As a result of their chemical and mineralogical characteristics, bottom ashes from municipal solid waste incinerators can, in principle, be used as aggregates in the production of normal strength concrete. However, because the ashes contain concrete-damaging components, such as chlorides, sulphates and organic compounds, or excessive quantities of fines, aluminium and waste glass, recycling becomes problematic. In particular, inclusions of aluminium in the ash particles and a glass content of about 15% cause considerable cracks and spalling in concrete specimens within a very short time. The harmful substances can be reduced or removed by additional treatments, such as upstream sieving and washing, waste glass separation, and lye treatment with sodium hydroxide solution. Tests on concretes with 2–32 mm bottom ash as coarse aggregates indicate that the quality of the ash is actually improved by the additional processing. Thus concretes with a compressive strength of C20/25 can easily be produced. Similar to concretes made with recycled aggregates, these concretes exhibit 15% lower compressive strength and E-modulus but twice the porosity of control specimens containing exclusively natural sand and gravel. However, only those concretes that were made with ash with a low aluminium content as a result of lye treatment remained free of damage.

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