Steel slags as the basis for CO2-neutral binders Slag to netzero – S2NET0

• Applicant: Professor Dr.-Ing. Horst-Michael Ludwig
• Subject Area: Architecture, Building and Construction History, Construction Research, Sustainable Building Technology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 542147582

It was identified that the way to net-zero concrete requires efficient usage of waste materials. 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 characterized using newly adopted methods such as for example electron backscatter diffraction (EBSD) in the SEM. Special focus is laid on the chemical-crystallographic characterization of C2S. In addition, the phase assemblage of slags is reconstructed using thermodynamic modelling. Subsequently, the hydration behavior 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 analyzed and modelled. Furthermore, the reactivity of synthesized 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 behavior is monitored by various analytical methods and thermodynamically modelled. With these findings, three strategies are then pursued to improve the reactivity of the slags: 1) By using complexing agents, which inactivate ions that inhibit hydration. 2) By carbonation of slags and variously doped C2S it is tested if the reactivity of carbonated slags is maybe better suited to use them as SCM. In addition, the carbonation of iron-containing phases (wüstite, C2F), which has been little considered so far, will be investigated. The idea is to bind reactivity reducing slag elements (Fe, Al, other metals) in carbonates. 3) Conditioning of slags during melting by targeted addition of oxides. The types and quantities of oxides added will be in a first step thermodynamically modelled. Reactivity of slags will be tested in alkaline environment measuring isothermal heat release curves. Finally, optimized slags will be compared by measuring reactivity with and without activators. This will reveal which of the slag optimization strategies (complexation agents, using carbonated slags or using conditioned slags) is best suited to increase performance of slags in mortars. Ultimately, mortar tests are carried out on optimized systems. The results of the work with contribute 1st and 3rd main objectives of the SPP call, namely: 1) pursuing a net-zero concrete based on alternative binders, and 3) carbonation strategies.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2436: Net-Zero Concrete