Resource utilization of ultrasonic carbonated MSWI fly ash as cement aggregates: Compressive strength, heavy metal immobilization, and environmental-economic analysis

Ultrasonic carbonation technology is effective for municipal solid waste incineration fly ash (MSWI FA) pretreatment. However, the subsequent disposal route is unclear. In this study, the mechanical strength and heavy metal immobilization effects of raw FA (RFA), washed FA (WFA), conventional carbonated FA (CFA), and ultrasonic carbonated FA (UFA) as cement aggregates in the preparation of concrete were investigated in comparison. The reaction kinetic analysis showed that ultrasonic excitation promoted the internal diffusion phase of the carbonation reaction. The compressive strength of the 28-day solidified bodies was in the order of UFA (32.2 MPa) > CFA (22.8 MPa) > WFA (16.4 MPa) > RFA (12.6 MPa). The 3-day UFA solidified body showed the highest degree of hydration and polymerization through 29Si solid-nuclear-magnetic analysis. The nano-CaCO3 in the UFA provided more nucleation sites and hydration reaction sites for the hydration reaction. In addition, the fine UFA particles filled the pores between cement particles, enhancing the strength of the product. Heavy metal leaching tests described that CFA and UFA met the criteria for resource utilization (HJ/T 1134-2020), and the immobilization efficiency of Cu, Pb, and Zn in the UFA solidified body was above 95.6%, 99.3%, and 99.2%, respectively, mainly by the dual immobilization of carbonate chemical precipitation and cement hydration. Considering 11 environmental impact factors and economic benefits, life-cycle-assessment and cost-benefit analysis were selected. The integrated environmental-economic results show that cement usage is the main influencing factor. UFA scenario was recommended as engineering applications with low global warming potential.