## 4 Conclusions  
In this study, the leaching behaviour of CDWs and RAs was evaluated by means of a statistical analysis, and the possible influence of grain size and volumetric reduction on the release of contaminants from MRAs and RCAs sampled from two CDW treatment plants was evaluated by applying the UNI EN 12457-2 (d max \(\leq\) 4 mm) and UNI EN 12457-4 (d max \(\leq\) 10 mm) leaching test methods. The analysed recycled aggregates were also tested in conditions of simple sieving and crushing, and the release of pollutants was compared to the legal limits imposed by the Ministerial Decree 186/2006. According to the results obtained from the statistical analysis performed, the following conclusions are drawn:  
* The pH of both CDWs and RAs is generally alkaline, with maximum values up to the legal limit of 12. This alkalinity is mainly due to the presence of cementitious fractions (EWC 170101) which show median values equal to 11. Instead, mixed CDWs (EWC 170904), soil and rocks (EWC 170504), and bituminous mixtures (EWC 170302) showed lower pH values (median values equal to about 7.5-8), and this is due to the presence of ceramic and soil fractions [8]. * In terms of release of contaminants, total chromium, mercury, and COD are the most critical parameters for the recovery of CDWs and RAs. In particular, total chromium is mainly released by cementitious materials (EWC 170101) due to the presence of cement and adhered mortar. Specifically, the main sources of chromium in cement are the raw materials and fuels used during its production process [57]. Instead, lower chromium release comes from mixed CDWs (EWC 170904). * High mercury releases were detected in soil and rocks fractions (EWC 170504), while high releases of COD are mainly associated with both cementitious and soil and stone fractions. In particular, this is due to the presence of decomposable organic matter. * Finally, maximum concentrations of nitrates, fluorides, sulphates, chlorides, nickel, and cadmium above the regulatory limits were detected in CDWs, while maximum concentrations of Pb were found in RAs. The results obtained from the experimental leaching tests carried out on MRAs and RCAs produced by two CDW treatment plants led to the following conclusions:  
* In line with the results obtained from the statistical analysis, the pH is always alkaline (pH \(>\) 10) for both MRAs and RCAs. In particular, compared with MRAs, RCAs show higher pH values. * Total chromium and COD are the most critical parameters for recovery, as they showed concentrations above the regulatory limits of 50 \(\upmu\)g/L and 30 mg/L, respectively. In particular, higher chromium concentrations were detected in RCAs, while COD in MRAs. * The grain size of the tested recycled aggregates (i.e., d max less than 4 or 10 mm) does not significantly affect the pH values for both MRAs and RCAs. Conversely, in terms of release of contaminants, the finer particles (d max \(\leq\) 4 mm) generally tend to release higher pollutant concentrations in the eluate than the coarse fractions (d max \(\leq\) 10 mm). This is mainly due to the larger specific surface area of the fine particles, which favours a greater interaction between the leaching agent and the solid matrix. * Different from some findings available in the literature [32], in this study the crushing of the material before being subjected to leaching tests does not always influence the release of contaminants into the surrounding environment. In particular, from the results obtained in plant A, the sieving process contributes to a greater release of contaminants. Conversely, regarding plant B, the material crushing results in a higher release of pollutants (especially for fluorides, sulphates, chromium, and COD). In conclusion, about this last aspect, the authors believe that the results obtained should be further investigated with additional experiments. In fact, as stated in the introduction, the research studies that address and evaluate the influence of the RA crushing process on the release of contaminants are still limited.