Influence of the eluent on the leaching of heavy metals from concrete

• Applicant: Professorin Dr.-Ing. Anya Vollpracht
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: from 2014 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 254458234

The environmental compatibility of building materials during service life and recycling is becoming increasingly important. Cementitious building materials contribute to the input of trace elements into soil and groundwater via leaching. Numerous research projects dealing with this subject have been carried out in Germany and the Netherlands during the last 20 years. In both countries the findings were used to develop evaluation concepts for the leaching of concrete. The basis of the evaluation are in both countries leaching tests which are performed with deionised water. The influence of substances which are dissolved in the eluent and of changing pH-values is not taken into account. Current research results show that the solvent attack on concrete which is caused by the deionised water has a significant effect on the leaching of trace elements. Therefore leaching tests with real groundwater are necessary to generate realistic predictions of the release of pollutants, but so far no test results are available. Since groundwaters differ widely concerning their constituents, different regions have been choosen whose groundwaters cover the ranges of the relevant parameters in Germany. With these groundwaters as well as with deionised water and defined test waters the leaching rates shall be determined as a function of the contact water.Due to its soluble components, its adsorption capacity and buffering capacity, the soil has an impact on the eluent and therefore on the leaching of concrete. This is not considered in the German evaluation concept. However it was assumed that the release into saturated soil is lower in case of pure water, because the amount of leachate is lower. Therefore the leaching rate was multiplied with the effective porosity of the soil. Meanwhile this assumption has been disproved using diffusion calculations. A proposal how to account for this aspect in the evaluation model in the future has not been developed. A sub-goal of this project is to develop a database and recommendations for the practical implementation.Other factors with influence on leaching are the temperature and in case of precast concrete ele-ments the carbonation of concrete prior to installation. These factors will be integrated in the testing programme.The aim of this project is to improve the knowledge of leaching mechanisms of concrete in contact to groundwater. Against the background of the current discussions on a new aggravation of the in-significance thresholds for groundwater and more stringent evaluation criteria probably resulting from that, it appears expedient and necessary to find a more realistic basis for the evaluation of the environmental compatibility of concrete than the leaching in deionised water.

• DFG Programme: Research Grants