The fate of iron during hydration of white Portland cement-fayalitic slag

There is increasing interest in using Fe-rich materials in cement, yet the Fe speciation in hydrated cement paste remains poorly understood. This study quantitatively investigates the fate of amorphous Fe in hydrated white Portland cement-fayalitic slag pastes with varying Fe contents cured for 3, 28, and 90 days. The results show that Fe precipitated as ferrihydrite and potentially adsorbed on calcium (alumino)silicate hydrate (C-(A)-S-H) after 3 days of hydration. Afterwards, Fe stabilised in Fe-siliceous hydrogarnet and Fe-adsorbed C-(A)-S-H phases, accounting for ∼15% and ∼ 85% of total reacted Fe, respectively, after hydration for 90 days. The high Fe uptake by C-(A)-S-H was mainly attributed to Fe(III) adsorption rather than Fe(II). Thermodynamic modelling combined with microstructural analysis supported the predominant distribution of Fe on the C-(A)-S-H phase. These findings can advance understanding in using reactive Fe-containing materials in Portland cement pastes and the development of their chemical and physical properties.