Alkali-activation of CaO-FeO_x-SiO₂ slag: Formation mechanism from in-situ X-ray total scattering

The pursuit of low-CO₂ technologies has led to a surge in research on alternative cementitious materials, of which alkali-activated materials are a large family. In recent years alkali-activated materials have expanded to encompass Fe-rich precursors in addition to the more commonly employed aluminosilicate precursors. The formation mechanism of alkali-activated materials from two Fe-rich synthetic slags has been assessed by employing in-situ X-ray total scattering and subsequent pair distribution function analysis. The evolution of the local atom-atom correlations reveals three reaction stages. After the dissolution of Fe-silicate clusters from the slag, a binder phase is formed with Fe in both Fe²⁺ and Fe³⁺ oxidation states. The Fe²⁺ state is present in the form of trioctahedral layers, similar to those in Fe(OH)₂, while the Fe³⁺ is likely located in the polymerized silicate network. Exposure to air causes the Fe²⁺ species to transition to the Fe³⁺ state.