Chemical performance of phosphogypsum and municipal solid waste incineration bottom ash blends for potential use as road base

The reuse of waste products such as acidic phosphogypsum (PG) and alkaline municipal solid waste incineration (MSWI) bottom ash (BA) in construction applications like road base continues to gain global popularity. While each material has been studied independently, their combination represents a novel approach to resource recovery and waste diversion from landfilling or stockpiling. This study evaluated three BA sources blended with PG, focusing on total and leachable concentrations of trace metals and fluoride. Compared to United States federal regulatory risk thresholds for direct exposure, elevated levels of As were found in all samples (4.71–18.6 mg/kg), and Pb in MSWI BA samples (571–790 mg/kg). Blending with PG reduced the total Pb concentrations in several mix designs (91–673 mg/kg). Leaching analysis of each material revealed As, Cd, Sb, Cu, Pb, and fluoride concentrations as constituents of potential concern from either BA or PG samples. Blending PG (pH 2.31) and BA (pH 9–11) resulted in more neutral eluent that ranged from pH of 5.5–10.6, decreasing with increasing PG replacement. A notable reduction in the concentrations of As, Cd, Sb, and fluoride was observed with the blends, attributed largely to the pH modification in the final eluent or precipitation or sorption mechanisms. For each constituent evaluated, at least one BA-PG blend reduced concentrations below the regulatory screening limit. Fresh MSWI BA was most effective at reducing mobile Sb and fluoride concentrations, attributed to the elevated ettringite and portlandite found in fresh ash at an elevated pH. These findings suggest that with appropriate mix design and consideration of site conditions, BA-PG blends may offer a practical reuse pathway with reduced environmental impact. Future investigations involving site-specific dilution and attenuation in soil and groundwater can serve to evaluate the complete risk assessment of the blending approach.