Enhanced geopolymerization of MSWI fly ash through combined activator pretreatment: A sustainable approach to heavy metal encapsulation and resource recovery.

With the rapid pace of socioeconomic progress, there has been a continuous rise in the generation of municipal solid waste incineration (MSWI) fly ash. Enhancing the efficiency of MSWI fly ash utilization, minimizing expenses, and developing more cost-effective and environmentally sustainable cementitious materials represent critical scientific challenges that warrant immediate attention. The research commenced with the preliminary treatment of MSWI fly ash using a blend of chemical activators and a water rinsing method. Following this, quicklime and coal fly ash were incorporated to formulate a geopolymer derived from MSWI, aimed at stabilizing the MSWI fly ash. Throughout this investigation, a selection of five chemical activators-calcium hydroxide, sodium hydroxide, sodium silicate, dilute sulfuric acid, and citric acid-was employed in conjunction with a water-washing procedure to condition the MSWI fly ash. Three single factor experiments denoted A, B, and C, were designed, and the corresponding geopolymer samples were prepared. In this research, the analysis focused on the solidified MSWI fly ash-based geopolymer samples, examining their physical and chemical characteristics, compressive strength, levels of heavy metal release, and the structure at a microscopic level. The findings revealed that the B3 series exhibited the most substantial 28-day compressive strength, reaching 1.692 MPa. Concurrently, the concentrations of leached heavy metals, including zinc (Zn), copper (Cu), chromium (Cr), lead (Pb), cadmium (Cd), and nickel (Ni), were all beneath the regulatory thresholds stipulated by the GB16889 and GB18598 criteria, signifying a notably effective solidification outcome. During the geopolymerization process, gel phases such as calcium aluminosilicate hydrate (C-A-S-H) and calcium silicate hydrate (C-S-H) are formed within the geopolymer matrix. This leads to the formation of a compact and cohesive cementitious material, which showcases superior mechanical strength and a robust capacity for heavy metal sequestration. This research contributes to the safe management of MSWI fly ash by not only facilitating its non-hazardous disposal but also enabling the efficient containment and elimination of heavy metals, chloride ions, and elemental aluminum.