## 4 Test Results and Analysis
### pH Changing Zone
Moreover, the pH value increases gradually with the increase in carbonation depth and increases with the increase in CO\({}_{2}\) AA by the cement mortar. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Carbonation** & **0** & **2** & **4** & **6** & **8** & **10** & **12** & **14** & **16** & **18** & **20** \\
**Depth/mm** & & & & & & & & & & & & \\ \hline CO\({}_{2}\) 0\% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.6 \(\pm\) 0.2 & 9.8 \(\pm\) 0.2 & 11.2 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.0 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 12.9 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 \\ CO\({}_{2}\)0.44\% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.7 \(\pm\) 0.2 & 10.0 \(\pm\) 0.2 & 11.5 \(\pm\) 0.2 & 13.3 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 \\ CO\({}_{0}\) 8.88 \% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.7 \(\pm\) 0.2 & 10.4 \(\pm\) 0.2 & 12.0 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 12.9 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 \\ CO\({}_{2}\) 1.32\% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.8 \(\pm\) 0.2 & 10.9 \(\pm\) 0.2 & 12.3 \(\pm\) 0.2 & 13.0 \(\pm\) 0.2 & 13.2 \(\pm\) 0.2 & 13.0 \(\pm\) 0.2 & 13.3 \(\pm\) 0.2 & 13.2 \(\pm\) 0.2 \\ CO\({}_{1}\) 1.76\% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 9.0 \(\pm\) 0.2 & 11.2 \(\pm\) 0.2 & 12.7 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.2 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 \\ CO\({}_{2}\) 2.20\% & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 8.5 \(\pm\) 0.2 & 9.0 \(\pm\) 0.2 & 11.5 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.2 \(\pm\) 0.2 & 13.0 \(\pm\) 0.2 & 13.1 \(\pm\) 0.2 & 13.2 \(\pm\) 0.2 \\ \hline \end{tabular}
\end{table} 
Table 6: pH value at 28 d in carbonized area of cement mortar with different CO\({}_{2}\) absorption (ultrasonic vibration agitation). Figure 10: pH distribution of CO\({}_{2}\)-absorbing cement mortar for 28 days (ultrasonic vibration agitation). Figure 9: pH distribution of CO\({}_{2}\)-absorbing cement mortar for 28 days (mechanical agitation). (3) Comparative analysis between two stirring methods  
In the early carbonation stage, the pH value of specimens at carbonation depths of 0-4 mm is 8.5 under both mechanical agitation and ultrasonic vibration agitation stirring. This zone is called the complete carbonation zone. The pH value of specimens at carbonation depths of 4\(\sim\)12 mm increases to about 13.0 (Figures 9 and 10). When the carbonation depth increases to 12 mm, the pH of cement mortar remains constant at about 13.0, reaching a stable state. It indicates that the Ph value of concrete increases with the increase in carbonation depth under both mixing methods, and the alkalinity increases. In addition, with the increase in the amount of CO\({}_{2}\) absorbed by the freshly mixed cement paste under the two mixing methods, the pH value of the hardened cement mortar in the carbonation zone changes in reverse order. Under mechanical agitation, the pH value of the carbonation zone generally decreases with the increase in CO\({}_{2}\) AA by the fresh cement mortar. However, the pH value of the carbonation zone under ultrasonic vibration agitation stirring is relatively stable and almost presents a rising trend. The major reasons for this observation are introduced as follows. Before molding of specimens, there is no variation law of pH at the same carbonation depth with the increase of CO\({}_{2}\) AA under mechanical agitation, due to the insufficient distribution uniformity in the paste. Instead, the pH value declines gradually. Influenced by the ultrasonic \"cavitation effect\", the internal structural distribution is more uniform and more compacted under ultrasonic vibration agitation stirring. With the increase of CO\({}_{2}\) AA, the porosity of specimens decreases continuously. When the specimens are further carbonized with CO\({}_{2}\) in air, the invasion speed of CO\({}_{2}\) into the specimens decreases with the decrease of internal porosity. As a result, the pH value presents a rising trend.