## 5 Concrete Carbonation Mechanism Analysis
### Carbonation Microstructure Analysis  
(1) Molding under mechanical agitation  
In the early hydration stage, cement and water react to produce C-S-H gels. When the cement begins to mold, water in the hardened paste evaporates continuously, finally forming multiple pores with different sizes and shapes in the hardened body (Figure 14a). For the hardened body under mechanical agitation, the fresh paste has absorbed CO2 before molding and reacts with Ca(OH)2 in the paste to produce CaCO3, thus increasing the hydration product in the cement. When the paste is hardened, the internal pore area decreases relatively, thus decreasing the CO2 AA. Furthermore, there are small gel pores in the C-S-H gel through which some CO2 may enter into hardened cement paste (Figure 14b).  
(2) Molding under ultrasonic vibration agitation  
The internal structure of fresh cement before molding under ultrasonic vibration agitation is shown in Figure 15a. The internal structure is similar to that in Figure 14a and it is not introduced here again. For the hardened body formed under ultrasonic vibration agitation, the fresh paste absorbs CO2 before molding and reacts with Ca(OH)2 in the paste to produce CaCO3. Under ultrasonic vibration agitation, the produced CaCO3 crystals are separated into \"nano-level\" CaCO3 nuclei that fill in spaces among the cement particles effectively. Due to the adsorption of nuclei, hydration products are adhered onto nuclei, such that internal spaces of the hardened paste are filled with nucleus-flocculating constituents. Hence, it is very difficult for CO2 to enter the hardened body of the fresh paste. Moreover, there are small gel pores in C-S-H gels, through which some CO2 may enter the hardened cement (Figure 15b).  
Figure 14: Mechanism of carbonation by mechanical agitation molding. (**a**) Non-invaded CO2; (**b**) Invasion of CO2.  
Figure 15: Carbonation mechanism of ultrasonic vibration agitation forming. (**a**) Non-invaded CO2; (**b**) Invasion of CO2.  
3. Microstructure analysis of CaCO\({}_{3}\) nuclei before molding of cement mortar with CO\({}_{2}\) absorption under ultrasonic vibration agitation Due to the assistance provided by the ultrasonic \"cavitation effect\", cement particle surfaces that are hydrated in the cement paste are broken continuously. As a result, gels that are produced on the surface peel off quickly, thus creating new cement particles. Since CaCO\({}_{3}\) flocculating constituents are segmented effectively under an ultrasonic wave, the abundant \"nano-level\" crystals are distributed among the cement particles uniformly (Figure 16a). When the slurry absorbs 0.44% CO\({}_{2}\), the \"nano-level\" CaCO\({}_{3}\) nuclei among cement particles increase. Moreover, a significant amount of hydration products adhere to each crystal nuclei (Figure 16b). When the CO\({}_{2}\) AA reaches 1.32%, the CaCO\({}_{3}\) crystals continue to increase, thus making flocculating constituents among the cement particles more compact (Figure 16c). It can be seen from Figure 16d that when the CO\({}_{2}\) AA is 2.20%, the CaCO\({}_{3}\) crystals cover all of the flocculating constituents, thus making them more compact.  
Based on the above mechanism analysis, the hardened body of fresh slurry formed under ultrasonic vibration had better carbonation resistance than the hardened body of slurry formed under mechanical stirring. Before molding, the higher CO\({}_{2}\) absorption volume generates more \"nano-scale\" CaCO\({}_{3}\) crystals which distribute uniformly in pores. This improves porous structure of cement mortar effectively. Since CaCO\({}_{3}\) has filling and activation effects, it can increase compaction degree of cement-based materials, thus decreasing the diffusion rate of CO\({}_{2}\) and improving the carbonation resistance of the cement-based materials.