Geopolymer sound-absorbing materials from municipal solid waste incineration fly ash and foundry ash: Dioxin detoxification, heavy metal stabilization and acoustic performance

Sound-absorbing materials are widely used to mitigate noise pollution. In this study, a geopolymer sound-absorbing material was prepared using municipal solid waste incineration fly ash (MSWIFA) and foundry ash (FA), and its dioxin and heavy metal characteristics were investigated. The toxicity equivalent quantity (TEQ) of dioxins in raw MSWIFA was 78 ng TEQ kg^-1, with 2,3,4,7,8-PeCDF contributing 34 ng TEQ kg^-1. Detoxification using calcium hydroxide combined with pyrolysis at 400°C reduced the TEQ in MSWIFA to 0.62 ng TEQ kg^-1, achieving a removal rate of 99.21%. A geopolymer was prepared by mixing low-temperature detoxified fly ash (LTDFA, pyrolysed at 400°C) with activated foundry ash (AFA, calcined at 800°C) at a 1:1 mass ratio, resulting in LTDFA-FABG with a TEQ of 12 ng TEQ kg^-1 - well below the 50 ng TEQ kg^-1 limit specified in the <i>Technical Specification for Pollution Control of Fly Ash from Municipal Solid Waste Incineration</i> (HJ 1134-2020). Compared to LTDFA-FABG, the geopolymer prepared using LTDFA, AFA and lime (LA-LTDFA-FABG) showed significantly reduced leachate concentrations of Cd and Mn, from 0.115 and 0.206 mg L^-1 to below detection limits. LA-LTDFA-FABG exhibited a sound absorption bandwidth of 400-1000 Hz, a noise reduction coefficient of 0.49, and a compressive strength of 0.34 MPa, representing increases of 28.9% and 13.3%, respectively, over a commercial sound-absorbing panel. With an average pore size of 13.71 nm and a closed-pore structure, LA-LTDFA-FABG enhances contact and friction between sound waves and air, improving low-frequency sound absorption. These findings demonstrate that MSWIFA and FA can be effectively combined to produce geopolymer sound-absorbing materials, offering a promising route for the resource utilization of MSWIFA.