Chlorellestadite-enriched waste-to-energy fly ashes in cementitious systems: Implications of ash treatment on end use

Waste-to-energy (WTE) plants generate fly ashes that are often destined for the landfill. These ashes are rich in chlorine, zinc, and various heavy metals. Here, we investigate the feasibility of their incorporation within cementitious systems in the context of a novel treatment process that chemically binds chlorines into chlorellestadite. Untreated ashes enhance the hydration kinetics and strength, likely due to excess Cl. However, the treated ashes retard the hydration because of the presence of ZnO (2-6 wt%). This retardation was successfully mitigated with up to 2 % calcium nitrate additive, resulting in cement-ash pastes that had mechanical strength comparable to 100% cement pastes. The untreated ashes leached excess Cl and led to the precipitation of hydrocalumite, whereas the treated ash-cement pastes were free from this phase. Finally, for ashes incorporated in hardened cement pastes, the leaching of heavy metals such as Pb and Cr was well under the toxicity limit, for both treated and untreated ashes. In summary, untreated ashes may find end use in unreinforced concrete applications, and treated ashes may be used within reinforced concrete, at least up to 10% cement replacement level.