## 4 Discussion  
It was found that the replacement rate and the recycling number of RRCA and freeze-thaw cycles of RRAC played critical roles in the carbonation resistance of RRAC used in the micro-frozen regions, as shown in Figure 11 and Table 2. The carbonation resistance of all three generations of RRAC gradually deteriorated with the growth of the replacement rate and recycling number of RRCA, and the freeze-thaw cycles.  
At the same replacement rate, whether or not it went through the freeze-thaw cycles, the carbonation depths of RRAC increased with the growth of the recycling number of RCA. The carbonation depths of RAC-1-25, RAC-2-25, and RAC-3-25 under the non-freeze-thaw condition were 1.13, 1.40, and 1.89 mm, respectively. After 100 times of freeze-thaw cycles, the carbonation depths of RAC-1-100, RAC-2-100, and RAC-3-100 were 2.05, 2.38, and 4.83 mm, respectively. This was attributed to the deterioration of RRCA quality, such as the decreasing apparent density, increasing attached mortar content and crushing index, and the increasing of recycling number [21]. Compared with NCA, the apparent density of RCA-1, RCA-2, and RCA-3 decreased by 5.9%, 9.4%, and 15.9%, respectively, and the attached mortar content of RRCA increased from 40.3% to 55.4%, as reported in a previous study [21]. The poor qualities of RRCA caused higher porosity and cracks in RRAC than the NAC one, which facilitated the diffusion of CO\({}_{2}\) in RAC.  
In a micro-frozen region, the freeze-thaw damage is another inevitable factor contributing to the degradation of carbonation resistance for RRAC. The more serious such damage, the worse the carbonation depth of RRAC. After 100, 200, and 300 times of freeze-thaw cycles, the carbonation depth of RAC-3-100 increased by 32.3%, 68.5%, and 117.3% compared to that of concrete not subjected to freeze-thaw cycles. As reported by [PERSON] et al. [49] and [PERSON] et al. [29], the freeze-thaw damages of concrete are usually manifested as the growth of porosity and micro-cracks caused by the appearance of frost heaving and osmotic pressure. These damages result in cracking and spalling of cement mortar on the surface of concrete, thus providing more convenient channels for CO\({}_{2}\) penetrating into concrete. It should be noted that, according to equivalent principle between laboratory freeze-thaw cycle and that in natural environment proposed by [PERSON] et al. [50], the damage caused by 300 times of accelerated freeze-thaw cycles indoors is equal to the damage of concrete used in micro-frozen areas for 50 years. No RRACs, even RAC-3-100, suffered freeze-thaw failure after such freeze-thaw cycles, which has been verified by [PERSON] et al. [21].  
Although the carbonation resistance of RRAC was degraded due to the freeze--thaw damage, RRCA replacement rates and recycling numbers, and the carbonation depth of all RRAC, even RAC-3-100, were still much lower than the protective thickness of reinforced concrete based on the standard for design of concrete structure durability of GB/T 50476-2019 [51]. This suggested that the three generations of RRAC can be used in micro-frozen regions for engineering applications for at least 50 years.