## 4 Test Results and Analysis
### pH Changing Zone  
Since the carbonation depth measurement method based on the phenolphthalein indicator cannot distinguish changes in pH value within incomplete carbonation zones accurately [26], the carbonation process of the hardened paste is represented more clearly and intuitively through the length of the pH changing section. This has very important significance to elaborate the carbonation process of hardened paste accurately [27; 28]. The pH distribution patterns of hardened pastes with different CO\({}_{2}\) AA under mechanical agitation and ultrasonic vibration agitation stirring are introduced as follows. The test results of pH values at different depths are compared. Therefore, the pH distribution and variation laws of the carbonation zone of the hardened paste under ultrasonic vibration agitation with the increase of CO\({}_{2}\) AA are disclosed more clearly. (1) Mechanical agitation  
The test results and distribution patterns of the pH value of the carbonation zone of the hardened paste after CO\({}_{2}\) absorption under mechanical agitation (28 d) are shown in Table 5 and Figure 9. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Carbonation** & \multirow{2}{*}{**0**} & \multirow{2}{*}{**2**} & \multirow{2}{*}{**4**} & \multirow{2}{*}{**6**} & \multirow{2}{*}{**8**} & \multirow{2}{*}{**10**} & \multirow{2}{*}{**12**} & \multirow{2}{*}{**14**} & \multirow{2}{*}{**16**} & \multirow{2}{*}{**18**} & \multirow{2}{*}{**20**} \\
**Depth/mm** & & & & & & & & & & & & \\ \hline CO\({}_{2}\) 0\%. & 8.5 \(\pm\) 0.5 & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.5 & 9.5 \(\pm\) 0.7 & 11.7 \(\pm\) 0.8 & 12.0 \(\pm\) 1.1 & 13.0 \(\pm\) 1.2 & 13.2 \(\pm\) 1.1 & 13.1 \(\pm\) 1.2 & 12.9 \(\pm\) 1.0 & 13.0 \(\pm\) 1.1 \\ CO\({}_{2}\) 0.44\%. & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.7 & 8.5 \(\pm\) 0.6 & 9.6 \(\pm\) 0.8 & 11.0 \(\pm\) 0.9 & 12.2 \(\pm\) 1.1 & 13.1 \(\pm\) 1.0 & 13.0 \(\pm\) 1.2 & 13.0 \(\pm\) 1.1 & 13.1 \(\pm\) 1.2 & 13.1 \(\pm\) 1.0 \\ CO\({}_{2}\) 0.88\%. & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.7 & 8.5 \(\pm\) 0.8 & 9.3 \(\pm\) 0.6 & 10.5 \(\pm\) 0.9 & 11.6 \(\pm\) 1.0 & 13.1 \(\pm\) 1.3 & 12.9 \(\pm\) 1.3 & 13.2 \(\pm\) 1.0 & 12.9 \(\pm\) 1.1 & 13.1 \(\pm\) 1.1 \\ CO\({}_{2}\) 1.32\%. & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.7 & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.8 & 9.7 \(\pm\) 0.9 & 11.1 \(\pm\) 0.9 & 13.0 \(\pm\) 1.2 & 13.1 \(\pm\) 1.2 & 12.9 \(\pm\) 1.1 & 13.0 \(\pm\) 1.2 & 13.1 \(\pm\) 1.2 \\ CO\({}_{2}\) 1.76\%. & 8.5 \(\pm\) 0.5 & 8.5 \(\pm\) 0.7 & 8.5 \(\pm\) 0.8 & 9.0 \(\pm\) 0.2 & 10.2 \(\pm\) 1.0 & 10.9 \(\pm\) 1.1 & 12.9 \(\pm\) 1.2 & 13.0 \(\pm\) 1.1 & 13.0 \(\pm\) 1.2 & 13.2 \(\pm\) 1.2 & 13.1 \(\pm\) 1.3 \\ CO\({}_{2}\) 2.02\%. & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.7 & 8.5 \(\pm\) 0.6 & 8.5 \(\pm\) 0.6 & 9.5 \(\pm\) 1.0 & 10.8 \(\pm\) 0.9 & 12.8 \(\pm\) 1.1 & 13.1 \(\pm\) 1.1 & 13.0 \(\pm\) 1.0 & 13.1 \(\pm\) 1.1 & 13.2 \(\pm\) 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: pH value at 28 d in carbonized area of cement mortar with different CO\({}_{2}\) absorption (mechanical agitation). Figure 8: Carbonization depth curves of different CO\({}_{2}\) absorption stages. The pH values at carbonation depths of 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 mm when the CO\({}_{2}\) AA is 0%, 0.44%, 0.88%, 1.32%, 1.64%, and 2.20% were tested. According to the tested pH values at different carbonation depths, zones with great variation in pH value are mainly concentrated within the carbonation depth range of 6 to 12 mm. It can be seen from Figure 9 that the pH of the carbonation zone decreases with the increase of CO\({}_{2}\) AA by the cement mortar. (2) Ultrasonic vibration agitation stirring  
The test results and distribution patterns of the pH value of the carbonation zone of the hardened paste after CO\({}_{2}\) absorption under ultrasonic vibration agitation stirring (28 d) are shown in Table 6 and Figure 10.  
It can be seen from Figure 10 that zones with large pH variations are concentrated within the carbonation depth range of 6 to 12 mm, which is similar to that under mechanical agitation.