Key Takeaways & Executive Findings
- •• • Water washing achieves 88%–95.5% chloride removal; at liquid-to-solid ratio 5 and 20 min washing, soluble chloride in the final aerated concrete is controlled below 1% (mass fraction), meeting HJ 1134-2020 limits and preventing durability degradation. • • CO2 curing (65 °C, 0.6 MPa, 2 h) effectively immobilizes heavy metals by densifying the pore structure; leaching of Cr, Pb, Cd, and Zn decreases with lower fly ash content, but Cu leaching anomalously increases when fly ash is absent (0% content). • • Adding 17.5% organic sulfur stabilizer (DTC) achieves superior heavy metal stabilization compared to Na2S, ensuring all heavy metal contents meet national standards without significantly affecting compressive strength (1.50 MPa) or carbonation efficiency. • • Optimal formulation (bottom slag:fly ash:slag = 40:20:40, aluminum powder 0.04%, water-to-cement ratio 0.36) yields aerated concrete with compressive strength of 1.50 MPa, meeting A1.5, B03 grade requirements per GB/T 11968-2020.
Abstract
This study systematically evaluates the environmental risks associated with the resource utilization of municipal solid waste incineration (MSWI) fly ash in the production of aerated concrete, co-prepared with multiple solid wastes. The focus is on the leaching behavior and total content of heavy metals (Cr, Pb, Cd, Cu, Zn) under CO2 curing and chelating agent stabilization. Water washing pretreatment parameters (liquid-to-solid ratio, washing time, ash-to-slag ratio) were optimized for chloride removal. Results demonstrate that CO2 curing suppresses the leaching of most metals; leaching concentrations of Cr, Pb, Cd, and Zn decrease with reduced fly ash content, whereas Cu leaching increases when fly ash is absent. The addition of 17.5% organic sulfur stabilizer (DTC) significantly outperforms inorganic sulfide (Na2S) in immobilizing heavy metals, achieving compliance with national standards without compromising compressive strength or carbon sequestration. Water washing effectively reduces soluble chloride content to below 1% (mass fraction), meeting the HJ 1134-2020 regulatory limit. Optimal parameters include a liquid-to-solid ratio of 5, washing time of 20 min, and a raw material ratio of incineration bottom slag:fly ash:slag = 40:20:40. Under these conditions, the final product exhibits a compressive strength of 1.50 MPa, with heavy metals and soluble chlorides fully compliant. This work provides key technical support for the safe recycling of MSWI fly ash in building materials.
1. Introduction
The global surge in municipal solid waste has made incineration a dominant treatment technology, yet the resulting fly ash—over ten million tons annually in China—contains hazardous soluble chlorides and heavy metals, posing severe environmental and health risks. Traditional disposal via solidification and landfilling suffers from high volume increase and limited landfill capacity, while high-temperature melting is energy-intensive and risks secondary pollution from volatile heavy metals. Consequently, the resourceful and high-value utilization of fly ash is imperative for both environmental protection and structural carbon reduction.
Co-preparing aerated concrete with fly ash and other solid wastes leverages the pozzolanic components (CaO, SiO2, Al2O3) in fly ash, but its high chlorine and heavy metal content fundamentally restricts its use in building materials. Existing pretreatment methods, such as water washing and chemical stabilization, have shown promise, yet a systematic environmental risk assessment of the final product—particularly regarding heavy metal leaching and soluble chloride content—remains lacking. This study addresses that gap by evaluating the environmental safety of aerated concrete produced under optimized conditions, including CO2 curing and DTC stabilization, thereby providing a technical foundation for safe industrial application.
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SHI Songwei, ZHANG Wenpei, ZHAO Cheng, SUN Yansong, ZHANG Yan, YAN Mi (2026). Environmental Risk Assessment of Aerated Concrete Prepared by Synergistic Utilization of Incineration Fly Ash and Multi-Source Solid Wastes. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202507029
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Frequently Asked Questions
What is the maximum allowable fly ash content in the mix that still meets the compressive strength requirement of 1.50 MPa?
The optimal mix ratio is bottom slag:fly ash:slag = 40:20:40, corresponding to a fly ash content of 20%. Under this condition, the compressive strength reaches 1.50 MPa, meeting the A1.5, B03 grade. Increasing fly ash content beyond this may reduce strength due to higher chloride and heavy metal content, though the study did not test higher contents.
How does the addition of DTC stabilizer affect the long-term stability of heavy metals under leaching conditions?
DTC (organic sulfur stabilizer) at 17.5% addition forms strong chelates with heavy metals, significantly reducing their leachability compared to Na2S. The study confirms that under optimized conditions, heavy metal contents comply with national standards, but long-term durability and leaching behavior under field conditions were not evaluated, which is a noted research gap.
What is the mechanism by which CO2 curing reduces heavy metal leaching?
CO2 curing promotes the formation of calcium carbonate and densifies the pore structure, physically encapsulating heavy metals and reducing their mobility. This is evidenced by decreased leaching concentrations of Cr, Pb, Cd, and Zn with lower fly ash content, though the anomalous increase in Cu leaching at zero fly ash suggests a complex interaction that warrants further investigation.
Can the water washing process be scaled up industrially without excessive water consumption?
The study used a liquid-to-solid ratio of 5 and a washing time of 20 minutes, achieving chloride removal of 88%–95.5%. For industrial scale, water recycling and treatment are essential to minimize freshwater use and avoid secondary pollution. The process is already being demonstrated in engineering projects with capacities of 3×10^5 to 1.5×10^6 tons per year, indicating feasibility.
What are the limitations of this study regarding organic pollutants like dioxins?
The study focused solely on inorganic pollutants (heavy metals and chlorides). The fate of persistent organic pollutants such as dioxins during the aerated concrete production was not assessed. This is a critical gap because dioxins can be present in fly ash and may pose environmental risks during product use or disposal. Future research should address their migration and degradation.
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