Urban Mining in Luxembourg: Integrating Geology and Engineering for Reliable Recycled Aggregate Concrete

Increasing urbanisation and stricter environmental regulations have significantly restricted the exploitation of new gravel quarries as well as the local extraction of natural hard rocks and cement raw materials (lime, marl, clay), posing major challenges for the resource-intensive construction sector. In response, urban mining is gaining importance as a key strategy for circular construction. While natural aggregates from primary quarries provide well-established and consistent quality for concrete production, recycled aggregates (RA) and alternative cement raw materials derived from construction and demolition waste exhibit highly variable performance, strongly governed by source material characteristics and processing routes.

Luxembourg offers a particularly relevant case study due to its pronounced geological diversity and building heritage. The country is divided into the Palaeozoic Eisleck in the north, dominated by schistose rocks affected by Variscan deformation, and the Mesozoic Guttland in the south, characterised by an alternation of sandstones, limestones, dolomites, and marls with limited tectonic overprint. Most of these lithologies were historically used as local building stones, particularly in rubble stone masonry, which was constructed up to the early 20th century. As limestone and marl quarries supplying the cement industry become increasingly depleted or impossible to expand, construction and demolition waste from decommissioned buildings is becoming a significant secondary raw material source.

RA obtained through urban mining originates from highly heterogeneous feedstocks, including demolished concrete, manufactured masonry units, and natural rubble stone masonry. The suitability of rubble stone masonry for structural recycled aggregate concrete (RAC) depends on geological origin, mineralogical composition, the amount and properties of adhering mortar, and potential chemical pre-contamination, particularly by sulphates and chlorides. Porosity and pore-size distribution govern water absorption, workability, and strength development, while mineralogical factors such as alkali–silica reactivity critically affect durability. In addition, the presence of potentially toxic constituents may further limit reuse options.

This contribution presents an integrated geological–engineering approach for the evaluation of locally sourced RA. A material matrix for systematic lithological classification is proposed, linking geological characteristics with processing requirements and concrete performance. Adapted treatment chains - including selective demolition, targeted pre-sorting, and controlled crushing and screening - are identified as essential to ensure consistent RA quality.

Within the regulatory framework of EN 206, EN 206/DNA-LU, and EN 12620, the study demonstrates that properly processed rubble stone masonry can serve as a technically robust and normatively compliant raw material for RAC, supporting sustainable resource management through urban mining.