## 3 Results
### Carbonation Behavior of RRAC after Freeze-Thaw Cycles
#### 3.2.1 The First Generation  
The time-evolution of carbonation depths for RAC-1 at different replacement rates is displayed in Figure 6a. It is no doubt that the carbonation depths of concrete increased as time went on, and the carbonation resistance of RAC-1 became poor when the replacement rate increased. On the whole, the carbonation depth of RAC-1-25 and RAC-1-75 was similar to that of NAC, indicating that the carbonation resistance of RAC-1 with non-100% of replacement rate was comparable to that of NAC. Especially at low replacement rates (25%), the carbonation depths of RCA-1 were even lower than the NAC ones. This was due to the attached mortar on the surface of RCA, which reacted with CO2 to form the carbonation products. These products filled in the micro-pores and micro-cracks in the attached mortar to enhance the weak ITZs [42], thus blocking the CO2 penetration in concrete to some extent. But with the increase of attached mortar content namely replacement rate, the poor performance of attached mortar and ITZs in RAC began playing the leading role. The increasing pores and cracks in the attached mortar and the loose ITZs made it easier for CO2 to penetrate into RAC [43]. Therefore, when the total attached mortar content in RAC exceeded a certain degree, the carbonation depth of RAC was be greater than that of NAC [44].  
The concrete structures used in micro-frozen regions are inevitably subjected to more or less freeze-thaw cycles. Predictably, freeze-thaw damage for concrete accelerates its carbonization. Figure 6b shows the 28-d carbonation depth variations of NAC and RAC-1 after different freeze-thaw cycles. The carbonation depth of all concretes increased gently as the freeze-thaw cycle went on. For NAC, the carbonation depth grew slowly before 600 freeze-thaw cycles, and the depth value increased by 41.4% compared to that of specimens under non-freeze-thaw conditions. But when freeze-thaw cycles increased unceasingly to 800 times, the carbonation depth reached 2 times as much as that of unfreeze-thaw specimens. Such a variation trend was also applicable to RAC-1-25. The carbonation depth of RAC-1-25 was close to the NAC one, showing that RAC-1 with low replacement rate owned comparable carbonation resistance durability of NAC. At a 75% replacement rate, the carbonation depth of RAC-1 increased rapidly after 300 freeze-thaw cycles. When the replacement rate was 100%, the carbonation depth of RAC-1 showed an exponential growth trend. This indicated that freeze-thaw damage and high contents of RCA-1 weakened the carbonation resistance of RAC-1. But for the first generation RAC, its carbonation resistance was acceptable especially at 25% and 75% replacement rates. In addition to the attached mortar of RCA retarding the CO2 penetration in RAC, the relatively few micro-cracks and micro-pores in RCA accommodated more free water, which greatly alleviated the seepage pressure and expansion stress caused by icing in the early stage of freeze-thaw process. Minor freeze-thaw damage appeared in the RAC-1, resulting in a lower carbonation depth. But with the raise of replacement rate, the contents of attached mortar, micro-cracks, and micro-pores increased, which provided more channels for water and CO2 media, leading to major freeze-thaw damage and poor carbonation resistance.  
Figure 8: Carbonation depth of RAC-3 with different replacement rates (**a**) before and (**b**) after freeze–thaw cycles.