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The European Union is currently aiming towards a circular economy. This economic model focuses on reusing materials and creating added value with recycled products using smart solutions. In this research, such an advanced approach was also chosen. Recovered municipal solid waste incineration bottom ash (MSWI BA) was used to create aggregate-like products for civil engineering structures. The aim was to potentially increase the value and the image of this waste-derived aggregate in civil engineering applications. The use of waste-derived aggregates in civil engineering is especially attractive and beneficial. Natural and crushed rock aggregates are scarce in many locations and vast amounts of materials are required in different types of structures. Conversely, these drivers can also cause drawbacks in the appropriate use of waste-derived aggregates. For example, building highway noise barriers from such aggregates can hardly be considered as utilization; in fact, it is merely dumping the potentially high-value material from one site to another, and thus, avoiding high waste tax costs for landfilling. In this study, the aggregate-like products from recovered MSWI BA were therefore designed as a replacement of natural and crushed rock aggregates in the structural layers of road and field structures. In such structures, the costs of natural and crushed rock aggregates are generally higher than, for example, those used in the noise barriers, and therefore, added value from reusing this waste-derived aggregate could be gained.Europe produces approximately 20 million tonnes of MSWI BA every year. Untreated MSWI BA contains large amounts of both non-ferrous (NF) and ferrous (F) metals. As a result, the development for metal recovering technologies has been of interest to many parties for the past decade. In this study, an advanced Dutch dry treatment technology called ADR (Advance Dry Recovery) was used to treat approximately 60.000 tonnes of MSWI BA from one waste incineration plant in Finland between the years 2013 - 2014. The treatment process efficiently separates NF and F metals from MSWI BA, generating 75 – 85 % of mineral fractions in different grain sizes (0-2, 2-5, 5-12 and 12-50 mm). These mineral fractions were first characterized thoroughly based on their technical and environmental properties. The aim was to properly understand what type of materials were generated from the process. Thereafter, different material mixtures were designed from these mineral fractions using the mathematical proportioning of aggregates. The aim was to create aggregate-like products for different structural layers (filtration, subbase and base layers) of, for example, road and field structures. Three products were designed, which were considered the most well suited based on their correspondence to the grain size distribution requirements of respective natural and crushed rock aggregates. These products were further analysed from their technical, mechanical and environmental points of view in the laboratory. The aim of these analyses was to understand the possibly unique material properties (e.g., sensitivity to changes in moisture content) that can affect the usability and constructability of these aggregate-like products in civil engineering. Finally, the laboratory findings were verified with a field performance study in which an interim storage field was built within a waste treatment centre using these aggregate-like products designed from recovered MSWI BA.Based on the technical and mechanical properties of the material, the aggregate-like products from recovered MSWI BA were considered the most suitable to be used in the lower structural layers of road and field structures. For base layers, the material cannot be recommended unless an additional base layer of natural aggregate or alternatively thicker asphalt pavement is constructed on top. The main reason for this is that MSWI BA particles are prone to crushing and most likely unable to resist the high stresses occurring in the upper parts of road and field structures. This study also demonstrated that the stiffness and strength properties of recovered MSWI BA were strongly dependent on the material’s aging and the changes in its moisture content, especially when the material dries out. For example, in this study the resilient modulus (Mr) was even quadrupled when the material’s moisture content decreased 5-7 %. With respect to the environmental properties of ADR recovered MSWI BA, the leaching of antimony (Sb) and chloride (Cl-) were identified as the main possible hindrances to the utilization of this material in civil engineering in Finland. On the other hand, the leaching behaviour of these substances showed consistency between the laboratory experiments and the field studies. These results therefore supported the reliability of laboratory leaching test results on which the utilization decisions in real construction projects are generally based.In general, the findings of this study indicated that the aggregate-like products designed from recovered MSWI bottom ash can be a valuable replacement of natural aggregates in certain structures as long as their unique properties are taken into account. When properly used, the attractiveness of replacing natural aggregates with the recovered MSWI BA can be increased, and the dumping of this material in landfill sites and noise barriers can be decreased. Finally, this study has also provided important background data for the finnish policymakers in order for them to decide that recovered MSWI BA can be added to the scope of a renewed Government Decree (YM14/400/2016). This Decree will further facilitate the use of waste-derived aggregates such as recovered MSWI BA in civil engineering in Finland. |
