Gaps in predicting water quality impacts of unbound air-cooled blast furnace slag utilized for roadway construction

Air-cooled blast furnace slag (ACBFS) is often used as road construction aggregate. Several incidences of adverse environmental effects have been reported in Indiana where unbound slag has been used. The aggregates that caused issues in the field had been previously approved for use by passing State of Indiana standard slag leaching procedures. The study goal was to test several hypotheses posed to the Indiana Department of Transportation and authors by the industry about slag leaching, and to better understand water quality impacts. Ten slags from two suppliers underwent leaching procedures adopted by two U.S. states, but additional test procedures and material and leachate chemical characterization were also conducted to better interpret results. Aggregates varied in size and age (24 h to 2 years old). Like prior literature reports, the chemical composition among the slag samples was similar. However, water quality impacts observed at the bench-scale in this study differed substantially across the slags. Results were consistent with those previously reported at Indiana field sites, but not with literature reported results from standardized leaching tests (i.e., TCLP, percolation, etc.). In the present study, no relationship was found between slag age and leachate composition. Leachate pH, conductivity, and color levels exceeded thresholds of regulatory and aquatic toxicity significance. For one slag, leachate pH reached 12.9, but the effluent was colorless. During both stagnation and rinsing tests, aqueous chloride, sulfate, and zinc concentrations sometimes exceeded regulatory water quality and aquatic toxicity levels. Repeated rinsing of the slag resulted in water conductivity and sulfate concentration differences. To better understand and predict the impacts of unbound ACBFS, additional fundamental studies and field investigations are recommended. Unbound ACBFS is not recommended for use near environmentally sensitive areas, drinking water sources, habituated areas, and where it may have water contact (i.e., waterways, wetlands, reservoirs, and groundwater).