# Preparation of Cementitious Material with Wet Fly Ash by Hydrothermal Reaction and Calcination
## 3 Results and Discussion  
### Hydrothermal Synthesis System Analysis  
#### 3.1.1 Reaction Products Analysis  
Figure 3 shows the XRD patterns of the specimens with different autoclave conditions. It can be seen from Figure 3 that the autoclave specimens contain nullite (A\({}_{3}\)S\({}_{2}\)) and quartz (SiO\({}_{2}\)) in fly ash, and the newly formed substances are Ca(OH)\({}_{2}\), C\({}_{2}\)SH, C\({}_{3}\)AH\({}_{6}\), and C\({}_{3}\)ASH\({}_{4}\). Ca(OH)\({}_{2}\) is formed by the autoclave system's reaction between calcium oxide and water. Due to the different conditions of the autoclave process, the content of each newly formed mineral composition varies. Figure 3a demonstrates that C\({}_{2}\)SH has started to form when the autoclave temperature is 120 \({}^{\circ}\)C for 4 h. When the autoclaving time is extended to 6 h, the C\({}_{2}\)SH diffraction peak is enhanced, but when the autoclaving time reaches 8 h, the intensity of its diffraction peak decreases instead. This is because C\({}_{2}\)SH is a kind of hydration product easily generated under a high calcium-silica ratio [36]. When the autoclaving time is short, the amount of dissolved silicon and aluminum in fly ash is less, the calcium-silica ratio in the system is higher at this time, and C\({}_{2}\)SH starts to form and gradually increases. With the extension of the autoclaving time, the amount of silicon and aluminum in the fly ash continues to dissolve in large quantities, the calcium-silica ratio decreases, and the stability of C\({}_{2}\)SH decreases and gradually decomposes to form CSH gel [37]. The dispersion peak of 20 between 25\({}^{\circ}\) and 30\({}^{\circ}\) is evident in the XRD pattern at this time. The intensity of the C\({}_{2}\)SH diffraction peaks was maximized at four hours when the autoclave temperature wasincreased to 140 \({}^{\circ}\)C and 160 \({}^{\circ}\)C, indicating that the formation of C\({}_{2}\)SH was accelerated by the rapid dissolution of silicon and aluminum from fly ash under a high temperature and pressure, and by the strengthening of the hydrothermal reaction driving force due to the high temperature. The intensity of the C\({}_{2}\)SH diffraction peaks was maximized at four hours when the steaming temperature was increased to 140 \({}^{\circ}\)C and 160 \({}^{\circ}\)C, indicating that the formation of C\({}_{2}\)SH was accelerated by the rapid dissolution of silicon and aluminum from fly ash under a high temperature and pressure, and by the strengthening of the hydrothermal reaction driving force due to the high temperature. C\({}_{3}\)AH\({}_{6}\) diffraction peaks appeared in the XRD patterns of all the specimens. C\({}_{3}\)AH\({}_{6}\) diffraction peaks appeared in the XRD patterns of all the evaporated specimens. C\({}_{3}\)AH\({}_{6}\) is an easily formed and relatively stable compound in the CaO-Al\({}_{2}\)O\({}_{3}\)-H\({}_{2}\)O system between 20 \({}^{\circ}\)C and 225 \({}^{\circ}\)C [38]. The C\({}_{3}\)ASH\({}_{4}\) diffraction peaks gradually increased with the pressure evaporation time at the same pressure evaporation temperature. When the autoclave temperature was increased to 160 \({}^{\circ}\)C, the intensity of the C\({}_{3}\)ASH\({}_{4}\) diffraction peaks increased more obviously compared with the autoclave temperature of 120 \({}^{\circ}\)C and 140 \({}^{\circ}\)C, indicating that the longer the autoclaving time in the high-temperature environment, the more thoroughly the calcium hydroxide in the autoclave system reacts with the silica and alumina continuously dissolved in the fly ash, and the higher the produced amount is. Figure 4a shows that the diffraction peak of C\({}_{2}\)SH gradually enhanced with the increase in the autoclave temperature when the autoclaving time was four hours. The intensity of the C\({}_{2}\)SH diffraction peaks was most significant at 120 \({}^{\circ}\)C when the autoclaving time was six and eight hours. The intensity of the C\({}_{2}\)SH diffraction peaks decreased as the autoclave temperature increased.