Reusability Potential of Selected Soil Types as Supplementary Cementitious Material: Case Studies with Particular Regard to Tunnel Excavation Material

Economic development is coupled with an ever-increasing demand for primary mineral resources. In this context, recycling of soil has gained importance. Intense research efforts have shown that calcined clays as supplementary cementitious materials (SCMs) meet the requirements to represent a viable solution in mitigating CO2 emissions related to cement production. Efficient use and recycling of mineral raw materials can thus significantly reduce the demand for primary mineral resources and improve resource efficiency. In the framework of the sustainable product policy in the EU, tunnel construction projects represent a promising feature for recycling of materials in production instead of using primary resources, as large amounts of excavated soil accumulate locally and over short time periods. In soft ground, tunnels are usually built with tunnel boring machines (TBM) with liquid or earth pressure supporting the tunnel face. During the process excavated soil often needs conditioning, TBM muck thus may need to be processed prior to reuse or disposal. Excavated material can be subdivided into material classes suitable for raw material substitution, for use in earthworks and traffic route construction, or as material for landscape structures. The aim of recycling should always be to use soil as a building material in the most ecologically sensible way possible. The focus of the present study is (1) on the determination of geotechnical properties of raw materials (i.e. clayey soils) from large infrastructure construction projects, and (2) the suitability of the processed material as a starting material for binding agents in mineral building materials. Geotechnical index tests of raw soils comprised determination of soil-mechanic parameters like water binding capacity, particle size distribution, Atterberg Limits and organic content. Characteristic crystalline mineral phases of raw soils and their calcined products was conducted via XRD coupled with Rietveld refinements. A diverse suite of clayey soils defined by disparate clay mineralogy was investigated. Calcination temperatures ranged from 650-850°C, with times of temperature exposure varying between 1-4h. The experimental strategy comprised exploration of reactivity of calcined clays in combination with low-carbon Portland-composite cement CEM II/C-M (S-LL) 42.5N as reference. The reference cement was replaced by calcined clays in supplementary proportions ranging from 10-50% by mass, at a constant water/binder ratio of 0.5. Commercial Centrilit NC powder was used as a standard for metakaolin. To rule out filler effects, powdered limestone was used as a secondary standard. Evaluation of pozzolanic activity was conducted via determining flexural and compressional strength. Results show that in terms of strength development, blended mortar mixtures with cement replacement rates of up to 20% reach equal strength or even exceed that of CEM II/C-M (SLL) 42.5N, proving that calcined clays can significantly contribute towards the eco-efficient transition of the cement industry.