Blending phosphogypsum to mitigate radionuclide leaching for sustainable road base applications

Production of phosphoric acid generates a calcium sulfate byproduct known as phosphogypsum (PG). PG is not considered a suitable standalone road base material because of concerns such as strength and presence of radionuclides. This paper investigates the latter, specifically the influence of blending PG with common alkaline road base aggregates - limerock (LR) and recycled concrete aggregate (RCA) - on radionuclide leaching. Radionuclide leaching from several PG sources was assessed for gross alpha, gross beta, uranium, and combined radium (226 + 228). Solution pH affected Ra²²⁶ mobility, with minimum concentrations exhibited at a pH in the range of 6 to 8. Mobile Ra²²⁶ concentrations in RCA blends decreased compared to original PG; Ra²²⁶ mobility initially increased at low LR replacements but decreased with increasing mass of LR (50 %-75 %). The data suggest an additional mechanism beyond pH alone impacted Ra²²⁶ mobility from the blends, possibly the binding or substitution of radium by elevated concentrations of Ba, Sr, or Ca. Blending with RCA resulted in radionuclide concentrations below respective drinking water thresholds, mitigating leaching concern from PG-RCA road base blends. PG-LR blends can meet regulatory limits when incorporating appropriate PG sources, providing an avenue for PG-amended road base materials. The blending approach reduced Ra²²⁶ mobility from PG-amended base, accommodating more PG use, serving as an alternative scenario to end-of-life stacking.