###### Abstract
In this context, [PERSON] [12] investigated the leaching performance of recycled concrete aggregates (RCAs) and reported that the pH of the concrete material, although strongly alkaline, gradually decreases over time due to carbonation, which is a natural well-known aging process that occurs when the hardened concrete comes in contact with atmospheric carbon dioxide and wetting-drying cycles [23; 28]. In particular, in the case of recycled materials, carbonation occurs when the RAs (specifically the RCAs) are crushed and stored in heaps before being used in the desired applications. The crushing process, in fact, leads to a substantial increase in the specific surface of the crushed concrete and accelerates the carbonation process [29]. Based on this, the aging process can change the RCA pH from an alkaline pH of about 12-14 (reference for a fresh non-carbonated material) to a slightly basic pH of about 9-10 (reference for a fully-carbonated material) [12; 16; 27]. This is mainly due to the degradation of the alkaline hydrate phases by the reaction between the dissolved Ca(OH)\({}_{2}\) and CO\({}_{2}\) to form stable CaCO\({}_{3}\)[30]. In reference to this, the authors [PERSON] et al. [8] found and highlighted that the carbonation level of RCAs stored outdoor for several months is, however, partial and not total. The pH, in fact, decreases but is always higher than 9-10. In addition to the crushing process and storage time, [PERSON] et al. [31] stated that the RCA carbonation level is also related to the carbonation degree of the original concrete structure associated with its manufacturing year and its service life exposure. However, the authors [PERSON] et al. [20] found small differences in the pH values of RCAs originated from concrete structures manufactured in 1960s and 1990s (pH of approximately 11.9) and those of RCAs originated from the demolition of new concrete samples (pH of approximately 12.9). Thus, despite the different manufacturing period, the RCAs were found to be partially carbonated (i.e., pH values higher than 10). As reported above, this may be due to the crushing of the old concrete that lead to the formation of new non-carbonatedsurfaces which come from the core of the concrete block and were not exposed throughout the lifetime of the building and never interacted with air or water [32]. Based on all these considerations, a high concrete carbonation degree or a high amount of ceramic materials (e.g., bricks, tiles) and soil in CDWs and RAs lead to lower pH values of the eluates [8; 27]. As stated by [PERSON] [12], these pH variations (i.e., more or less alkaline) affect the leachability of many chemical substances contained in both CDWs and RAs. Several studies, in fact, identified high releases of Cr, As, Zn, Se, Mo, and Sb at neutral or alkaline pH values [21; 33; 34]. In particular, the leaching of As, Se, and Cr is related to the degradation of hydrated attached mortar on the RA surface [33]. [PERSON] and co-workers [20] also highlighted that at neutral or alkaline pH most of the total Cr is released as hexavalent chromium (Cr (VI)), which is harmful and can cause allergies, nasal lesions, bronchitis, and especially dermatitis [35]. Conversely, when the pH is less than 5, Cr is leached as Cr (III). High releases of Cu, Ni, Pb, Cd, and Zn (metal cations) are instead detected at lower pH values (i.e., from pH 7 to pH 4) [36]. In particular, as stated by [PERSON] et al. [33], Zn is highly released in both acidic and alkaline environments. Obviously, as shown by previous research studies, the release of pollutants into the surrounding environment is also strictly related to the merceological fractions present in both CDWs and RAs. [PERSON] et al. [33] assessed the leaching behaviour of inorganic elements from concrete and gypsum CDWs. As compared with concrete CDWs, the highest concentration of inorganic pollutants was found in gypsum CDWs according to the order SO4 > Ca > CI > Mg > Zn > Cu > Fe.