# Properties and Mechanism of Hydration of Fly Ash Belite Cement Prepared from Low-Quality Fly Ash
## 4 Discussion
### Hydration Activity of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S in FABC Pastes  
Compared with C\({}_{2}\)S usually synthesized at higher than 1300 \({}^{\circ}\)C, the highly active C\({}_{2}\)S (\(\alpha^{\prime}\)L-C\({}_{2}\)S) synthesized at 800 \({}^{\circ}\)C had smaller grains, larger specific surface area, higher porosity, higher hydration speed, and smaller later strength advantage. The highly active C\({}_{2}\)S synthesized at a low temperature was responsible for the relatively high strength of fly ash belite cementitious material. However, the early strength development of FABC pastes was still slow. Especially, the early strength development from 1 to 3 days was slow. Hence, the early activity of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S was far lower than that of C\({}_{3}\)S in Portland cement [21; 22; 23]. The analysis of porosity and microstructure showed that the low strength of FABC pastes was affected by high porosity. Although the proportion of harmful holes was smaller than that in Portland cement pastes, the harmful influence of porosity was more than that in Portland cement pastes because of high porosity.  
Hydration Mechanism of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S -C\({}_{12}\)A\({}_{7}\)-\(\alpha\)So\({}_{4}\)-System  
The hydration products of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S single minerals were C-S-H gels and Ca(OH)\({}_{2}\), and the hydration products of the \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S-C\({}_{12}\)A\({}_{7}\)-CaSO\({}_{4}\) system were C-S-H gels, AFt, and C\({}_{3}\)AH\({}_{6}\). The coexistence of CaSO\({}_{4}\) and C\({}_{12}\)A\({}_{7}\) promoted the hydration of C\({}_{2}\)S, This was because the hydration of C\({}_{12}\)A\({}_{7}\) released Al(OH)\({}^{4-}\), which reacted with Ca(OH)\({}_{2}\) to form C\({}_{3}\)AH\({}_{6}\), thus reducing the concentration of Ca\({}^{2+}\) and accelerating the hydration of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S. In the presence of CaSO\({}_{4}\), SO\({}_{4}\)\({}^{2}\)- combined with Ca\({}^{2+}\) and Al(OH)\({}^{4-}\) in the liquid phase to form ettrinigite. It reduced the content of hydrated calcium aluminate and Ca(OH)\({}_{2}\), accelerating the hydration of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S further. It indicated that CaSO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O changed the hydration products of C\({}_{12}\)A\({}_{7}\) and improved the hydration degree of FABC to a certain extent.  
Hence, the coexistence of CaSO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O and C\({}_{12}\)A\({}_{7}\) accelerated the hydration of \(\alpha^{\prime}_{L}\)-C\({}_{2}\)S [24; 25; 26]. Clinker with a porous and loose structure had a high water requirement, which also resulted in the poor strength properties of FABC hydration pastes. FABC and Portland cement can be added to the composite to improve the hydration degree and strength of pastes, while Portland cement provided the alkaline environment and accelerated the hydration of FABC pastes. This was a very good and effective way to reduce the porosity influence on FABC, protect clay resources, and improve the utilization rate of low-quality fly ash.