## 3 Results  
### Statistical Evaluation of Leaching Behaviour of CDWs and RAs  
The assessment of potential environmental impact of CDWs and RAs is a topic of great interest around the world, as the main goal of most countries is to achieve high recovery rates of these materials [31]. In this context, to ensure environmental protection and to evaluate the release of pollutants into the surrounding environment, these materials are subjected to leaching test during their recovery process. One of the main goals of the leaching test is to evaluate the potential pollution of a material by comparing the released contents of contaminants with the legal limits defined by national legislation [16]. In this study, the leaching behaviour of CDWs and RAs was evaluated according to the compliance test established by UNI EN 12457-2 and the release of pollutants was compared with the limits imposed by M.D. 186/2006. Table 4 reports the concentrations of released compounds in terms of minimum-maximum range and average value. Figure 4: Samples of RAs subjected to leaching tests according to EN 12457-2 and 4 standards. Figure 3: Scheme of the RA sampling method adopted. As research studies reported [14; 40; 48; 52; 56], the pH of both CDWs and RAs is generally alkaline with maximum values equal to the regulatory limit (i.e., pH 12) (Figure 5). Compared with the available regulatory limits, the average concentrations are generally lower except for mercury and COD in CDWs. Nitrates, fluorides, sulphates, chlorides, nickel, cadmium, and total chromium are also critical parameters for CDWs in terms of maximum concentrations. These results have also been confirmed by other authors [13; 19; 27; 33]. Regarding RAs, critical maximum concentrations were detected for total chromium, lead, mercury, and COD. To identify the main sources of the most critical compounds for recovery (i.e., total chromium, mercury, and COD), the box-plot graphs resulting from the statistical analysis are shown in Figures 6-8. Figure 5 illustrates the variation of pH values in CDWs and RAs. Both materials are alkaline with 50% of the values ranging between 7 and 10, but values up to the regulatory limit (i.e., 12) were also detected. This alkalinity is mainly due to the cementitious fractions present in the CDWs and, consequently, in the produced RAs. In fact, EWC 170101 \"cement\" has the highest pH values, with a median value of approximately 11. Although to a lesser extent, mixed CDWs (EWC 170904) are also responsible for alkalinity, as they may contain concrete and cement fractions (median value of approximately 8), while CDWs with EWC 170504 (soil and stones) and EWC 170302 (bituminous mixtures) have significantly lower pH values (median values of about 7.5). These results are confirmed by [PERSON] and [PERSON] [14], [PERSON] et al. [52], and [PERSON] et al. [8]. In particular, [PERSON] et al. [8] highlighted that CDWs or RAs samples containing cement and concrete fractions have pH values between 11 and 13. Instead, mixed CDWs show slightly lower pH values due to the presence of ceramic and soil fractions. The results obtained also show that RAs generally have pH values higher than CDWs. This may be due to the fact that the RAs--subjected to crushing--present new non-carbonated surfaces coming from the core of the concretes that have higher pH values [32].