## 3 Results
### Influence of Particle Grain Size and Crushing Conditions on the Leaching Behaviour of RAs
Compared with MRAs, RCAs have higher pH values according to the results reported by several authors [8; 27; 52]. For both MRAs and RCAs, the particle size of the material (d\({}_{\text{max}}\) less than 4 mm or 10 mm) does not significantly affect the pH values. Only slightly lower pH values are recorded for recycled aggregates with a maximum particle diameter equal to 4 mm. On the contrary, the crushing process greatly affects the pH values of MRAs produced by both treatment plants. In particular, regarding the MRAs produced by plant A, the pH of the only sieved material varies between 10.9 (d\({}_{\text{max}}\)\(\leq\) 4 mm) and 11 (d\({}_{\text{max}}\)\(\leq\) 10 mm), while the pH of the crushed material varies between 11 (d\({}_{\text{max}}\)\(\leq\) 10 mm) and 11.1 (d\({}_{\text{max}}\)\(\leq\) 4 mm). Similarly, the MRAs produced by treatment plant 2 show pH values of the sieved material equal to 10.4 (d\({}_{\text{max}}\)\(\leq\) 4 mm) and 10.6 (d\({}_{\text{max}}\)\(\leq\) 10 mm), while equal to 10.7 (d\({}_{\text{max}}\)\(\leq\) 4 mm and 10 mm) for the crushed material. The influence of the crushing process is not, however, evident in the RCA eluates. For these samples, in fact, the sieving process contributes to higher pH values (11.4 for d\({}_{\text{max}}\)\(\leq\) 4 mm and 11.5 for d\({}_{\text{max}}\)\(\leq\) 10 mm) than those of crushed samples (11.3 for d\({}_{\text{max}}\)\(\leq\) 4 mm and 11.4 for d\({}_{\text{max}}\)\(\leq\) 10 mm). Figures 10 and 11 show the pollutant concentrations normalized to their respective regulatory limits for recovery (M.D. 186/2006). As for pH, Figure 10 shows the release of contaminants from MRAs and RCAs produced by plant A in terms of particle grain size and test conditions (i.e., sieving or crushing). As shown, the only critical parameter for recovery is total chromium released by the crushed RCA (d\({}_{\text{max}}\)\(\leq\) 4 mm) which, with a concentration of 67 ug/L, exceeds the regulatory limit of 50 ug/L. Generally, the results clearly show that the greatest concentration of chromium comes from RCAs, confirming the results obtained by other authors [13; 21; 41; 45]. Overall, for both MRAs and RCAs, the coarser fraction (d\({}_{\text{max}}\)\(\leq\) 10 mm) releases lower pollutant concentrations than the finer fraction (d\({}_{\text{max}}\)\(\leq\) 4 mm), except for copper and COD. These results confirm the data observed by [PERSON] and [PERSON] [48], [PERSON] et al. [32], and [PERSON] et al. [49]. In particular, as stated by [PERSON] et al. [49], this high release of contaminants from fine particles is due to their larger specific surface area, which allows a greater interaction between the aqueous solution and the recycled aggregate. In fact,  
Figure 9: Trend of pH values in relation to the type of RA (RCA or MRA), particle sizes, and test conditions (i.e., material only sieved or crushed). as highlighted by [PERSON] et al. [66], the available specific surface area exhibit a direct correlation with the release of contaminants. Regarding the test conditions, the results obtained in plant A show that, compared with the crushing process, the sieving procedure generally involves a higher release of pollutants both in MRAs and RCAs, except for total chromium (in samples with \(\mathrm{d}_{\mathrm{max}}\leq 4\mathrm{mm}\)) and COD (in the MRA with \(\mathrm{d}_{\mathrm{max}}\leq 10\mathrm{\ mm}\)). For these samples, the highest concentrations of pollutants were recorded under condition of volumetric reduction. As for plant A, Figure 11 shows the release of contaminants from MRAs produced by plant B in terms of particle grain size and test conditions. From the results obtained, COD is the only critical parameter for both the fraction with \(\mathrm{d}_{\mathrm{max}}\leq 4\mathrm{\ mm}\) and \(\mathrm{d}_{\mathrm{max}}\leq 10\mathrm{\ mm}\). In particular, with respect to the regulatory limit, the concentration of COD increases from 3% to 20%.