Benefits of pre-treating MSWI fly ash before alkali-activation

The significant amount of heavy metals and soluble salts contained in municipal solid waste incineration fly ash (MSWI FA) and the corresponding risk of leaching make them classified as hazardous waste. Nowadays, the ashes are usually landfilled, but another application can be the inertization and valorization of MSWI FA in building materials. Geopolymer can be a potential material because it can naturally entrap heavy metals physically and chemically inside its matrix. This research investigated how the use of electrodialytic treatment compared to a water washing procedure, considered as pre-treatments for MSWI FA, influenced a metakaolin-based geopolymer as an encapsulation matrix for MSWI FA. Geopolymers with metakaolin and 5, 10, 20 wt% of raw or pre-treated MSWI FA were compared with a sample containing only metakaolin for environmental, mechanical, and physical properties. The formation of aluminosilicate gel was confirmed for all geopolymers, although with varying degrees of impurities. 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 metakaolin-based geopolymer or with geopolymer with washed ash. A decrease in leaching for all the metals in the samples confirms the encapsulation capacity of geopolymers. The electrodialytic pre-treatment decreases, particularly the leaching of Cr, Zn and chloride in geopolymers, confirming the potential use of this pre-treatment to improve the environmental and mechanical characteristics of geopolymers with MSWI FA.