From Excavated Material to Construction Material: Research Project Investigates How Soil and Rock Could Become a Sustainable Cement Substitute

According to STUVA statistics, tunnel construction projects in Germany alone generate more than four million tonnes of excavated material every year. While hard rock can be reused, for example as aggregate in concrete, the utilisation of cohesive soils such as silts and clays is considerably more challenging. To develop a sustainable solution for these materials as well, the research project is focusing on the process of calcination. In this process, clay-rich material is dried, ground and heated in a kiln. This thermal treatment alters the structure of the clay minerals, making them reactive and capable of acting as a binder. The consortium, comprising the two research institutions STUVA e. V. and TH Köln, together with the three industry partners IMM Maidl & Maidl Beratende Ingenieure GmbH & Co. KG, Brameshuber + Uebachs Ingenieure GmbH and MC-Bauchemie Müller GmbH & Co. KG, combines expertise in construction materials technology, mineralogy, geotechnics, chemistry and civil engineering.

Two Pathways to Low-Carbon Binders

To unlock the potential of these calcined clays, the partners are pursuing two technological approaches. In low-clinker cement systems, the treated material is intended to replace a proportion of conventional cement clinker. Of particular interest is determining the percentage of clinker that can be substituted by calcined clay without compromising performance. As cement production is responsible for around eight per cent of global greenhouse gas emissions, according to the German Environment Agency, this substitution approach can significantly reduce the CO₂ footprint of concrete. At the same time, the consortium is also investigating entirely cement-free binders. In these systems, strength development is initiated by so-called activators, such as water glass (sodium silicate), a mineral-based compound. The result is an alternative binder that represents a particularly resource-efficient solution for the construction industry.

From Laboratory Samples to a Mobile Calciner

As part of the Geo-BAB project, various unconsolidated and hard rock materials from Germany’s low mountain ranges, metropolitan regions and major international construction sites are being investigated. The materials are first characterised and their strength development assessed under laboratory conditions. In parallel, the project is examining how the technology can be implemented in practice. For this purpose, a mobile calciner is planned to be deployed directly on site, enabling material processing at a technically relevant scale on construction projects. The potential applications of the new binders derived from tunnel excavation material are wide-ranging. They include use in tunnelling itself, for example in annular gap grouts between the tunnel lining and the surrounding ground. Other potential applications include concrete paving blocks for landscaping projects and ballast concrete for hydraulic engineering works.

About the Project

The collaborative project Geo-BAB (Calcination of Geological Secondary Raw Materials from Tunnel Construction for the Production of Alternative Binders) is being funded for a period of two years by the German Federal Highway Research Institute (BASt) under its Innovationsprogramm Straße (“Road Innovation Programme”), with funding of approximately €650,000.