## 3 Results
### SEM Analysis  
It can be seen from the above that RAC-1 possessed the best carbonation resistance which was equivalent to that of NAC. Therefore, RAC-1 with different replacement ratessubjected to freeze-thaw cycle for 600 times and carbonized for 28 d was chosen for SEM analysis, which are shown in Figure 9. After freeze-thaw and carbonation processes, the micro-morphology of RAC-1 had changed with different replacement rates. When the replacement rate was 25%, the internal structure of RAC-1 was relatively dense with almost no pores and micro-cracks, and a small amount of carbonation reaction products such as CaCO\({}_{3}\) existed on the surface to fill in the pores. With the increase of replacement rate, the internal structure of specimen began to become loose and the cracks appeared. When the replacement rate was 100%, cracks in concrete were formed and developed in large quantities to provide the reaction channels for carbonation. In addition, a large number of carbonation reaction products were attached on the surface of concrete, but these carbonation products were not enough to fill these cracks (Figure 9c). Therefore, the carbonation resistance of RAC-1-100 was worse than that of RAC-1-25 and RAC-1-75.  
This is mainly because that the freeze-thaw cycle is essentially a process in which the hydration products of concrete change from compact to loose. The freeze-thaw damages of RAC were aggravated with an increase of the replacement rate due to the inferior properties of RCA, which have been verified as previous work [6]. These damages intensified the development of internal pores and micro-cracks in concrete, thus providing more channels for CO\({}_{2}\) to initiate a carbonation reaction, thus decreasing the carbonation resistance of RAC.  
To explore the effect of the freeze-thaw cycles on the carbonation resistance of RAC, the morphologies of RAC-2-75 carbonated for 28 d after different freeze-thaw cycles were observed by SEM (Figure 10). After 200 times of freeze-thaw cycles, RAC-2 was still relatively dense. Its surface contained a small amount of CaCO\({}_{3}\), SiO\({}_{2}\), and other substances formed by carbonation. But micro-cracks and pores began to form inside of RAC-2, and RAC-2 became loose and porous after 400 times of freeze-thaw cycles, which accelerated the carbonation of concrete. A growing number of carbonation products appeared on the concrete surface. When the freeze-thaw cycle reached 600 times, the pores generated in the early stage of concrete gradually expanded, forming large holes and cracks, causing the structure of RAC to significantly deteriorate. The ITZs in concrete gradually became fuzzy, and a large number of carbonation products attached to the surface of concrete.  
With fewer freeze-thaw cycles, the presence of micro-pores and micro-cracks in RCA provided more space for free water; thus, RAC showed good freeze-thaw resistance. However, with the increasing number of freeze-thaw cycles, free water migrated and was frozen repeatedly inside the pores of RAC-2. The freeze-thaw damages caused by hydrostatic pressure on pore wall were gradually accumulated and the internal structure gradually became porous [48]. These porous structures caused by freeze-thaw damages provided quick channels for CO\({}_{2}\) to accelerate the carbonation reaction of concrete. Therefore, the freeze-thaw of concrete was the key factor to determine the carbonation resistance durability of RAC at the same replacement rate in a micro-frozen region.  
Figure 9: SEM images of RAC-1 with different replacement rates carbonated for 28 d after 600 times of freeze–thaw cycles: (a) 25%; (b) 75%; (c) 100%.