Suitability of Soil and Tunnel Excavations as SCM - Comparison of Thermal and Mechano-Chemical Activation

Construction and demolition waste represents one of the largest waste streams in the European Union. The soil and stone fraction accounts for the biggest share of these materials. This fraction includes waste materials from tunnelling and excavations, which often contain significant amounts of clay minerals. The demand for raw materials containing clays will increase significantly due to the growing interest for calcined clay as emerging SCM. As the extraction of primary raw materials is to be reduced on the way to a circular economy, secondary materials containing clay must be tapped and new processing methods developed. In this study, different samples from drill cores of planned tunnelling operations and soil excavations were characterized with focus on their mineralogical composition. All investigated materials show a high carbonate and a low to moderate clay content, dominated by 2:1 and 2:1:1 clay minerals. The samples were activated by thermal and mechanochemical activation and the alteration of the physical and mineralogical properties was analysed. The effects of mechanochemical activation on particle characteristics and crystal structures known from studies of pure clays could be confirmed. Mechanochemical activation was proven to significantly enhance the pozzolanic reactivity of a material with a clay content of 40 wt. %, while it showed no benefit regarding the activation of materials with clay contents of 20 wt. % or below. The study demonstrated that mechanochemical activation can have great potential as an alternative activation method for secondary clays, while the scale-up and associated energy demand still need to be proved.