4.1. 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
Figure 2,
Figure 3 and
Figure 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.
(3) Carbonation age at 28 d
The carbonation of cement mortar after CO
2 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 CO
2 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 CO
2 AA. When the CO
2 AA is 0.44%, the carbonation depth is 1.8% lower compared to that before CO
2 absorption. Subsequently, CO
2 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 under mechanical agitation can influence the carbonation resistance of concrete to some extent. However, such influences are not obvious.
- (2)
Ultrasonic vibration agitation
(1) Carbonation age at 3 d
The carbonation of cement mortar after CO
2 absorption under ultrasonic vibration agitation at 3 d is shown in
Figure 5. The average carbonation depth under different CO
2 AA was calculated according to the test results in
Figure 5,
Figure 6 and
Figure 7. The results are listed in
Table 4. It can be seen from
Figure 5 that, with the increase of CO
2 AA, the area of color-developing zones increases to some extent. It can be calculated from
Table 4 that, for each increase in CO
2 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 CO
2 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 CO
2 AA, the area of color-developing zones still increases to some extent. It can be calculated from
Table 4 that with the increase of CO
2 AA, the carbonation depth at 7 d decreases by 5%, 5.2%, 5.6%, 2.9%, 6.1%, and 6.5%, respectively.
(3) Carbonation age at 28 d
The carbonation of cement mortar after CO
2 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 CO
2 AA. It can be calculated from
Table 4 that with the increase of CO
2 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 CO
2 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 CO
2 AA. The carbonation depth of cement mortar under ultrasonic vibration agitation declines quite considerably with the increase of CO
2 AA. This reveals that the carbonation depth of hardened paste at different ages is negatively related to the CO
2 AA under ultrasonic vibration agitation.
In a word, when CO
2 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 CO
2 absorption volume increased to 2.2% of cement mass, the extended degree of cement mortar was 103.23mm, which decreased by 5.4% compared to that before CO
2 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 CO
2 absorption rate, and the neutralization rate of Ca(OH)
2 hydration product are increased significantly, thus enabling the construction and development of a more uniform and compacted microstructure of cement paste [
14].
4.2. 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.
The pH values at carbonation depths of 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 mm when the CO2 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. 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.
- (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 4~12 mm increases to about 13.0 (
Figure 9 and
Figure 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 CO2 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 CO2 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 CO2 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 CO2 AA, the porosity of specimens decreases continuously. When the specimens are further carbonized with CO2 in air, the invasion speed of CO2 into the specimens decreases with the decrease of internal porosity. As a result, the pH value presents a rising trend. This proves that under ultrasonic vibration agitation stirring, the carbonation slows down after the cement-based materials absorb more CO2, which is conducive to improving the carbonation performance of cement-based materials.
4.3. XRD Analysis of Hardened Paste
XRD spectra of cement mortar absorbing 2.20% CO
2 and the cement mortar without CO
2 absorption under two stirring methods are shown in
Figure 11. The XRD spectra and phase analysis results of cement paste without CO
2 absorption under mechanical agitation, cement paste with 2.20% of CO
2 absorption under mechanical agitation, and cement paste with 2.20% of CO
2 absorption under ultrasonic vibration agitation stirring are shown in
Figure 11a–c, respectively.
- (1)
Pure cement paste
It can be seen from
Figure 11a that there is a diffraction peak of Ca(OH)
2 (Portlandite) in the XRD spectra of pure cement paste, indicating that this specimen contains abundant Ca(OH)
2 crystals in hydration products at 28 d. The diffraction peaks of CaCO
3 (Calcite) and 3CaO·Al
2O
3·3CaSO
4·32H
2O (Ettringite) are relatively low [
29,
30].
- (2)
Cement paste after CO2 absorption under mechanical agitation
After 2.20% CO
2 is supplied to the fresh paste under mechanical agitation (
Figure 11b), the diffraction peak of Ca(OH)
2 in the diffraction spectra is lower than that of the pure cement mortar. However, the reduction amplitude is very small, indicating that Ca(OH)
2 crystals decrease only to some extent. This reveals that the supplied CO
2 into the fresh cement paste reacts with and partly consumes some of the Ca(OH)
2 in the cement, thus reducing the diffraction peak of Ca(OH)
2 (Portlandite) to some extent. Furthermore, the diffraction peak of CaCO
3 (Calcite) of pure cement paste increases compared to that on the XRD spectra of the cement paste after CO
2 absorption. This is because some of the CaCO
3 is produced from the reaction of the supplied CO
2 and substances in the cement paste during stirring.
- (3)
Cement paste after CO2 absorption under ultrasonic vibration agitation stirring
After 2.20% CO
2 is supplied to the fresh paste under ultrasonic vibration agitation stirring (
Figure 11c), the diffraction peak of Ca(OH)
2 declines only slightly compared to that under mechanical diffraction. This is mainly because the crystal number of Ca(OH)
2 in cement paste after CO
2 absorption under ultrasonic vibration agitation changes and Ca(OH)
2 can come into contact to react with CO
2 better. Hence, Ca(OH)
2 is consumed and CaCO
3 crystals are produced.
Additionally, the cement paste also may react with CO
2 in the air in the curing tank. Since ultrasonic vibration agitation increases the compaction of curing specimens, the CO
2 AA from air into the paste decreases accordingly, thus resulting in small influences on Ca(OH)
2 during the curing process [
31,
32,
33].
It can be seen from
Figure 11 that the diffraction peak of Ca(OH)
2 under ultrasonic vibration decreased to some extent compared to that under mechanical stirring (Portlandite). This proves that CO
2 absorption by the cement mortar under ultrasonic vibration had small influences on Ca(OH)
2. Additionally, peaks of the CaCO
3 crystal might be increased to some extent. To sum up, cement mortar after CO
2 absorption under ultrasonic vibration has small influences on (Portlandite) Ca(OH)
2, and CaCO
3(Calcite) might increase to some extent.