Long-term stability in landfills of Municipal Solid Waste Incineration fly ashes solidified/stabilized by hydraulic binders

In order to understand the long-term behavior of pollutants in the solidified/stabilized (S/S) Municipal Solid Waste Incineration (MSWI) fly ashes using cement, raw MSWI fly ashes, as well as aged and fresh S/S samples were studied. Materials were examined through chemical analyses, leaching and permeability tests, petrographical observations, X-ray diffraction, EDS-based TEM and SEM. Drastically high amounts of Cl− and Pb are leached from the solidified/stabilized MSWI fly ashes, due to both the presence of soluble minerals and a network of micro-cracks leading to a very low cohesion and an impermeability loss of the material. Beyond portlandite and calcite, the main major minerals identified in aged S/S MSWI fly ashes are hydrocalumite, ettringite and ordered silicate-hydrates mostly free of heavy metals. The ordered silicate-hydrates are missing in fresh samples. The large variety of heavy metal-bearing compounds is distributed between inherited stable phases (metallic alloys, spinels, glasses, refractory oxides, titanates, phosphates, etc.) and inherited or neo-formed unstable phases (amphoteric metals, hydroxides, carbonates, chlorides, chromates, etc.). Combining the results from major phase identification and heavy metal speciation, it emerges that Zn and partly Cr are trapped in stable compounds whereas Cl−, SO42− and Pb mineral species are characterized by an instability which is not enhanced by maturation. The high Cl− activity and the high alkali conditions play a critical role in Pb-forming minerals and, more generally, the evolution of major mineral phases and the differentiation of heavy metal-bearing species in huge monolith are governed by an exposure to temperatures close to 100 °C for several years.