# 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 Hydration Behavior of Oil Well Cement  
The hydration reaction is the most important physical and chemical reaction of cement and is directly related to the strength of hardened cement pastes. The change in the mineral composition of cement during the aging process inevitably affects its hydration. The early hydration behaviors of the A0, A7, A14, and A28 samples at 20 \({}^{\circ}\)C were analyzed using the heat flow thermal method, as shown in Figure 7.  
With the aging time increased, the intensity of the main heat release peak decreased. This could be attributed to the prehydration of C\({}_{3}\)S and C\({}_{3}\)A during the aging process. The decrease of C\({}_{3}\)S and C\({}_{3}\)A led to the decrease in hydration heat release. A0 sample had only one heat release peak, but A7 and A14 samples showed a second heat release peak. This was caused by the decrease in gypsum content, which could not fully react with C\({}_{3}\)A. After the gypsum was depleted, the remaining C\({}_{3}\)A continued the hydration reaction, and a second hydration heat release peak occurred [10; 26]. According to the analysis of mineral composition and specific surface area, the specific surface area and the clinker phases of aged cement decreased [27]. This meant that the hydration nucleation sites of the cement decrease, which caused a decrease on the hydration rate of cement. Moreover, the reduction in the specific surface area meant that the surface energy of the cement powder and the energy that cement can release during the hydration reaction reduced. These factors led to a reduction in the cumulative heat evolution during cement hydration, which reduced the compressive strength of the hardened cement pastes. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** &
\begin{tabular}{c} **Mass Loss (\%)** \\ **A0** \\ \end{tabular} & **A28** & **Added Mass (\%)** & **Origins of Mass Loss** \\ \hline
25–140 & 0.50 & 1.00 & 0.50 & Physically adsorbed water \\
140–400 & 0.10 & 0.60 & 0.50 & Decomposition of calcium sulfate hemihydrate \\
400–550 & 0.20 & 0.30 & 0.10 & Decomposition of calcium hydroxide and calcium aluminosulfate hydrate \\
550–1000 & 0.60 & 0.85 & 0.25 & Decomposition of calcium carbonate and C-S-H \\
25–1000 & 1.40 & 2.75 & 1.35 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The specific mass loss of cement samples A0 and A28. With increasing aging time, the cement sorbed H\({}_{2}\)O and CO\({}_{2}\) in the air, and the agglomeration of the cement particles led to a decrease in the specific surface area and surface energy of the powder. After water was sorbed, the clinker phase and gypsum in the cement continued to undergo a slight hydration reaction and produced hydration products, reducing the reaction phase with the mixing water. Therefore, the rate of nucleation of the cement hydration products decreased, and the heat release of hydrated cement decreased accordingly. Slight hydration during the aging process had an effect on the hydration of cement. Measured using an FBT-9 Blaine air permeability apparatus (Shanghai Yichang), the specific surface area of cement decreases with aging time. Meanwhile, the thickening time of cement paste also decreases. This phenomenon can be attributed to the decreased gypsum content in aged cement, which inadequately limits the rapid hydration of C\({}_{3}\)A [28; 29]. The longer the aging time is, the lower the gypsum content. Consequently, the inhibition of gypsum to the rapid hydration of C\({}_{3}\)A was weakened, the longer the aging time, the shorter the cement thickening time. As the specific surface area decreased, the fluidity and rheological properties of the unaged oil well cement improved [30]. Conversely, the aging process negatively impacted the aged cement. This phenomenon can be attributed to a reduction in the surface energy of the cement powder during aging, coupled with the formation of hydration products that increase particle friction.