Utilization of municipal solid waste incinerator fly ash under high temperature sintering and alkali excitation for use in cementitious material

Abstract

It is crucial to have a secure and dependable approach to treat and reuse municipal solid waste incinerator (MSWI) fly ash to prevent potential hazards. High temperature sintering is regarded as an efficient method for solidifying heavy metals, and alkali excitation provides a practicable solution for utilizing MSWI fly ash. In this work, the activation mechanism of MSWI fly ash was figured out under high temperature sintering. Besides, the preparation and optimization of sintered MSWI fly ash with metakaolin for use in alkali–activated cementitious material were explored based on Box–Benhnken design (BBD), considering different modulus and alkali equivalent of alkaline activator, as well as different contents of sintered MSWI fly ash and metakaolin. The mechanical performance and alkali excitation mechanism of the cementitious material were investigated. The results showed that the pozzolanic activity of MSWI fly ash treated by high temperature sintering was increased by 81.7 %, which further was improved by alkali excitation. The optimal mix proportion of alkali–activated sintered MSWI fly ash–metakaolin cement mortar consisted of a modulus of 1.332, an alkali equivalent of 1 %, a sintered MSWI fly ash content of 20 %, and a metakaolin content of 10.5 %, whose flexural and compressive strength were 8.5 and 57.3 MPa, respectively. Moreover, SiO32− and OH− in alkaline activator and Al(OH)4–, SiO2(OH)22−, and SiO(OH)32− in metakaolin promoted Ca2+ in sintered MSWI fly ash to generate C–S–H and C–A–S–H. Finally, the alkali–activated mortar made from sintered MSWI fly ash demonstrated high safety and eco–friendliness, with a synthesis toxicity index (STI) of 25.59.