# Improvement of Recycled Aggregates Properties by Means of CO\({}_{2}\) [PERSON]
## 4 Results and Discussions
### Parametric Study of the Accelerated Carbonation Test on RCAs
#### 4.1.4 Effect of the RCA Size on the CO2 Uptake  
Figure 9a,b presents the effect of the RCA size on the CO2 uptake (with 15% CO2). The plots also show the effect of the water content. For OC 1-4 (Figure 9a), the optimum water content to achieve the maximum CO2 uptake of 21.8 g/kg was about 4.0%. For OC 12-20, the maximum CO2 uptake of 9.6 g/kg was achieved with a water content of about 2.8%. For HPC 1-4 (Figure 9b), the optimum water content to achieve the maximum CO2 uptake of 11.4 g/kg was about 3.5%, compared with an optimum water content of about 2.2% to achieve the maximum CO2 uptake of 6.5 g/kg for HPC 12-20.  
Furthermore, we observed that the 1-4 mm RCA had a greater capacity to store CO2 than the 12-20 mm RCA for both types of RCAs. First, this difference can be explained by the physical effect of CO2 diffusion in the 1-4 mm RCA, which depends on the diameter of the RCA particles. The diffusion time of CO2 in a fine RCA is lower than in a coarse RCA. Moreover, the distribution of the phases able to be carbonated differs in the two fractions. The formation of calcium carbonates at the surface of a coarse RCA can hinder the diffusion of CO2 more than in a fine RCA, where the total volume is carbonated uniformly. Regardless of the granular fraction, the OC RCA had the highest CO2 uptake. This result may be correlated with the porosity of the OC concrete. Secondly, the portlandite rates of the RCAs differed according to the granular fraction, due to the chemical compositions of the RCAs. The difference in the portlandite rate is explained by the cement paste content, as estimated by the soluble fraction in salicylic acid (SFSA). For the OC RCA, OC 1-4 had the highest capacity for CO2 uptake because it had a higher cement paste content (45%) and a higher portlandite rate (4%) than OC 12-20, whose cement paste content was equal to 32% and portlandite rate was 2.3%. The cement paste content and the chemical composition of the RCA have a chemical effect on the CO2 uptake. In conclusion, the physical and chemical effects combined to alter the effect of the RCA particle size on the CO2 uptake. However, for the HPC RCA, the two fractions had a similar portlandite rate and the same  
Figure 9: Effect of the RCA size on the CO2 uptake (**a**) for OC and (**b**) for HPC (one accelerated carbonation test for these RCAs at these water contents).  
cement paste content as estimated by SFAS, so the difference in their CO2 uptakes was low. The hypothesis of C-S-H carbonation is difficult to confirm by TGA. Although the chemical effect had less influence on the CO2 uptake for these RCAs, the physical effect of the diameter of the RCA had an influence on the difference of the CO2 uptakes between HPC 1-4 and HPC 12-20.  
In the literature, [PERSON] et al. [35] suggested that fine RCAs have a higher potential CO2 uptake than coarse ones. [PERSON] et al. [29] showed that the maximum CO2 uptake is achieved with fine RCAs rather than coarse RCAs. They concluded that the difference is related to the cement content of the RCA. Our results are consistent with this conclusion.