Toward low NaOH and silicate concentrations for alkali-activated municipal solid waste incineration fly ash

Municipal Solid Waste Incineration Fly Ash (MSWIFA) is a hazardous waste characterized by a high concentration of heavy metals and a high soluble fraction of salts. It is commonly stabilised by the addition of cement. This paper proposes an alternative method of solidification/stabilisation involving room-temperature alkali activation as a low-carbon, cement-free solution. This industrially feasible method retains a waste content of over 50 wt% in the wet mix and uses the low concentrations of NaOH and sodium silicates to activate the material by adding Ground Granulated Blast Furnace Slag (GGBFS). The formation of reaction products like calcium aluminosilicates hydrate gels (CASH) and Friedel's salt resulted in good early-age mechanical properties, with compressive strength reaching 7 MPa after 91 days. Heavy metals leaching was significantly reduced by the activation of the waste, as well as the soluble fraction. Notably, the formation of Friedel's salt improved chloride immobilisation in the samples. This work also highlights how low activator concentrations mitigate the risks of increased porosity, sulphate species leaching, and heavy metal immobilisation degradation. Thus, this study contributes to the understanding of the S/S process for hazardous waste by presenting a cost-effective, low-carbon alternative to cement with a high waste content.