Morphological and chemical characterization of calcium-hydrate phases formed in alteration processes of deposited municipal solid waste incinerator bottom ash

During a study which investigates the exothermal heating of municipal solid waste incinerator bottom ash (MSWI) in a landfill samples were taken at a solid waste incinerator in southern Germany operating at ~1000 °C. The chemical and mineralogical bulk composition was determined by X-ray fluorescence (XRF) and X-ray diffraction (XRD). Single bottom ash particles were investigated by optical microscopy and scanning electron microscopy with quantitative energy-dispersive X-ray microanalysis (SEM/EDX). The fresh bottom ash consists of ash (42%), melting products (40%), metallic components (8%), usually aluminum, iron, and copper, and residual parts (10%). The main phase of the bottom ash is glass (~ 40%) with relics (e.g. quartz) and quench phases (e.g. gehlenite). The main crystalline phases are silicates (e.g. gehlenite, augite, diopside, quartz), oxides (e.g. magnetite, spinel, hematite), carbonates (e.g. calcite, metal-carbonates), and salts (e.g. chlorides and sulfides). In the deposited bottom ash endothermic and exothermic alteration processes are observed (dissolution/precipitation of salts, glass corrosion, hydration and oxidation reactions of metals, slaking of lime, cementation and carbonation processes). In the course of these processes new mineralogical phases are formed. Among these are e.g. anhydrite, portlandite, calcite, iron oxides and hydroxides, or gibbsite. These minerals are always accompanied by different calcium-hydrate phases with different mineralogical and chemical properties. The hydrate phases were morphologically and chemically characterized by SEM/EDX. The single crystals have a fibrous, ribbonlike, or tabular habit. Ca, Al, Si, and Fe were found as main components, with minoramounts of Na, K, Mg, Cu, Zn, Ti, Mn, and Cl, S, P.