Unsustainable consumption and production patterns, together with industrialization and population growth, have increased the generation of municipal solid waste (MSW), causing several environmental problems. Current global MSW generation levels are approximately 1.3 billion tonnes per year, and are expected to increase to approximately 2.2 billion tonnes per year by 2025 [1]. In this sense, waste management is one of the most complex environmental challenges faced by modern societies. The European Waste Framework Directive (WFD) [3] is promoting a more sustainable performance, reducing waste generation and connecting resource use and waste residuals. WFD sets waste prevention, preparation for reuse and waste recycling as priority strategies. Nevertheless, recycling and reuse are not always suitable for all waste streams; and thus, a great amount of MSW ends up in landfills and waste-to-energy (WtE) plants, considered by European policies as last-resort options. WtE plants are one of the most robust waste treatment, which not only reduces waste volume, but also allows efficient recovery of energy [2]. However, incineration results in various types of solid wastes, primarily including bottom, boiler and fly ashes [3]. Fly ashes (FAs) represent only a few percent of the input waste. They are fine and are normally characterised by a high content of chlorides and significant amounts of dangerous substances, such as heavy metals or organic compounds. Due to its composition, FA must be treated in order to reduce its toxicity and to avoid negative impacts on the environment and human health. The application of a certain treatment may have two approaches: ensure landfilling of FA in non-hazardous landfills or improve the possibilities of its valorisation [4]. Regarding the former, FA are often treated by means of separation processes, stabilisation/solidification (S/S), thermal treatments or combined treatments. Among S/S alternatives, stabilisation with cement and carbonation are one of the most popular.
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