# Properties and Mechanism of Hydration of Fly Ash Belite Cement Prepared from Low-Quality Fly Ash
## 3 Results
### Composition of FABC Hydration Products  
The FTIR patterns of FABC pastes hydrated for different ages are shown in Figure 5. The results showed that the wavenumbers and curves of 3-, 7-, 28-, and 90-day pastes were essentially the same, indicating that the hydration products did not change with the development of the hydration process. The infrared spectra of pastes of four ages showed the characteristics of hydration products in wide absorption bands. This was largely due to two reasons [19]: first, the hydration products of FABC had a lower degree of crystallization, resulting in a larger change in the vibration frequency of groups; second, it was a superposition of infrared spectroscopy from various hydration products in the similarity region.  
The strong bands at 872 and 964 cm\({}^{-1}\) were because of the asymmetrical stretching vibration (\(\
u_{3}\)) of [SiO\({}_{4}\)]\({}^{2-}\) and [AlO\({}_{4}\)]\({}^{-}\) groups from C-S-H and C-A-H gels, which had a disordered structure. [CO\({}_{3}\)]\({}^{2-}\) groups were observed at 1430 and 872 cm\({}^{-1}\) assigned to the asymmetrical stretching vibration (\(\
u_{3}\)) and out-of-plane bending vibration (\(\
u_{2}\)), respectively, indicating that carbonation easily occurred in hydration products. Another strong band at 3440 cm\({}^{-1}\) of crystal water was due to the asymmetrical stretching vibration (\(\
u_{3}\)). Different from most ordinary Portland cement pastes, the band at 3640 cm\({}^{-1}\) of the OH\({}^{-}\) group due to the asymmetrical stretching vibration (\(\
u_{3}\)) in portlandite was never found. The possible reason was that all Ca(OH)\({}_{2}\) was consumed in forming AFt according to Equation (1), until it was completely absorbed. With the increase in curing time, the bands at 900-1100 cm\({}^{-1}\) of [SiO\({}_{4}\)]\({}^{2-}\) groups were found to shift to higher frequencies, suggesting that the degree of silicate polymerization increased with the hydration time. Therefore, it was predicted that the Si-O bonds of Q\({}_{2}\) in C-S-H gels increased, and more hydration products were generated.  
XRD patterns of FABC pastes hydrated for 3, 7, 28, and 90 days are shown in Figure 6. After 3 days, C-S-H gels (20 = 28-30\({}^{\circ}\)) and AFt (20 = 9.2\({}^{\circ}\)) were generated in large numbers. In addition, unhydrated \(\alpha^{\prime}\)L-C\({}_{2}\)S (20 = 32-34\({}^{\circ}\)), unreacted quartz, mulilte, and AFt were found in FABC pastes. After 7-day curing, the diffraction peaks of AFt (20 = 9.2\({}^{\circ}\)) became sharper, proving that gypsum participated in the reaction sustainably. After 28-day curing, these products still existed as previously generated. With the increasing age, the intensity of the characteristic diffraction peaks of AFt was enhanced obviously. At the age of 90 days, \(\alpha^{\prime}\)L-C\({}_{2}\)S almost disappeared, the content of AFt had no significant change, and calcium carbonate was formed by carbonization reaction. In addition, no Ca(OH)\({}_{2}\) was found in  
Figure 5: Fourier transform infrared spectrometer (FTIR) patterns of pastes hydrated for different ages.  
this hydration product on analyzing the XRD patterns because \(\alpha^{\prime}\)\({}_{\text{L}}\)-C\({}_{\text{2}}\)S in the clinker contained a low content of CaO and Ca(OH)\({}_{\text{2}}\) completely reacted to form AFt. Finally, Equation (1) was as follows.  
\[\begin{split} 12\text{CaO}\cdot 7\text{Al}_{\text{2}}\text{O}_{\text{3}} +9\text{Ca(OH)}_{\text{2}}+21\text{CaSO}_{\text{4}}\cdot 2\text{H}_{\text{2}}\text{O}\\ \rightarrow 7(3\text{CaO}\cdot\text{Al}_{\text{2}}\text{O}_{\text{3}}\cdot 3 \text{CaSO}_{\text{4}}\cdot 32\text{H}_{\text{2}}\text{O})\end{split} \tag{1}\]