Batch leaching tests have been performed on two alkaline fly ashes collected from Australian power stations fuelled by chemically different coals. Fly ashes with a pH of 10.22 and 12.03 have been leached by solutions with initial pH values of 4, 7 and 10, to assess the effect of pH on leaching rates and mobilisation of chemical elements from the solid phase. During the leaching tests the pH of all solutions approached the pH values of the fly ash when wet with water alone. Hydrogeochemical modelling has been applied to obtain information about precipitation of solid phases formed during the leaching process. Ca and Sr carbonates, calcite and strontianite, seem to be the most important carbonate phases precipitated out from the solutions. No other metal carbonates have been found by modelling to precipitate from the leaching solutions. Some metals are expected to be precipitated as oxides and hydroxides. Several mineral phases may also precipitate, removing Ca and Na. Among the sulphates only barite and ettringite showed supersaturation. Formation of ettringite as a secondary mineral from alkaline fly ashes has been evaluated by hydrochemical methods. An application of hydrogeochemical modelling using PHREEQC software shows that ettringite is supersaturated, and theoretically can precipitate out from alkaline solutions derived from alkaline fly ashes. Knowledge of the formation of ettringite is important, as this mineral can incorporate into its structure oxyanions including B(OH)_4~-, SeO_4~(2-), AsO_4~(3-), CrO_4~(2-) and MoO_4~(2-) causing their immobilisation from fly ash. Ettringite formation, indicated by hydrogeochemical modelling, may therefore explain progressive decreases in the concentration of B, Mo, As and Se in alkaline leaching solutions, through substitution of relevant oxyanions in the ettringite structure. All these elements are toxic to various degrees in aquatic environments, and an understanding of ettringite formation is potentially significant in ash management programs.
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