Effect of pH on the Leaching of Elements from Highway Base Layers Built with Recycled Concrete Aggregates

摘要

Concrete waste is one of the most abundant waste materials, remains in the environment for a long period of time, and if not recycled, can significantly reduce the life of municipal landfill facilities. Crushing waste concrete to make recycled concrete aggregate (RCA) has been gaining more attention recently, as economic and environmental constraints on the production of natural aggregates increase. Recycling concrete also diverts construction and demolition debris waste streams from our landfills. However, RCA has been shown to contain heavy metals which may leach and cause potential contamination of the groundwater. Laboratory pH-dependent batch water leach tests at the pH range of 2-14 were conducted to investigate the leaching behavior of Ca, Zn, and Cu from two different recycled concrete aggregates used in highway constructions. Leached concentrations of Zn were below the detection limits, while the pH of the leachate was higher than 5. Ca showed a cationic leaching pattern, indicating that the concentrations of leached Ca decreased with an increase in pH while Cu and Zn showed amphoteric leaching pattern. The leached element concentrations were below the Environmental Protection Agency Maximum Contaminant Limits (EPA MCLs) with few exceptions. Geochemical modeling results from MINTEQA2 indicated that releases of the elements were solubility-controlled. For a given element, the solubility-controlling solids were found to be very consistent. Oxide and hydroxide minerals control the leaching of Ca, Zn, and Cu elements.