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Leaching issues in recycled aggregate concrete                    333


           fresh material (BpH 13) will fall down to values eventually around pH 8 due to the
           formation of calcium carbonates.
              The pH is a relevant chemical factor that controls the release of many contami-
           nants and, therefore, ageing has a profound effect of leaching (e.g., Dijkstra et al.,
           2006). Leaching of metal cations generally decrease upon ageing, but the leaching
           of some anions may increase (Huijgen and Comans, 2006). To study the effect of
           pH changes as a result of ageing, a dependence test is the most useful tool. This
           test allows the extrapolation of the results to different scenarios (different field con-
           ditions) in a semi-quantitative manner. The analysis of the pH-dependency allows
           analysing the effect of pH variations on release patterns of contaminants.
              Because the alkalinity and leaching mechanisms are quite similar throughout
           cementitious systems, the material is used as a surrogate for many other cementi-
           tious raw or waste materials, including demolition wastes (Van der Sloot, 2000;
           Van der Sloot, 1997; Engelsen et al., 2009; Engelsen et al., 2010).


           12.1.3 Concepts and mechanisms which control release
           Since RCA is a cement-based material, there are concerns related to potentially ele-
           vated leaching patterns due to the inherent high alkalinity of RCA (Chen et al.,
           2012). Studies by Aydilek (2015) focussing on the environmental suitability of
           RCA in highways have shown that in practical applications using cement-based
           material, wide pH ranges (7.5 12) due to both weathering of material and material
           alkalinity have been observed (Van der Sloot, 1997). Leachate from RCA with high
           pH value will bring about the potential degradation of vegetation and obstructed
           highway drainage through clogging, but more importantly varying heavy metal
           leaching characteristics (Engelsen et al., 2010; Van der Sloot and Dijkstra, 2004;
           Dijkstra et al., 2006).
              For this type of material, the main source of the constituents on leachates is the
           cement paste (Engelsen et al., 2010), as well as cement additives [as fly ash or steel
           according to Aydilek (2015)].
              The release of metals on leachates is derivative of the physical and chemical
           properties of RCA as well as the environmental conditions imposed. Thus, previous
           research works have proved that recycled aggregates have the tendency of leaching
           certain metals, such as zinc, chromium or copper (Engelsen et al., 2009, 2010).
              Chen et al. (2012) registered leachates from RCA materials with high chromium
           concentrations that exceeded 0.1 mg/L [the maximum contamination level imposed
           by the Environmental Protection Agency (EPA)]. This research proved that lea-
           chates from RCA present critical levels of certain toxic metals, and includes an
           evaluation of several factors such as curing time, freeze thaw effect, liquid-to-
           solid ratio, particle size distribution or the role of pH on the release levels of metals
           (Aydilek, 2015).
              The Leaching Assessment Research Group from the Vanderbilt University has
           been developing useful tools for characterising the leaching processes on solid
           materials. These research works have proved that the leaching process is indiscrimi-
           nate due to the release of the constituents produced by chemical phenomena which
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