The proposed NPDES regulations in the State of Washington, requiring a pH below 8.5 at the point of discharge from recycled concrete aggregate (RCA), may have the unintended consequence of prohibiting the use of recycled concrete materials in commonly accepted concrete recycling applications; e.g., as unbound base course or fill material or aggregate in ready-mix concrete. A more appropriate method to determine compliance with pH regulations would be to determine a “point of compliance” and enforce pH regulations at that point. However, selection of an appropriate point of compliance is hindered by disagreement in previous studies on the pH of leachate as well as its acid neutralizing capacity (alkalinity). Most laboratory studies and many field studies suggest that the leachate pH should be very high (e.g., >9) for extended periods of time; however, the NAICS data presented in Figure 1 and results of our own field studies (Chen et al., 2012; Chen et al., 2013) suggest that leachate pH values above 8.5 are actually infrequent. Here we propose to couple laboratory leaching studies, utilizing representative saturation and geochemical conditions, with results from a forensic examination of an RCA base course located at the MnROAD test facility to determine mechanisms that may limit the production of high pH of leachate.
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