# Preparation of Cementitious Material with Wet Fly Ash by Hydrothermal Reaction and Calcination
## 3 Results and Discussion
### Hydrothermal Synthesis System Analysis
#### 3.1.2 CaO Reaction Degree Analysis  
Figure 5 shows the specimens' DSC thermal analysis measurements and TG curves for different pressure evaporation conditions. The DSC curves show sharp heat absorption peaks around 120 \({}^{\circ}\)C, 350 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, 560 \({}^{\circ}\)C, and 750 \({}^{\circ}\)C. The TG curves all show an accompanying uniform weight loss from 0 \({}^{\circ}\)C to 100 \({}^{\circ}\)C, mainly due to the removal of adsorbed water from the pressure evaporation products as well as gel water and interlayer water in the form of water molecules. The heat absorption effect that occurs near 120 \({}^{\circ}\)C is due to the dehydration of C\({}_{2}\)SH. A prominent heat absorption peak was found near 450 \({}^{\circ}\)C, deep and wide, accompanied by a significant weight loss in the TG curve. This region mainly removes crystalline water from Ca(OH)\({}_{2}\) in the product. The reaction generating C\({}_{3}\)AH\({}_{6}\) also produced three heat absorption peaks in the DSC curve, accordingly [39], with a slight heat absorption effect at 350 \({}^{\circ}\)C and 560 \({}^{\circ}\)C. C\({}_{3}\)AH\({}_{6}\) crystals each shed part of their crystalline water, and when the temperature continued to rise to about 750 \({}^{\circ}\)C, the dehydration reaction of C\({}_{3}\)AH\({}_{6}\) crystals was complete, accompanied by the dehydration and weight loss of C\({}_{3}\)ASH\({}_{4}\)[40].  
Figure 3: XRD patterns of autoclaved samples at 120 \({}^{\circ}\)C (**a**), 140 \({}^{\circ}\)C (**b**), and 160 \({}^{\circ}\)C (**c**).  
Figure 4: XRD patterns of samples autoclaved for 4 h (**a**), 6 h (**b**), and 8 h (**c**).  
The DSC curve was extrapolated to determine the start and end points of the Ca(OH)\({}_{2}\) absorption peak dehydration reaction temperature, and the Ca(OH)\({}_{2}\) weight loss in the TG profile was combined to calculate the amount of CaO consumed during the pressure evaporation reaction. The degree of calcium oxide reaction in the compressed steam specimen is the consumption of CaO as a percentage of the initial mass fraction of CaO during the compressed steam reaction, so the degree of calcium oxide reaction during the compressed steam process in terms of the scorch base is calculated by the following formula:  
\[f(CaO)=\left(1-\frac{\frac{56\Delta m}{18}}{\frac{n}{100-LOI}}\right)\times 100\% \tag{6}\]  
where _f_(_CaO_) is the degree of calcium oxide reaction during the autoclave process,%; \(\Delta m\) is the weight loss of Ca(OH)\({}_{2}\) in the TG plot, wt%; \(n\) is the initial mass fraction of CaO in the system before autoclave,%; \(LOI\) is the loss on burning of fly ash, wt%; 18 is the relative molecular mass of H\({}_{2}\)O; and 56 is the relative molecular mass of CaO.  
The results of calculating the degree of reaction of calcium oxide in different specimens are shown in the Table 3. The degree of CaO reaction in two groups of autoclaved samples at 140 \({}^{\circ}\)C-8 h--22% and 160 \({}^{\circ}\)C-6 h--22% reached more than 75%. When the lime content is the same, the degree of CaO reaction increases with the autoclave temperature and time  
Figure 5: DSC (**a**) and TG (**b**) curves of autoclaved samples with fly ash.  
increase. At 22% lime dosing, the CaO reaction in the autoclaved samples reached over 70% and was significantly greater than that of the autoclaved samples at 26% and 30% lime dosing.