## 2 Materials and Methodology
### Effect of CO\({}_{2}\) Concentration (10%, 18%, and 25%)
#### 3.2.2 Recycled Concrete Aggregates (RCAs)  
In general, the analysis of the results presented in Figure 6 confirm the previous conclusion that the reduction in CO2 concentration decreased the CO2 capture content per cement paste. The exception is RCA-RP2, where the maximum capture was obtained with 10% CO2 concentration.  
The results of RCA-RP1 are similar at 10% and at 18%, with only a slight increase at 25%. At 10% CO2 concentration, the maximum amount of CO2 captured was 9.3%, while at 18% the maximum was 8%, and at 25% the maximum captured value increased to 13.3%. Similarly to prior outcomes for MRA, the reduction in CO2 concentration led to a lower content of CO2 captured by RCA-RP1. RCA-RP2 and RCA-IW1 are the samples with the highest content of Rc fraction; however, they presented different behaviours in these tests. For RCA-RP2, the decrease in CO2 concentration led to an increase in CO2 capture, from 5.2% at 25% to 9.5% at 10%. RCA-IW1 had an opposite effect to RCA-RP2 but similar behaviour to RCA-RP1, with the maximum content captured at 25% CO2 concentration (34.8%). This happened because the decrease in CO2 concentration led to less CO2 being captured during carbonation. This is due to the principle of chemical equilibrium and the driving force it creates for the carbonation reaction. During the carbonation process, which involves the reaction between CO2 and alkaline compounds in the sample matrix, the reaction seeks to establish a state of equilibrium. This equilibrium is defined by the  
Figure 7: Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each MRA at 25, 18, and 10%.  
concentrations of reactants and products in the reaction. When the concentration of one of the reactants, in this case CO2, is decreased, the equilibrium is disturbed, leading to a reduced formation of CaCO3, meaning that there is less blocking the path of the diffusing CO2 and slowing it down [7; 38]. However, there are also studies that suggest that high CO2 concentrations can result in less CO2 capture due to the obstruction originating from the formation of denser minerals [7]. These results agree with previous results (MRA), where lower concentrations lead to longer carbonation periods. The unexpected carbonation behaviour observed in RCA-RP2 can be attributed to several factors inherent to the complex nature of RCA. The heterogeneity of RCA and possible variations in concrete sources have a significant role in this phenomenon [49; 50]. Also, differences in initial carbonation states among the aggregates can greatly influence their susceptibility to further carbonation; the differences in weight loss before carbonation shown in Figure 8 are indicative of this [49]. Additionally, variations in moisture content across the samples can affect the carbonation process, as optimal moisture levels are crucial for efficient CO2 uptake [49]. Finally, differences in the age and degree of hydration of the original concrete sources may contribute to varying carbonation behaviours [51].