## 4 Test Results and Analysis  
### Carbonation Performances Analysis  
(1) Mechanical agitation  
(1) Carbonation age of 3 d  
The carbonation of cement mortar after CO\({}_{2}\) absorption under mechanical agitation at 3 d is shown in Figure 2. Clearly, the area of color developing zone in specimens decreases with the increase of CO\({}_{2}\) AA. The average carbonation depth under different CO\({}_{2}\) AA was calculated according to test results in Figures 2-4. The results are listed in Table 3. It can be seen from Table 3 that with an increase in CO\({}_{2}\) AA, the carbonation depths in the hardened body of fresh cement mortar at 3 d are 3.1, 2.9, 3.3, 3.5, 3.5, and 3.6 mm, respectively. The carbonation depth of cement mortar with 0.44% of CO\({}_{2}\) absorption is 6.9% lower than that of cement mortar without CO\({}_{2}\) absorption. However, the carbonation depth increases by 13.8%, 6.1%, 0%, and 2.9% with the continuous increase of CO\({}_{2}\) AA in the later stages. (2) Carbonation age at 7 d  
The carbonation of cement mortar after CO\({}_{2}\) absorption under mechanical agitation at 7 d is shown in Figure 3. Clearly, the area of color developing zone in specimens decreases compared to that at 3 d, but it changes slightly macroscopically with the increase of CO\({}_{2}\) AA. It can be seen from Table 3 that the carbonation depth of cement mortar with 0.44% of CO\({}_{2}\) absorption at 7 d is 2.3% lower than that of cement mortar without CO\({}_{2}\) absorption. When the CO\({}_{2}\) AA increases to 0.88% and 1.32%, the carbonation depths are increased by 4.7% and 2.2%, respectively. As the CO\({}_{2}\) AA continues to increase to 1.76%, the carbonation depth declines by 2.2%. When the CO\({}_{2}\) AA reaches 2.20%, the carbonation depth remains the same. In summary, the carbonation changes of hardened paste at 7 d are not stable due to influences by CO\({}_{2}\) AA. Figure 2: Image of carbonation depth under mechanical agitation (3 d). (3) Carbonation age at 28 d  
The carbonation of cement mortar after CO2 absorption under mechanical agitation at 28 d is shown in Figure 4. Clearly, the area of color-developing zone in specimens decreases compared to that at 7 d and it continues to decrease significantly with the increase of CO2 AA. At 28 d, carbonation depths of the hardened body of fresh cement mortar are 5.6, 5.5, and 5.7 mm as well as 6.0, 5.8, and 6.0 mm with the increase of CO2 AA. When the CO2 AA is 0.44%, the carbonation depth is 1.8% lower compared to that before CO2 absorption. Subsequently, CO2 AAs are increased by 1.8%, 7.1%, 3.6%, and 7.1%, respectively. In view of increasing and decreasing percentages of carbonation depths at different ages, the carbonation depth of hardened paste generally increases with the increase of CO2 AA. It changes slightly at 7 d, but it increases to some extent with increasing curing age and CO2 AA. This reveals that the hardened body of cement mortar with CO2 absorption  
Figure 4: Image of carbonation depth under mechanical agitation (28 d). Figure 3: Image of carbonation depth under mechanical agitation (7 d). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen No.** & **R0** & **R1** & **R2** & **R3** & **R4** & **R5** \\ \hline CO2 AA (\%) & 0 & 0.44 & 0.88 & 1.32 & 1.76 & 2.20 \\ 3 d/mm & 3.1 \(\pm\) 0.2 & 2.9 \(\pm\) 0.1 & 3.3 \(\pm\) 0.2 & 3.5 \(\pm\) 0.1 & 3.5 \(\pm\) 0.2 & 3.6 \(\pm\) 0.2 \\ 7 d/mm & 4.4 \(\pm\) 0.3 & 4.3 \(\pm\) 0.4 & 4.5 \(\pm\) 0.3 & 4.6 \(\pm\) 0.3 & 4.5 \(\pm\) 0.2 & 4.5 \(\pm\) 0.4 \\ 28 d/mm & 5.6 \(\pm\) 0.3 & 5.5 \(\pm\) 0.3 & 5.7 \(\pm\) 0.4 & 6.0 \(\pm\) 0.4 & 5.8 \(\pm\) 0.4 & 6.0 \(\pm\) 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Absorb different amounts of CO2 and carbonize depth at different ages (mechanical agitation). under mechanical agitation can influence the carbonation resistance of concrete to some extent. However, such influences are not obvious.