Reducing Pb and Cl Mobility in Waste-to-Energy Fly Ashes via Chlorellestadite Formation

Sustainably sourced development minerals are vital to meeting the demand for low-carbon construction materials. A potential waste material that can be a low-carbon construction material for cementitious systems is waste-to-energy fly ash (WTE ash), a byproduct of waste incineration. WTE ash contains chloride-bearing species and heavy metals, which can promote steel reinforcement corrosion and present long-term leaching risks. Here, we present a new multistep ash treatment protocol involving dissolution and heating to transform the nonreadily removable chlorides in WTE ash to chlorellestadite [Ca10(SiO4)3(SO4)3Cl2]. Our results suggest that this transformation significantly reduces chloride release from WTE ash in an alkaline cement-like environment by over 2 orders of magnitude, from 1500 mg/L to less than 10 mg/L. This transformation also significantly reduces Pb mobility after treatment (from >5 mg/L to <0.5 mg/L), likely caused by the crystallo-chemical incorporation of Pb in chlorellestadite. In contrast to the existing ash treatment methods, the proposed ash treatment protocol minimizes heavy metals volatilization while simultaneously reducing Pb mobility and chloride release through chlorellestadite formation. These findings overcome limitations preventing the commercial use of WTE ash and thus opening up pathways toward a circular economy.