Benefits of pre-treatments on MSWI fly ash before alkali-activation

Municipal solid waste incineration fly ash (MSWI FA) is classified as hazardous waste for the high content of soluble salts and heavy metals, and the corresponding risk of leaching. Nowadays the ashes are usually landfilled, but another possibility is to inertize and recover the FA in building materials. Geopolymer can be a potential material for its natural capacity to entrap the heavy metals physically and chemically inside its matrix. In this research it was investigated how the use of pre-treatments compared with no treatments on the MSWI FA influence the environmental and mechanical stability of a metakaolin-based geopolymer as encapsulation matrix for FA. The MSWI FA originated from the Amager Bakke incinerator plant of Copenhagen. The pre-treatments applied were an electrodialytic treatment and a water washing procedure. The treated and untreated ash and metakaolin were characterized. Geopolymers with 5, 10, 20 wt% of MSWI FA were compared with a sample containing only metakaolin and the compressive strength and the leaching of heavy metals and salts from the geopolymers were investigated. The results show low compressive strength for the sample with washed ash, but good mechanical strength for geopolymers with raw and electrodialytic treated ash compared to only metakaolin-based geopolymer and a decrease in leaching for all the metals, confirming the encapsulation capacity of geopolymers. The electrodialytic treatment decreases in particular the leaching of Cr, Zn and chlorides confirming the potential use of this pretreatment to improve the environmental and mechanical characteristics of geopolymers that use MSWI FA.