The central objective is to develop low-CO₂ binders based on burned tailings while clarifying their role in cementitious and alkali-activated hardening reactions. Two complementary approaches are pursued. First, the clinker content in cement-based systems is to be significantly reduced through the incorporation of processed tailings, including the design of multicomponent systems. Second, clinker-free alkali-activated formulations are developed using burned tailings as precursor materials. In both cases, thermodynamic modelling and empirical mix design concepts are applied to preselect promising formulations, which are then experimentally validated and refined

A fundamental part of the project is understanding the temperature-dependent phase evolution of the burned tailings. Because the dumps experienced heterogeneous and partly uncontrolled high-temperature exposure, their phase composition is highly complex. Controlled calcination experiments on model mixtures and original parent materials are conducted to systematically study mineral transformations, amorphisation, recrystallisation, and non-equilibrium phase formation. Thermodynamic high-temperature modelling (FactSage) supports interpretation by simulating phase stability and melting behaviour. The goal is to link mineralogical composition with pozzolanic reactivity and to identify indicators for reactive versus inert fractions

Parallel to the fundamental investigations, representative tailings dumps from different coal regions are screened and characterised. Drill cores are taken to assess chemical, mineralogical, and physical heterogeneity throughout the dump bodies. Reactivity testing (ASTM C1897-20), XRD, XRF, and particle characterisation are used to correlate composition and fineness with performance. Based on these findings, targeted processing strategies are developed to enrich reactive components. Both wet and dry separation techniques—such as jigging, optical sorting, and combined X-ray/optical sorting—are evaluated. The aim is to establish a technically and economically viable pilot-scale preparation route for producing reactive binder components while minimising environmental impact

The processed materials are subsequently integrated into binder systems. In cementitious systems, thermodynamic modelling (GEMS) is used to predict phase assemblages and hydration products, especially C-(A)-S-H formation. Experimental validation includes XRD, TGA, calorimetry, NMR investigations of pore structure, and compressive strength testing up to 90 days. In alkali-activated systems, aluminium and silicon solubility under highly alkaline conditions is examined, and empirical strength prediction models are adapted to the burned tailings. Microstructural analyses (MAS NMR, FT-IR, Raman spectroscopy, H-NMR) provide insight into gel formation and water incorporation

Overall, the project combines mineralogical analysis, thermodynamic modelling, processing technology, and binder development to enable the efficient utilisation of burned coal mining dumps as secondary mineral resources, contributing to CO₂ reduction and circular material strategies in construction.

Applicants: Vollpracht & Vraetz, RWTH Aachen