To design a sustainable binder, it is reasonable to utilize secondary raw material streams effectively. Steel slags are available in large quantities worldwide and are currently landfilled or used as aggregate even though their composition indicates hydraulic capabilities. Steel slags contain large amounts of C2S, which is one of the major hydraulic phases in Portland cement. For reasons that are still unclear, the C2S in slags shows a reactivity far below expectations. In addition, delayed setting may occur when used in combination with Portland cement. The aim of the project is to produce a clinker-free or ultra-low clinker binder with low CO2 emissions from steel slags. To achieve this aim, various European steel slags will first be characterised using newly adopted methods such as for example electron backscatter diffraction (EBSD) in the SEM. Special focus is laid on the chemical-crystallographic characterisation of C2S. In addition, the phase assemblage of slags is reconstructed using thermodynamic modelling. Subsequently, the hydration behaviour of the slags is investigated. The aim is to reveal the cause of the low reactivity of the C2S in slags and the cause of slag retardation effects on Portland cement. Therefore, the slag hydration behavior is analysed and modelled. Furthermore, the reactivity of synthesised C2S with varying content of minor elements and in combination with the slag pore solution is investigated. These results will help to elucidate the key factors for hindered C2S and slag reactivity. The hydration behaviour is monitored by various analytical methods and thermodynamically modelled. With these findings, several strategies are then pursued to improve the reactivity of the slags. The final goal is to produce a (almost) CO2-free prototype binder from steel slag.

Applicants: Ludwig, BUW