###### Abstract  
Construction and demolition wastes represent a primary source of new alternative materials which, if properly recovered, can be used to replace virgin raw materials partially or totally. The distrust of end-users in the use of recycled aggregates is mainly due to the environmental performance of these materials. In particular, the release of pollutants into the surrounding environment appears to be the aspect of greatest concern. This is because these materials are characterized by a strong heterogeneity which can sometimes lead to contaminant releases above the legal limits for recovery. In this context, an analysis of the leaching behaviour of both CDWs and RAs was conducted by applying a statistical analysis methodology. Subsequently, to evaluate the influence of the particle size and the volumetric reduction of the material on the release of contaminants, several experimental leaching tests were carried out according to the UNI EN 12457-2 and UNI EN 12457-4 standards. The results obtained show that chromium, mercury, and COD are the most critical parameters for both CDWs and RAs. Moreover, the material particle size generally affects the release of contaminants (i.e., finer particles showed higher releases), while the crushing process does not always involve higher releases than the sieving process. construction and demolition wastes; recycled aggregates; leaching behaviour; statistical analysis; crushing process +
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only treated and recovered in treatment plants which are subsequently stored without any actual reuse in the economic market. This is mainly due the fact that end-users still prefer to use natural aggregates rather than recycled aggregates (RAs) because, to date, they are still cost-effective resources and are considered more reliable in terms of technical and environmental performance. In particular, the possible release of contaminants into the surrounding environment by the RAs is one of the main reasons for distrust in the use of these materials in the construction sector. This is mainly due to the high heterogeneity rate of both CDWs and RAs linked to the different origin of the materials contained (e.g., bricks, concrete, gypsum, wood, masonry, plastics, metals, and more) [4]. As highlighted and confirmed by [PERSON] et al. [5] and [PERSON] et al. [6], these materials may contain environmentally hazardous compounds that can potentially filter and contaminate the underlying soil as well as surface and groundwater [7; 8]. Generally, concrete and masonry are usually the prevailing material fraction in CDWs, especially in Mediterranean countries [9]. Concrete derives from foundations, load-bearing structures, pavements, road structures, or sidewalk [10], while masonry and ceramic materials come from covering elements, such as roof tiles and brickworks [11]. Minor fractions of plastics, metals, gypsum, and wood may also be found depending on the source segregation level and the recovery technology adopted [12]. In the last few decades, several research studies have investigated the leaching of organic and inorganic compounds by CDWs and RAs [12; 13; 14; 15; 16; 17; 18; 19; 20]. Generally, CDW and RA eluates have alkaline pH (pH \(>\) 7) [17; 21]. As stated by [PERSON] et al. [22], pH values of concrete and mixed CDWs--as well as of the respective RAs produced--is strongly alkaline with pH values around 12.5 or even higher [23]. As stated by [PERSON] et al. [24] and [PERSON] et al. [25], this alkalinity is due to high content of cement paste and mortar in concrete. These compounds are mainly composed of portlandite and calcium-silicate-aluminate (C-S-H) which are hydrated in an aqueous solution to produce Ca(OH)\({}_{2}\) with high alkalinity [26]. Conversely, the pH values of brick CDW leachates are neutral or slightly alkaline (pH ranging from 7 to 8.5) [27].