# Properties and Mechanism of Hydration of Fly Ash Belite Cement Prepared from Low-Quality Fly Ash
## 3 Results  
### Effect of Gypsum Content on the Performance of FABC Pastes  
The optimal amount of gypsum was investigated by observing the performance of pastes with different gypsum contents as 3%, 5%, 7%, and 10%; the results are shown in Table 2. As the gypsum content increased, the water requirement of cementitious materials decreased, and the setting time was slightly prolonged. In general, the setting time of FABC with different gypsum contents was very short. Even for the cement with a gypsum content of 10%, the initial setting time was only about 25 min, and the final setting time was only about 40 min. The main reason was that the activity and content of C\({}_{12}\)A\({}_{7}\) in the clinker were higher than those of C\({}_{3}\)A in Portland cement, and the reaction speed was very high. The water requirement of FABC was more than that of Portland cement, while the water requirement of RC was 28.6%. FABC had a higher porosity and specific surface area. Further, the clinker reacted immediately with water, which had a \"drying\" effect on the slurry and further increased the water requirement. The optimal amount of gypsum was about 7% of cement by weight. Meanwhile, the 3-day and 28-day compressive strength of cement pastes was 13.6 MPa and 60.2 MPa, respectively. This was because gypsum provided enough ettringite for covering and isolating the clinker particles, thus having a retarding effect on cement hydration and the setting of hydraulic minerals. In summary, the optimal amount of gypsum was about 7% of cement by weight.  
The mechanical properties of mortars with different gypsum contents were studied at the same time, and the results are shown in Figure 3. The results showed that the compressive strength and flexural strength were the highest when the gypsum-adding rate increased to 7%. The experimental results showed that the 3-, 28-, and 90-day compressive strengths of mortars reached 6.0, 25.9, and 28.4 MPa, respectively. In addition, the 3-, 28-, and 90-day flexural strengths of mortars reached 1.6, 4.6, and 5.8 MPa, respectively. The setting time of FABC was very short in the early stage because gypsum, as a sulfate admixture, promoted the reaction of alumina phase with Ca(OH)\({}_{2}\) to generate a more aciculate ettringite (AFt, 3 CaO\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)3 CaSO\({}_{4}\cdot\)32H\({}_{2}\)O) crystal and increased the strength at the early ages, which was stable at the later ages. However, the compressive and flexural strengths increased initially and then decreased with the increase in gypsum doses. The main reasons for this change in strength were as follows. On the one hand, AFt formed from the aluminate phase, Ca(OH)\({}_{2}\), and gypsum filled the porous space, which together with C-S-H gels shortened the setting time and improved the early strength. Mostly calcium aluminate hydrates, such as C\({}_{3}\)AH\({}_{8}\) and C\({}_{4}\)AH\({}_{16}\), were observed, without significant AFt using lower contents of gypsum. On the other hand, the extremely high content of gypsum in the clinker did not support the development of its later-stage strength due to the lack of hydraulic clinker. Especially, it resulted in expansion damage through delayed ettringite (DEF) formation. To summarize, the numerical results indicated that the optimum content of gypsum in FABC was 7%.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Gypsum (\%)**} & \multirow{2}{*}{
\begin{tabular}{c} **Water** \\ **Requirement (\%)** \\ \end{tabular} } & \multicolumn{2}{c}{**Setting Time (min)**} & \multicolumn{2}{c}{**Compressive Strength (MPa)**} \\ \cline{4-7}  & & & **Initial** & **Final** & **3-day** & **28-day** \\ \hline
1 & 3 & 43.9 & 16 & 22 & 10.7 & 36.3 \\
2 & 5 & 43.1 & 18 & 25 & 12.7 & 42.0 \\
3 & 7 & 42.5 & 20 & 30 & 13.6 & 60.2 \\
4 & 10 & 41.0 & 25 & 40 & 11.4 & 53.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance of fly ash belite cement (FABC) with different gypsum contents.