# Improvement of Recycled Aggregates Properties by Means of CO\({}_{2}\) [PERSON]
## 4 Results and Discussions  
### Parametric Study of the Accelerated Carbonation Test on RCAs  
#### 4.1.1 Effect of the Initial Natural Carbonation of an RCA on the CO\({}_{2}\) Uptake  
The initial natural carbonation affects the potential CO\({}_{2}\) uptake of an RCA. This state can be determined based on the color change after spraying phenolphthalein over the RCA. A non-carbonated RCA has a pH of between 12 and 13, and a pink color is obtained. A carbonated RCA has a pH of less than 9, and the phenolphthalein indicator is colorless. The effect of the initial natural carbonation of an RCA was confirmed by the color change after phenolphthalein was sprayed over the RCA (Figure 3). CB 1-4 had low natural carbonation, as almost all the aggregate turned pink after being sprayed with phenolphthalein. RB 1-4 C had partial natural carbonation because it was sourced from RB 10-20, only the surface of which had been naturally carbonated. RB 1-4, which had been stored for 6 years, remained colorless, indicating that this material was already naturally fully carbonated. Figure 2: Drying curve of an RCA with respect to time to obtain the SSD state. Figure 4 presents the effect of the initial natural carbonation and the water content on the CO\({}_{2}\) uptake after an accelerated carbonation test. First of all, the water content had a marked effect on the CO\({}_{2}\) uptake. In addition, the shape of the plot was similar for each type of RCA, regardless of its initial natural carbonation status or source. As expected, and as was mentioned above, a high water content slowed the diffusion of CO\({}_{2}\) because of the presence of water in the pores, while a low water content limited the dissolution of CO\({}_{2}\) because of a lack of water. The maximum CO\({}_{2}\) uptakes were obtained for optimum water contents of 5.2, 5.4 and 7.2% for RB 1-4 C, CB 1-4 and RB 1-4, respectively (see Table 4). On both sides of the CO\({}_{2}\) uptake maxima, there were CO\({}_{2}\) uptake minima for the low and high water contents. Similar shapes of plots were also found by [PERSON] et al. [28], who also reported that there is an optimum water content for a maximum CO\({}_{2}\) uptake. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **CB 1–4** & **RB 1–4 C** & **RB 1–4** \\ \hline CO\({}_{2}\) concentration (\%) & 100\% & 100\% & 100\% \\ Initial natural & \_ 1 & + 1 & ++ 1 \\ carbonation & \_ & & \\ Optimum water & & & \\ content (\%) & 5.4 & 5.2 & 7.2 \\ CO\({}_{2}\) uptake (g/kg) & 49.9 & 17.3 & 8.6 \\ \hline \hline \end{tabular}  
* No natural carbonation, + Partial carbonation, ++ Total natural carbonation.  
\end{table} 
Table 4: Effect of initial natural carbonation of an RCA on CO\({}_{2}\) uptake. Figure 4: Effect of the initial natural carbonation of an RCA on CO\({}_{2}\) uptake as a function of the water content (one accelerated carbonation test for these RCAs at these water contents). Figure 3: Phenolphtalein spray on RCA (**A**) CB 1–4, (**B**) RB 1–4 C and (**C**) RB 1–4. CB 1-4 was not naturally carbonated and therefore exhibited the highest CO\({}_{2}\) uptake (49.9 g/kg). For RB 1-4 C, which was partially naturally carbonated, the value of the CO\({}_{2}\) uptake was 17.3 g/kg. RB 1-4, after 6 years of storage, had the lowest CO\({}_{2}\) uptake (8.6 g/kg). The values we obtained were in line with the portlandite rates of the RCAs (Table 3). The effect of the initial natural carbonation status on the CO\({}_{2}\) uptake was also demonstrated by [PERSON] et al. [33], who found that in the case of an RCA that has been stored for a long time, partial carbonation can affect the potential CO\({}_{2}\) uptake, rendering it lower compared with those sourced from recently deconstructed concrete. This partial carbonation depends on the duration of storage. In addition, it was possible to evaluate the potential CO\({}_{2}\) uptake of an RCA even with a low water content (around 4%).