Fe(II) interaction with cement phases: Method development, wet chemical studies and X-ray absorption spectroscopy

Area of Science:

• Geochemistry
• Materials Science
• Environmental Science

Background:

• Understanding iron(II) interactions with cement phases is crucial for predicting long-term material stability and environmental impact.
• Iron(II) readily oxidizes in alkaline cementitious environments, complicating its study.

Purpose of the Study:

• To investigate the interaction mechanisms of iron(II) with major cement phases, specifically calcium-silicate-hydrates (C-S-H) and AFm phases.
• To develop a methodology for stabilizing iron(II) in alkaline conditions for sorption experiments and spectroscopic analysis.
• To determine the binding sites and coordination environment of iron(II) within cementitious materials.

Main Methods:

• Co-precipitation and sorption experiments were employed to study iron(II) interaction with cement phases.
• X-ray absorption fine structure (XAFS) spectroscopy and X-ray diffraction (XRD) were used to characterize the iron(II) species and their binding locations.
• A novel methodology was developed to stabilize iron(II) against oxidation during experiments.

Main Results:

• X-ray diffraction (XRD) indicated that iron(II) is incorporated into the interlayer of calcium-silicate-hydrates (C-S-H), increasing interlayer spacing.
• Wet chemical experiments using a 55Fe radiotracer showed linear sorption of iron(II) on C-S-H, with distribution coefficients (Kd) significantly lower than for iron(III).
• XAFS spectroscopy revealed iron(II) binding within C-S-H in an octahedral coordination, suggesting structurally bound species rather than simple surface complexation, with evidence for both surface and interlayer binding.

Conclusions:

• Iron(II) is primarily incorporated into the interlayer of C-S-H phases in cementitious systems.
• The binding of iron(II) to C-S-H is structurally controlled and distinct from iron(III) interactions.
• The developed methodology enables reliable investigation of iron(II) behavior in alkaline cementitious environments.