Hydration behaviour of hydrogen-based DRI-EAF slags and their application as supplementary cementitious materials

The steel and cement industries are among the largest global sources of CO 2 emissions, and both must be addressed to meet the European Union’s legally binding net-zero target by 2050. While hydrogen-based direct reduced iron (H 2 -DRI) combined with electric arc furnace (EAF) steelmaking represents a low-carbon alternative to the Blast Furnace – Basic Oxygen Furnace (BF–BOF) route, the cement industry can benefit from the valorisation of the resulting slags through clinker replacement. However, these novel slags generated in the H 2 -DRI-EAF process require an in-depth characterisation to assess their potential valorisation paths, particularly as an alternative binder. This study investigates the hydration behaviour of H 2 -DRI–EAF (H 2 -DRI-EAFS) slags in cementitious systems, with emphasis on mineralogical and microstructural changes. Their suitability as supplementary cementitious materials (SCMs) was assessed by replacing 20% of Portland cement. Furthermore, nucleation-seeding strategies (i.e., C–S–H, ettringite and Al-sulphate additions) were used to mitigate the early strength loss. Results show that H 2 -DRI–EAF slags exhibit slow hydration kinetics. After 28 days, dicalcium silicates reacted progressively along with a limited dissolution of RO-phase, representing a major phase, yielding hydrotalcite–sjögrenite, katoite, C–S–H and minor portlandite. Mechanical tests confirmed their potential as SCMs, although a 16% decrease in compressive strength was observed at 28 days compared with Portland cement. Seed additions improved early hydration, enabling mechanical performance comparable to Portland cement pastes. This research provides critical insights into the hydration, phase assemblage and behaviour of H 2 -DRI–EAF slags in cement environments, highlighting opportunities and limitations for their further studies as SCMs. The findings could support strategies for integrating steel slags into cement systems, ultimately helping to reduce clinker demand and CO 2 emissions.