Assessing the impact of contact time on leachate chemistry from recycled concrete aggregates
This addresses environmental risks for construction applications by providing data on RCA leachate over time, but it is incremental as it fills a gap in existing studies without major breakthroughs.
The study tackled the problem of understanding how contact time affects leachate chemistry from recycled concrete aggregates (RCA), finding that leachate pH decreases over time from over 10, calcium ion concentration increases rapidly then gradually, and alkalinity stabilizes at 50-65 mg CaCO3/L after an initial increase.
Recycled concrete aggregate (RCA) is recognized as a readily available, mechanically sufficient construction and demolition waste product that is suitable as a base course substitute for natural, virgin aggregate in pavement construction. Environmentally responsible applications of RCA must consider the high alkalinity, high pH leachate, and heavy metal leaching risks reported in the literature. The existing body of literature does not address discrepancies between field and laboratory measurements of RCA leachate pH, nor are there any existing studies of aged RCA leachate composition as a function of time. To consider the influence of contact time on RCA leachate, the present study evaluates recovered RCA base course samples from the Minnesota Road Research highway construction study site using modified batch test methodology. Leachate pH, alkalinity, and calcium ion (Ca2+) concentration were monitored for 24 hours to understand RCA leachate chemistry during the initial contact period. Leachate pH is high upon initial contact with water (pH > 10) and decreases over time as it reacts with atmospheric carbon dioxide. Calcium ion concentration increases rapidly in the initial contact period, then more gradually as calcium saturation is reached. Alkalinity stabilizes (50-65 mg CaCO3/L) after a dramatic increase during the initial contact period.