0.39 & 1.06 & 0.53 \\ K\({}_{2}\)O & 0.28 & 0.71 & 0.32 \\ SO\({}_{3}\) & 0.18 & 0.53 & 1.90 \\ Loss & 10.08 & 1.97 & 1.01 \\ Density, g/cm\({}^{3}\) & N/A & 2.82 & 3.12 \\ Blaine specific area, & 438.3 & 585 & 332 \\ \multicolumn{2}{c}{} \\ Residual, \(>\)45 \(\upmu\)m/wt \% & 2.4 & 4.5 & 6.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and physical properties of fly ash, clinker, and reference cement (RC). Figure 1: X-ray diffraction of belite cement clinker. The microstructure of the clinker is shown in Figure 2. As shown in the figure, the clinker minerals were made of small particles, which were mostly less than 100 nm.