## 4 Test Results and Analysis
### Carbonation Performances Analysis
(2) Ultrasonic vibration agitation  
(1) Carbonation age at 3 d  
The carbonation of cement mortar after CO2 absorption under ultrasonic vibration agitation at 3 d is shown in Figure 5. The average carbonation depth under different CO2 AA was calculated according to the test results in Figures 5-7. The results are listed in Table 4. It can be seen from Figure 5 that, with the increase of CO2 AA, the area of color-developing zones increases to some extent. It can be calculated from Table 4 that, for each increase in CO2 AA, the carbonation depth at 3 d decreases by 7.7%, 8.3%, 4.5%, 9.5%, 5%, and 0%, respectively. (2) Carbonation age at 7 d  
The carbonation of cement mortar after CO2 absorption under ultrasonic vibration agitation at 7 d is shown in Figure 6. The area of color-developing zones is smaller compared to that at 3 d. With the increase of CO2 AA, the area of color-developing zones still increases to some extent. It can be calculated from Table 4 that with the increase of CO2 AA, the carbonation depth at 7 d decreases by 5%, 5.2%, 5.6%, 2.9%, 6.1%, and 6.5%,  
(3) Carbonation age at 28 d  
The carbonation of cement mortar after CO2 absorption under ultrasonic vibration agitation at 28 d is shown in Figure 7. The area of color developing zones further decreases compared to that at 7 d. Similar to the observations at 3 d and 7 d, the area of color-developing zones still increases to some extent with the increase of CO2 AA. It can be calculated from Table 4 that with the increase of CO2 AA, the carbonation depth at 28 d decreases by 1.9%, 4%, 6.4%, 2.2%, 4.5%, and 4.8%, respectively. (3) Comparative analysis of two stirring methods  
To disclose the effects of ultrasonic vibration agitation on carbonation depth clearly, the carbonation depths of specimens with different CO2 AA s at different ages under mechanical agitation and ultrasonic vibration agitation stirring are compared and analysed. The results are shown in Figure 8. Obviously, the carbonation depths of cement mortar specimens at 3 d, 7 d, and 28 d under mechanical agitation increase only slightly with the increase of CO2 AA. The carbonation depth of cement mortar under ultrasonic vibration agitation declines quite considerably with the increase of CO2 AA. This reveals that the carbonation depth of hardened paste at different ages is negatively related to the CO2 AA under ultrasonic vibration agitation. In a word, when CO2 absorption volumes of the cement mortar before carbonization were 0.44%, 0.88%, 1.32%, 1.76%, and 2.20% (28 d), the carbonization depth under ultrasonic vibration decreased by 5.5%, 12.3%, 21.7%, 20.7%, and 26.7% compared to those under mechanical stirring, respectively. When the ultimate CO2 absorption volume increased to 2.2% of cement mass, the extended degree of cement mortar was 103.23 mm, which decreased by 5.4% compared to that before CO2 absorption. Additionally, the carbonation resistance of cement mortar molded under ultrasonic vibration agitation is improved. The reasons are introduced as follows. Since ultrasonic vibration agitation increases the collision probability of cement particles effectively, the hydration rate of cement particles, the CO2 absorption rate, and the neutralization rate of Ca(OH)2 hydration product are increased \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**No.** & **Q0** & **Q1** & **Q2** & **Q3** & **Q4** & **Q5** \\ \hline CO2 AA/\% & 0 & 0.44 & 0.88 & 1.32 & 1.76 & 2.20 \\
3 d/mm & 2.8 \(\pm\) 0.1 & 2.6 \(\pm\) 0.2 & 2.4 \(\pm\) 0.1 & 2.3 \(\pm\) 0.2 & 2.1 \(\pm\) 0.1 & 2.0 \(\pm\) 0.1 \\
7 d/mm & 4.2 \(\pm\) 0.3 & 4.0 \(\pm\) 0.2 & 3.8 \(\pm\) 0.3 & 3.6 \(\pm\) 0.2 & 3.5 \(\pm\) 0.3 & 3.3 \(\pm\) 0.2 \\
28 d/mm & 5.3 \(\pm\) 0.4 & 5.2 \(\pm\) 0.3 & 5.0 \(\pm\) 0.4 & 4.7 \(\pm\) 0.4 & 4.6 \(\pm\) 0.3 & 4.4 \(\pm\) 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Absorb different amounts of CO2 and carbonize depth at different ages (ultrasonic vibration agitation). Figure 7: Image of carbonation depth under ultrasonic vibration agitation (28 d).