Recycled use of municipal solid waste incinerator fly ash and ferronickel slag for eco-friendly mortar through geopolymer technology

Due to the rapid urbanization and increased population, the production of municipal solid waste incineration fly ash (MSWI-FA) has become enormous and attracted attention worldwide. This study feasibly utilizes MSWI-FA as an immobilization host to co-dispose of ferronickel slag (FNS) through geopolymer technology. Due to the high content of heavy metals in MSWI-FA and FNS, the leachate of the geopolymer was tested with two leaching methods, in terms of acetic acid buffer solution method (AAM) and sulphuric acid & nitric acid method (SNM). Results showed that the leaching concentrations of heavy metal ions in geopolymers were less than the upper limit of Chinese landfill standard. For instance, the value of Cu, Zn, and Cr leachate in geopolymers are far lower than 100, 100, and 5 mg/L. Furthermore, various microstructural tests were performed to evaluate the stabilization/solidification behavior of MSWI-FA and FNS geopolymer, including electrochemical impedance spectroscopy (EIS) test, scanning electron microscope (SEM)/energy dispersive spectrometer (EDS), X-ray diffraction (XRD) test, and mercury intrusion porosimeter (MIP) test. The EIS results showed that geopolymer with the combination of 60 wt% MSWI-FA, 20 wt% ground granulated blast furnace slag (GGBS), and 20 wt% silica fume (SF) exhibited a compact and dense microstructure, which was beneficial to the physical encapsulation of heavy metal ions. The MIP result also confirmed that the capillary pore structure of geopolymer is also developed. The SEM, EDS, and XRD results showed that the C-A-S-H, Friedel's salts, and NaCl were the main hydration products in geopolymers. The C-A-S-H was capable contributed to improve the physical encapsulation of heavy metal ions. Friedel's salts were observed to adsorb heavy metal ions on its surface and had the good binding capacity of heavy metal ions. Based on these findings, the feasibility of geopolymers are confirmed, which promote the co-disposal of MSWI-FA and FNS and provide valuable information on the MSWI-FA based construction materials.