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Task 05

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Environmental impact of bio ash concrete

Leaching test can be conducted as tank test where a monolithic concrete block is tested for the leachabillity through the surfaces of the concrete. Leaching of heavy metals can also be measured in a column test where a column is filled with crushed concrete and then percolated with water or it can be measured in a batch test PrEn 14429 where crushed concrete is extracted with water at different pH-values. All leaching tests are designed as accelerated tests, i.e. from experiments lasting a few weeks or months, the long-term leaching is predicted.

Focus will be on the leaching of heavy metals but also the leaching of phosphor will be investigated.

In this task focus is put on the batch test where crushed concrete is extracted with water, and to a lesser extent on the tank test. Before leaching tests samples of concrete are exposed to carbon dioxide in order to accelerate the natural uptake of carbon dioxide.

The focus on the potential leaching from the concrete at the end of the lifecycle reflects the fact that concrete after demolition is going to be utilised as crushed materials for road construction or similar purposes. Currently, in Denmark 95% of all demolished concrete is used for road construction purposes. Even though the concrete recycling percentage may be lower in other parts of Europe, it is very likely that the rest Europe will obtain higher reuse rates of demolished concrete in the future.

A range of bio ash concrete mixtures will be mixed (the same mixtures will be used here as in Task 6). On these samples of concrete and on samples from the highway bridge already constructed with bio ash concrete, leaching tests will be carried out.

It is expected that even if the batch leaching tests are designed for the "end of life cycle" situation (demolished concrete) it will be possible on the basis of the test to recommend if there should be any restrictions to use of the bio ash concrete. This could be the case for e.g. concrete in contact with drinking water.

The batch leaching tests including the chemical analysis are carried out at the laboratories at the DTI. The tank leaching test and the evaluation of results are carried out by the Energy Research Centre of the Netherlands ECN, who has substantial experience with concrete leaching obtained from e.g. other EU-projects.

Progress Indicators – 1) Concrete specimens casts, 2) Leaching studies initiated, 3) Leaching studies completed.

Expected results – Documentation of the amount of leaching of heavy metals from various concrete types containing bio ash. The leaching expected to similar to leaching from traditional concrete, i.e. without any significant environmental implications. There is not a tremendous amount of experience with the leaching tests on concrete, as there has only been focus on this issue for a decade or so. Nevertheless, comparing leaching form bio ash concrete with that of traditional concrete will provide a good basis for evaluating the environmental impact from heavy metals’ leaching out of bio ash concrete.

Constraints – It will constrain the use of bio ash concrete if the leaching of heavy metals is higher than from traditional concrete. However, this information is valuable as it allow for the selection of bio ash concrete products that can be manufactured without being a liability to the environment.