# Improvement of Recycled Aggregates Properties by Means of CO\({}_{2}\) [PERSON]
## 4 Results and Discussions
### Parametric Study of the Accelerated Carbonation Test on RCAs
#### 4.1.3 Effect of the RCA Type on CO\({}_{2}\) Uptake  
For further testing, we chose a CO\({}_{2}\) concentration of 15%, as this is the concentration observed at cement kiln outlets, with the objective of transferring this laboratory study to the industrial scale. As in Figure 8, the maximum CO\({}_{2}\) uptake was obtained at an optimum water content. This last parameter depended on the type of RCA and its initial natural carbonation.  
The difference in the CO\({}_{2}\) uptake of OC 1-4 and HPC 1-4 can be explained by their portlandite content (Table 3). NC-OC 1-4 had a portlandite content of 4%, compared with 2.3% for NC-HPC 1-4, and thus contained more phases able to be carbonated. This explains the CO\({}_{2}\) uptake of 21.8 g/kg for OC 1-4 and 11.4 g/kg for HPC 1-4. Furthermore, NC-OC 1-4 was sourced from concrete with a water-to-cement ratio of 0.49, in contrast to NC-HPC 1-4, which was sourced from concrete with a water-to-cement ratio of 0.32. [PERSON] et al. [37] concluded in their study that the paste is more dense in the case of an RCA sourced from concrete with high mechanical strength like HPC 1-4. This reduces the diffusion of CO\({}_{2}\) and the effectiveness of accelerated carbonation and thus the CO\({}_{2}\) uptake. The higher water porosity of 10.9% of NC-OC 1-4, compared with 8.5% for NC-HPC 1-4, also increased the diffusion of gas through the pores of the RCA.  
If the source of the RCA, and hence the composition of the concrete (Table 1), is known, it is possible to determine the theoretical maximum CO\({}_{2}\) uptake (by Portland cement) as a percentage by applying [PERSON]'s theoretical formula (Equation (4)) [28] in kg of CO\({}_{2}\) per kg of solid residue on a dry basis [49]:  
\[\text{CO${}_{2\text{ max }}$}\left(\%\right)=0.785(\text{CaO}-0.7\text{SO${}_{3}$})+1.091\text{MgO}+1.420\text{Na${}_{2}$O}+0.935\text{K${}_{2}$O} \tag{4}\]  
According to this formula, the cement in OC concrete can store a maximum of 52.1% of CO\({}_{2}\), and the cement in HPC concrete can store a maximum of 51.3%. On the basis of a cement content of 45% for NC-OC 1-4 and 35% for NC-HPC 1-4, the CO\({}_{2}\) uptake could be calculated, giving a theoretical maximum (CO\({}_{2\text{ max}}\)) of 23 g of CO\({}_{2}\)/kg of RCA (or 51 g of CO\({}_{2}\)/kg of cement paste) for OC 1-4 and 18 g/kg of RCA (40 g CO\({}_{2}\)/kg of cement paste) for HPC 1-4. This result can be compared to the plot of CO\({}_{2}\) uptake of the two RCAs, where OC 1-4 could take up more CO\({}_{2}\), both theoretically and experimentally, than HPC 1-4. It is possible to calculate the carbonation percentage \(\varepsilon\) as described by [PERSON] et al. [25].  
Figure 8: Effect of the RCA type on CO\({}_{2}\) uptake as a function of the water content (one accelerated carbonation test for these RCAs at these water contents).  
This is the ratio (Equation (5)) between the experimental CO2 uptake (Equation (1)) and the theoretical CO2 uptake CO2 max of an RCA after accelerated carbonation (Equation (4)):  
\[\varepsilon=\frac{\text{CO}_{2}\text{ uptake}}{\text{CO}_{2}\text{ max}}\times 100\% \tag{5}\]  
The percentage carbonation \(\varepsilon\) was equal to 94% for OC 1-4 and 63% for HPC 1-4. This difference demonstrates that in the case of OC 1-4, all the RCA was almost totally carbonated, but for HPC 1-4, the RCA had not been totally carbonated by accelerated carbonation. Increasing the carbonation treatment time can be an option to enhance the uptake of CO2.