###### Abstract
[32] investigated the leaching behaviour of RCAs with particle sizes of 0/6.3 mm, 6.3/20 mm, 20/40 mm, and \(>\)40 mm by applying two different leaching test methods: (1) the EN 12457-2 standard, in which each fraction was crushed to a 4 mm particle size before the leaching experiment (whatever the initial particle size of the sample) and (2) a non-standardised leaching test without any supplementary crushing (RAs only sieved), in order to show the effect of grain size on the leaching behaviour. The results of the tests performed on samples crushed to a 0-4 mm showed different concentration trends for the chemical elements analysed (Si, Al, Na, K, Ca, Mg, Fe, P, Cr, Ti, S). In particular, the smaller fraction (0-6 mm) concentrates trace elements (Cr, Cu, and Pb) and SO\({}_{4}\) more often than the coarse fraction. The same behaviour was detected for RAs tested with the non-standardized method (i.e., only screened and not crushed). The results globally showed a decrease in the concentrations of K, SO\({}_{4}\), Cl, Cr, and Cu as the grain size rises. Based on this, the study showed that smaller fractions generally lead more than the coarse fractions. Moreover, by comparing the two leaching test methods, the volumetric reduction involves a greater release of contaminants. In this context, the authors recommended the implementation of leaching tests on samples of different grain sizes without prior volumetric reduction. This finding was also corroborated by [PERSON] and [PERSON] [48], who highlighted that leached concentrations of Ca, Ba, Cr, and SO\({}_{4}\) decreased with an increase in RCA particle size, and by [PERSON] et al. [49], who reported that leached metal concentrations increased when the particle size decreased. [PERSON] et al. [18] also demonstrated the importance of evaluating the release of contaminants from RAs in their commercial particle size to simulate their actual on-site conditions. In particular, MRAs with 0-32 mm grain size distribution were tested by applying the Under Compaction Leaching Test (i.e., procedure that analyses the samples without any pre-crushing or alteration of the grain-size distribution). Compared with traditional leaching tests (UNE EN 12457-3 and NEN 7343) that predict a reduction in particle size (d\({}_{\text{max}}<\) 4 mm), the results showed lower release levels. Within the aforementioned context, most of the scientific studies [7; 8; 14; 15; 17; 18; 20; 33; 39; 41; 48; 50; 51; 52] concerning the evaluation of the environmental properties of both CDWs and RAs are generally based on laboratory-scale research. Thus, one of the main purposes of this research was to evaluate the leaching behaviour of CDWs and RAs by adopting a statistical approach. In particular, several leaching test certificates related to CDWs and RAs were collected by two CDW treatment plants located in the Lombardy Region (northern Italy). The collected data were then statistically processed, and the leaching behaviour of both CDW and RAs was assessed in compliance with the limits defined by the Italian legislation for recovery of granular materials (Ministerial Decree 186/2006) [53]. Subsequently, in order to identify the sources of high release levels in both CDWs and RAs, the critical release of some contaminants was correlated with specific EWC codes (i.e., European Waste Code). The second goal of this research was to investigate whether and how the RA grain size, as well as the crushing process before the leaching test, can affect the release of contaminants. As reported above, the material crushing leads to higher pollutants release due to the increase of the contact surfaces between RAs and leaching agent, but, at the same time, sieving operations can also lead to greater fine fractions that cause high releases. Despite this, research studies that thoroughly evaluate these aspects are limited [18; 32; 48; 54]. Thus, the main aim was to further investigate these conditions and support or integrate the data obtained from previous studies.