# Improvement of Recycled Aggregates Properties by Means of CO\({}_{2}\) [PERSON]
## 4 Results and Discussions
### Impact of Accelerated Carbonation on the Properties of RCAs
#### 4.2.3 Changes in the Pore Size Distribution of an RCA by Mercury Intrusion Porosimetry  
Figure 18 shows the change in the pore size distribution of RB 1-4 C that had undergone various treatments: before carbonation (NC-RB 1-4 C), after accelerated carbonation with a CO\({}_{2}\) concentration of 15% (RB 1-4 C (15% CO\({}_{2}\))) and 100% CO\({}_{2}\) (RB 1-4 C (100% CO\({}_{2}\))). As expected, carbonation affected the porous structure of the RCA, decreasing its total porosity. The total porosity of NC-RB 1-4 C was about 17% and fell after accelerated carbonation to 15% and 14% for RB 1-4 C (15% CO\({}_{2}\)) and RB 1-4 C (100% CO\({}_{2}\)), respectively. Figure 17: Effect of the CO\({}_{2}\) concentration on the CO\({}_{2}\) uptake and WAC (one WAC measurement for RCAs after carbonation). Figure 18: Pore size distribution. The total decrease in porosity was around 20%. As for the pore size distribution, for NC-RB 1-4, the plot has a monomodal distribution with a wide peak between 30 and 220 nm, which corresponds with the capillary pores. After accelerated carbonation with 15% CO\({}_{2}\) (and a CO\({}_{2}\) uptake of 16.7 g/kg), this plot has a bimodal distribution, and the peak decreases. The porosity in this range also decreased when the CO\({}_{2}\) concentration was increased to 100% (CO\({}_{2}\) uptake of 17.3 g/kg). This decrease was undoubtedly due to the clogging of pores by the formation of calcium carbonates during carbonation, which has been observed in both cement pastes [40; 53; 54] and RCAs [55]. The calcium carbonates that are formed accumulate and fill large pores. They improve the porous structure and reduce the range of size of the capillary pores. [PERSON] et al. and [PERSON] et al. [53; 55] observed that carbonation reduced the number of pores over 200 nm or eliminated them. In our case, carbonation did not have a significant effect on the range of pore sizes because NC-RB 1-4 C was partially carbonated. Moreover, Figure 18 shows that carbonation led to an increase of pores of under 30 nm. [PERSON] et al. and [PERSON] et al. [40; 54] reported the same finding. This is the range of pore sizes, in particular those under 10 nm, which made up the pores in the C-S-H gel. The decalcification of C-S-H due to carbonation occurred in these pores, producing a coarse poral structure. This decalcification was greater with a CO\({}_{2}\) concentration of 100%, leading to a CO\({}_{2}\) uptake of 17.3 g/kg (Figure 18). This suggests that the increase in the number of pores of under 30 nm was greater with a CO\({}_{2}\) concentration of 100% than 15%. This conclusion is consistent with an optimum of concentration of CO\({}_{2}\). The pore volume fraction is presented in Figure 19. Four ranges of pores were identified:  
* 3.7-30 nm: micropores (pores between C-S-H gels and capillary micropores);
* 30-220 nm: capillary pores;
* 220-10,000 nm: macropores;
* Pores >10,000 nm: large pores (air bubbles due to production of the concrete). As we can see in Figure 19, carbonation increased the number of pores with a diameter of less than 30 nm due to decalcification of the C-S-H. At a CO\({}_{2}\) concentration of 100%, there were 2.5 times more of such pores in the carbonated RCA than in the reference RCA. The number of capillary pores was reduced by 33% at 15% CO\({}_{2}\) and 40% with 100% CO\({}_{2}\) due to CO\({}_{2}\) uptake and the formation of calcium carbonates. In both cases, these values were consistent with the decrease in the water absorption coefficient. In the case of RB 1-4 C, carbonation did not have a significant effect on the macropores or the large pores. The CO\({}_{2}\) concentration and CO\({}_{2}\) uptake also had an impact on the pore volume fraction,  
Figure 19: Pore volume fraction. leading to a significant increase in the presence of micropores and a significant decrease in the volume of capillary pores with a CO\({}_{2}\) concentration of 100%.