A study published in the Quarterly Journal of Engineering Geology and Hydrogeology this week outlines ways in which household waste can be recycled by using the products of its incineration as an aggregate for concrete.

Incineration is already a popular method of waste disposal. Although it reduces the volume of waste by around 95%, the products are still often disposed of in landfill sites, so finding alternative uses for the products of incineration is vital to reducing landfill and promoting recycling.

The study, carried out at the BAM Federal Institute for Materials Research and Testing in Berlin, used material from a modern waste-to-energy plant in Hamburg. One of the main products of incineration is bottom ash, the non-combustible material which is left behind in the furnace. This is already often reused in coarse form to construct roads, but has previously contained too many impurities to be strong enough for use in concrete, limiting the recycling opportunities. The study outlines various ways in which the quality of the ash can be improved so that it can be used to make concrete; a development which would both save natural resources and reduce the amount of waste destined for landfill.

Although bottom ashes are, in theory, suitable for use as aggregate in normal strength concrete, in reality they contain impurities that damage concrete. The most harmful substances are aluminium and glass, which cause cracks in a very short space of time. The researchers, led by Dr. Katrin Rübner, used several different methods for improving the ash properties. To remove glass components, transparent particles were automatically detected and removed mechanically, while storage in sodium hydroxide solution and subsequent washing, known as lye treatment, reduced the aluminium content to less than 0.4%.

The results were tested by producing concretes where the usual aggregates were replaced by bottom ashes treated in various ways. The engineering properties of the concrete, such as workability, compressive strength, elasticity and porosity, were tested, demonstrating that treating the bottom ash improves the quality of the concrete produced.