Cement recycling – Sustainability at the interface between climate and resource protection

The production of concrete or its primary component cement, is associated with substantial carbon emissions and high consumption of resources. To reduce these impacts, various solutions have been introduced in the last few years, such as innovative ternary composite cements or recycled concretes (RC), which make a significant contribution to climate and resource protection. However, climate and resource protection are often contradictory, because RC concretes, for instance, generally have a slightly higher cement content than normal concretes (in particular, if the latter ones are manufactured with fluvial round aggregates), with possibly increasing the specific carbon emissions. Notwithstanding the fact that processed aggregates made from concrete demolition waste can substitute natural aggregates to a large extent and thus making an important contribution to the protection of resources. So far, however, in the production of recycled aggregates from concrete debris up to 30 wt.-% of the demolition material are left over as crushed sand or powder with grain sizes smaller than 2 mm. This fraction, however, has a large potential to be used either as carbon sink through fast carbonization or as a carbon free source of CaO for the production of new cements. This is investigated in the URBAN joint project that is funded by the German Federal Ministry for Economic Affairs and Climate Action. Head of the consortium is the Institute for Technical Chemistry at the Karlsruhe Institute of Technology. Other partners involved are the Leibniz University of Hannover, Holcim (Deutschland) GmbH, Sika AG and Ehl AG.

The aim of the URBAN research consortium is to use new cement clinker manufactured of crushed concrete sands as raw meal constituent for the production of RC concrete, thus substituting normal cements to some extent. For this purpose, crushed sands, in particular from returns and/or from the recycling in the precast concrete and concrete goods industry, are ground to raw meal. This raw material is calcined to belite cement clinker in a newly developed low-temperature burning process. The method has already been used very successfully for the recycling of autoclaved aerated concrete and is now expanded to crushed concrete sand. The developed technology allows for significant reductions of carbon emissions due to lower process- and energy-based emissions and the possibility to upgrade recycled materials. Moreover, there is a process-related accumulation of carbon dioxide in a concentrated form, facilitating the combination with technologies used for carbonization. The low-temperature belite clinker produced was proportionally used as substitute for conventional Portland cement clinker and the influence on the rheological and mechanical characteristics of concrete were investigated. The findings reveal a promising reduction of the carbon dioxide and resource consumption in combination with promising technical characteristics of the concrete, thus enabling a closed loop of concrete that is ideally carbon dioxide neutral, in the near future.