## 3 Results and Discussion
### Thermogravimetric  
Figure 11 shows the TG and DTG curves of the paste samples of C100, 20 FA, 20 QZ, and 10 FA10 QZ at 28 days. The mass loss of these samples mainly occurred at three temperature stages: 1. The low-temperature peak (120-300 \({}^{\circ}\)C) is due to the dehydration of C-S-H and AFt. 2. The medium-temperature peak (400-500 \({}^{\circ}\)C) is related to the decomposition of CH. 3. The high-temperature peak (500-800 \({}^{\circ}\)C) is related to the decomposition of calcium carbonate [44]. The percentage of chemically bound water and CH was calculated based on the weight loss data.  
In the first stage, the mass loss is caused by the dehydration of C-S-H and AFt in the sample. When QZ is used to replace cement, the intensity of the dehydration peak decreases, mainly due to the reduction in OPC content and hydration products. It was further found that when FA is used to replace cement, the decomposition mass increases at this temperature, indicating that the secondary hydration reaction of FA produces additional hydration products.  
In the second stage, mass loss is caused by CH decomposition. The content of CH in the paste was determined using Weerdt [54] and the formula in Equation (5):  
\[\mathrm{CH}=\frac{\mathrm{W}_{400}-\mathrm{W}_{500}}{\mathrm{W}_{550}}\times \frac{74}{18}\times 100\% \tag{5}\]  
where \(\mathrm{W}_{400}\), \(\mathrm{W}_{500}\), and \(\mathrm{W}_{550}\) are the sample masses at 400, 500, and 550 \({}^{\circ}\)C, respectively. Table 3 shows the calculated mass percentage of CH in the paste samples at 28 days. According to the results, as SCMs are added, the mass fraction of CH gradually decreases, which is also confirmed by the XRD experiment. This is because the hydration product of C\({}_{3}\)S and C\({}_{2}\)S in OPC is CH, and the content of C\({}_{3}\)S and C\({}_{2}\)S decreases after OPC is replaced. In addition, CH and FA will undergo a secondary hydration reaction, which will also consume a portion of CH, thus reducing the CH content in the 20 FA and 10 FA10 QZ groups.  
Figure 11: Weight loss (%) registered by thermal analysis data: thermogravimetry—TG (**left**) and differential thermogravimetry—DTG (**right**) of the pastes studied at the ages of 28 days.  
The third stage is the mass loss peak caused by the decomposition of calcium carbonate. The decomposition peak appearing here mainly originates from the limestone in the cement. The decomposition peak of calcium carbonate decreased significantly with the use of the two SCMs. This is because FA and QZ do not contain calcium carbonate, so the decomposition peak of calcium carbonate decreases as OPC is replaced.  
[PERSON] [54] calculated the chemically bound water content in the paste samples using the method of Equation (6):  
\[\text{Wt}=\frac{\text{W}_{40}-\text{W}_{550}}{\text{W}_{550}}\times 100\% \tag{6}\]  
\(\text{W}_{40}\) and \(\text{W}_{550}\) represent the mass measured by TGA at 40 and 550 \({}^{\circ}\)C, and Wt is the calculated chemically bound water content. The calculation results are shown in Table 6. The bound water content is high to low in the order of C100, 20 FA group, 10 FA10 QZ group, and 20 QZ group. This is because the amount of OPC used in the experimental groups is the same, and the final content of chemically bound water will depend on the reactivity and reaction products of FA and QZ. The reactivity of QZ is much lower than that of FA, and it can only react with CH in cement to produce a small amount of C-S-H [55]. For the FA group, because FA has pozzolanic activity, as the curing age increases, the number of secondary hydration products increases, and the final content of chemically bound water will also increase.