Fe(III) uptake by calcium silicate hydrates

Wet chemistry and spectroscopic investigations were conducted to study Fe(III) uptake by calcium silicate hydrate (C-S-H) at different Ca/Si ratios. Wet chemistry experiments were carried out by using a Fe-55 radiotracer while Si-29 NMR and XAS spectroscopy were performed on C-S-H phases loaded with different Fe(III) concentrations. Sorption kinetics experiments indicate that equilibrium was attained within 30 days. Over the studied concentration range, Fe(III) sorption was linear, irrespective of the difference in pH of the suspension and the Ca/Si of C-S-H. In addition, Fe(III) sorption on C-S-H phases was significantly stronger than Al(III) sorption. The total Fe(III) uptake by C-S-H phases, however, was limited by the lower solubility of Fe(OH)(3) compared to Al (OH)(3); up to 1 mol Fe/kg C-S-H (molar Fe(III)/Si approximate to 0.001) could be taken up. Si-29 NMR and EXAFS data suggest Fe(III) uptake in octahedral coordination into the interlayer of the C-S-H phases with Ca/Si ratios 1.2 and 1.5. However, such an uptake mechanism appears unlikely in the case of the C-S-H phase with Ca/Si 0.8 because structural parameters of Fe(III) deduced from EXAFS are different.