Reuse of concrete waste: Challenge with hexavalent chromium (Cr6+)

The European Commission's Circular Economy Action Plan includes a legislative proposal on waste and provides long-term targets to reduce landfilling and increase recycling and reuse. One of the goals set out in the Action Plan... [ view full abstract ]

The European Commission's Circular Economy Action Plan includes a legislative proposal on waste and provides long-term targets to reduce landfilling and increase recycling and reuse. One of the goals set out in the Action Plan is to achieve a 70 % reuse of materials in building and construction projects. Concrete is a large fraction related to demolition waste from building and construction projects. Together with brick waste, concrete makes up approximately 13 % of the waste disposed off at Norwegian landfills.

To reduce the amount of concrete waste to landfills, the Norwegian Environmental Authorities have worked out guidelines for reuse of this waste fraction. The guideline includes threshold values (concentrations in solid samples) for chromium (total) and the much more toxic hexavalent chromium (Cr⁶⁺). When analyzing concrete waste, the Cr⁶⁺ value is often exceeded, and the concrete has been disposed off at a landfill rather than being reused.

At high pH, which is the case in concrete, chromium may occur in the hexavalent form. But knowledge about the fate of Cr⁶⁺ after release from concrete in contact with water and further environmental behavior is scarce. In addition, the chemical analysis of Cr⁶⁺ is challenging, where different standard methods for solid samples, give large variation in the results. This makes the decision of reuse versus landfilling somewhat haphazard, and we see that the lack of knowledge is preventing reuse of concrete. In cooperation with the Norwegian environmental Authorities, NGI is currently carrying out a project to investigate the environmental fate and speciation of Cr6+ released from concrete waste under realistic condition for reuse. Leaching tests with different types of soils are carried out to investigate the effect of total organic carbon (TOC) and pH. Furthermore, kinetic tests are included to study the oxidation rate of Cr⁶⁺ to the less toxic Cr³⁺. Analytical issues as well as a review of the chemical content of “clean” concrete from cement producers are also included in the work. Results from the study will be presented at the WASCON conference.