Optimizing Fe(II)/Fe(III)-bearing magnesium silicate glasses for applications in supplementary cementitious materials

This study investigates the potential of Fe-bearing magnesium silicate glasses as alternative supplementary cementitious materials (SCMs) for reducing CO ₂ emissions in Portland cement production, given the limited supply of traditional SCMs. The glass powders were synthesized using a low-temperature sol-gel process and ascorbic acid (AA) as a reducing agent. This route enables controlled partial reduction of Fe ³⁺ to Fe ^{2 +} and stabilizes a fraction of the reduced species during heat treatment, without requiring strongly reducing gases (e.g., H ₂ or CO). The reactivity of the glasses was evaluated through batch dissolution tests revealing significantly higher elemental solubility for the glasses synthesized with AA compared to those produced without additives. By combining dissolution experiments with ⁵⁷Fe Mössbauer spectroscopy and XPS, we establish a direct link between Fe redox state/coordination (Fe ²⁺ and tetrahedral vs. octahedral Fe ³⁺), silicate network depolymerization, and glass reactivity relevant for SCM use. The higher reactivity is particularly pronounced at low Fe concentrations, attributed to the reduction of Fe ³⁺ to Fe ²⁺, which acts as a network modifier and enhances the solubility. While the solubility slightly decreases with higher Fe concentrations among AA-assisted glasses, it remains remarkably higher than in glasses synthesized without AA. This is attributed to a greater proportion of octahedral Fe ³⁺, facilitated by chelation between AA and Fe. Tetrahedral Fe ³⁺ acts as a network former, whereas octahedral Fe ³⁺ acts as a network modifier that depolymerizes the silicate network and enhances elemental solubility. Overall, AA-assisted glasses with intermediate Fe contents (Fe/(Fe + Mg) = 7–15 mol%) exhibit the highest reactivity, attributable to their higher Fe ²⁺ fraction in the network, making them the most promising candidates for low-CO ₂ cement applications.