Arbejdsrapport fra Miljøstyrelsen 38/2006 – Miljøvurdering af nye betontyper

Miljøvurdering af nye betontyper

Summary and conclusions

Industrial residues are utilized in modern concrete as a substitute for a part of the clinker content in the cement. Especially fly ash from coal-fired power plants and mikrosilica have been used. Ash from incinerating sludge from wastewater treatment plants has also been utilized as a substitute to cement in concrete.

This utilization of industrial residues could possibly give rise to environmental problems at the end of the lifecycle of the concrete. In order to shed light on this, a series of tests of leaching of heavy metals from a number of different types of concrete has been carried out. The following types of passive concretes for indoor use were tested : an ordinary concrete without any residues, a concrete with fly ash and mikrosilica, a concrete with fly ash, and a concrete with ash from sludge incineration. Furthermore, two types of concrete with sludge ash from the aggressive environmental class (for outdoor use) were included in the tests.

All samples of concrete were carbonised in an accelerated way under laboratory conditions. During carbonation the concrete takes up carbon dioxide from the air. This seems to have a great impact on the leaching of heavy metals from the concrete. To which extent this accelerated carbonation of concrete is representative for the real situation is not clear.

Both carbonized and non-carbonized samples of concrete were put through the batch leaching test CEN prEN 12457-3 with a liquid-to-solid ratio L/S of 2 l/kg. This method is prescribed in the Danish government Statutory Order on utilization of residues and soil for construction works no. 655 27/06/2000. The test is economically acceptable to carry out, and it is well suited to comparing different types and residues and polluted soils. Furthermore, in the above Statutory Order the Danish authorities have established a number of limit values for concentrations of heavy metals released in this precise test.

This investigation covers a very small number of concrete samples, and since the mechanism of leaching is obviously very complicated, it is difficult to draw any clear conclusions. It therefore must be recommended to carry out further investigations in order to make it possible to draw a more unambiguous conclusion.

The heavy metals in concrete behave very differently. Some metals are fixed in new and non-carbonised concrete (chromium, vanadium, and antimony), while others are fixed in old concrete (lead and barium). These findings are in line with other reports regarding leaching from concrete.

The results of the investigation indicate that, compared to ordinary concrete without such constituents, the new types of concrete containing fly ash, mikrosilica, and sludge ash do not exhibit increased leaching of heavy metals at the end of the lifecycle of the concrete.

A single sample of carbonized concrete with low strength, however, slightly exceeded the limit values given in the Danish government Statutory Order on utilization of residues and soil for construction works no. 655 27/06/2000. This should be investigated further.