Characterisation of ferri-silicates using selective dissolution

Sodium iron silicate (NFS) samples were synthesised at initial Fe/Si molar ratios of 0.3 and 0.8 and equilibrated at 50 °C for up to 180 days to elucidate reaction pathways in high-Fe alkaline environments. The resulting solid phases comprised siliceous ferrihydrite, sodium‑iron-silicate-hydrate (N-F-S-H) and nontronite. N-F-S-H was isolated via selective dissolution using citrate-bicarbonate-dithionite (CBD). Its features include a broad X-ray diffraction (XRD) hump centred at ~38° 2θ, a Si O Fe stretching band at 965 cm −1 in Fourier transform infrared spectroscopy (FTIR), and a mass-loss feature at ~130 °C corresponding to water release. Mass balance calculations identified the early-age (7 and 28 days) N-F-S-H composition as (Na 2 O) 0.3 (Fe 2 O 3 ) 0.4–0.5 (SiO 2 )·0.7 H 2 O. Identifying these secondary, poorly crystalline phases preceding nontronite formation advances the understanding of Fe Si reaction pathways. These insights are highly relevant for Fe-rich blended cements, nuclear waste disposal, and reinforced concrete.