Investigating the Effect of Wet Milling on the Speciation of Al in Municipal Solid Waste Incineration Fly Ash and the Microstructure of Hydration Gels in Alkali-Activated Materials

Abstract

Municipal solid waste incineration fly ash (MSWI fly ash) typically contains metallic aluminum ( Al 0 ), which can induce expansion and strength deterioration in alkali-activated materials and cementitious materials, consequently increasing environmental leaching risks. In this study, wet milling was employed as a pretreatment method to reduce gas release from MSWI fly ash under alkaline conditions. The wet-milled MSWI fly ash then was used as a raw material for the preparation of alkali-activated materials to investigate its enhanced reactivity under alkali-activation conditions and its influence on the microstructure of hydration gels. The physicochemical properties, hydration process, and effectiveness of alkali-activated materials in immobilizing heavy metals were systematically analyzed. The results indicated that wet milling is an efficient method for mitigating gas release. During wet milling, metallic Al, feldspar, and gypsum were converted into ettringite. The use of wet-milled MSWI fly ash significantly improved the 7- and 28-day compressive strengths of the alkali-activated materials. At a 50% replacement level, the 28-day strength was 3.8 times that of the alkali-activated materials prepared with untreated MSWI fly ash, and maintained stable performance at high dosages. Gel polymerization was promoted by the enhanced amorphous content and preferential dissolution of aluminum derived from ettringite. This led to an increased Al/Si ratio in N─A─S─H/C─(N)─A─S─H gels and longer main chain lengths, along with increased formation of hydration gels. These microstructural changes resulted in excellent heavy-metals immobilization performance. This study not only proposes a novel pretreatment method for MSWI fly ash, but also improves its resource utilization efficiency, demonstrating potential for high-value applications.