# Effect of Aging on Class G High Sulfate-Resistant Oil Well Cement Under High Relative Air Humidity
## 3 Results and Discussion
### Influence of Aging on the Mineral Composition of Oil Well Cement  
Powder X-ray diffraction (XRD) was used to characterize the mineral composition changes throughout the cement aging process. The XRD patterns of samples A0, A7, A14, and A28 are shown in Figure 5, with the specific compositional changes shown in Table 6.  
Figure 4: The curves of particle size distributions of (**a**) sample A0, (**b**) sample A28.  
As the aging time increased, the C\({}_{3}\)S and C\({}_{3}\)A contents in the cement decreased, whereas the hydration product AFt emerged and continued to increase, along with an increase in the CH content. The total content of CaSO\({}_{4}\) decreased because of its involvement in the hydration of C\({}_{3}\)A [25].  
Thermogravimetric analysis (TGA) has provided a more comprehensive investigation of mineral composition changes during the aging process [3; 12; 14]. The TG-DSC curves of samples A0 and A28 are shown in Figure 6, the specific mass changes and the decomposition of compounds corresponding to different temperature ranges are shown in Table 7.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Cement Sample** & **C\({}_{3}\)S** & **C\({}_{2}\)S** & **C\({}_{3}\)A** & **C\({}_{4}\)AF** & **CaSO\({}_{4}\)+0.5H\({}_{2}\)O** & **CaSO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O** & **AFt** & **CH** & **CaCO\({}_{3}\)** \\ \hline A0 & 60.0 & 20.1 & 1.2 & 15.8 & 0.3 & 2.2 & 0.0 & 0.4 & 0.0 \\ A7 & 59.6 & 20.4 & 1.1 & 15.6 & 0.0 & 2.0 & 0.5 & 0.8 & 0.0 \\ A14 & 59.0 & 20.7 & 0.9 & 15.5 & 0.0 & 1.6 & 1.2 & 1.1 & 0.1 \\ A28 & 58.9 & 20.5 & 0.7 & 15.5 & 0.0 & 1.6 & 1.5 & 1.2 & 0.1 \\ \hline \end{tabular}
\end{table}
Table 6: The mineral compositions of cement samples after different aging time (wt.%).  
Figure 5: The curves of powder X-ray analysis of cement samples after different aging time.  
Figure 6: The curves of thermogravimetric analysis of **(a)** sample A0, **(b)** sample A28.  
Compared with sample A0, the mass loss of A28 increased by 1.35% between 50 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C. In the temperature range of 110 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, the mass loss of the A28 cement increased by 1% compared with that of the A0 cement, which accounted for 74% of the total increase. This mass increase was attributed primarily to the moisture absorption of the cement and the transformation of CaSO\({}_{4}\)\(\cdot\)0.5H\({}_{2}\)O into CaSO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O during aging. In the range of 400 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, the decomposition of hydration products, such as CH and calcium sulfoaluminate hydrate, led to an additional mass loss increase of 0.1%. During the aging process, C\({}_{3}\)S partially hydrated to form a C-S-H gel, and CH reacted with H\({}_{2}\)O and CO\({}_{2}\) to produce CaCO\({}_{3}\); both the C-S-H gel and CaCO\({}_{3}\) decomposed between 550 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, leading to a further increase of 0.25% in mass loss. This was consistent with the results of the XRD analysis.  
The CaCO\({}_{3}\) content of A28 sample analyzed by XRD increased by only 0.1% compared with that of A0 sample, which was consistent with the thermogravimetric results. This meant that, during the aging process, the cement samples sorbed more H\({}_{2}\)O than CO\({}_{2}\) from the air. [PERSON] [7] suggested that the H\({}_{2}\)O sorbed by cement samples from the air during aging was less than the CO\({}_{2}\) sorbed. The reason for the opposite was related to the aging condition of the closed curing box. In the box, the water from the saturated KCl solutions diffused into the air was constantly sorbed by the cement, whereas CO\({}_{2}\) was not replenished from the surrounding atmosphere.