Formation of sodium iron silicate hydrate (N-F-S-H) in Fe-rich cementitious systems

Na 2 O-Fe 2 O 3 -SiO 2 -H 2 O samples with initial bulk Fe/Si molar ratios of 0.1, 0.3, 0.5, 0.8, and 1.0 were produced by mixing high-purity FeCl₃·6H₂O, and NaOH with fumed silica and water. They were equilibrated at a pH between 10.7 and 13.2 at 25 and 50 °C. Phase evolution was investigated over 1 year at 25 °C and 6 months at 50 °C using XRD, FTIR, SEM-EDS and ICP-OES. At 25 °C, sodium iron silicate hydrate (N-F-S-H) initially formed, with FTIR confirming the destabilisation of siliceous ferrihydrite and no evidence of crystalline Fe hydroxides such as goethite ( α- FeO(OH)). At 50 °C, nontronite (Na x (Si 4- x Fe x 3+ )Fe 2 3+ O 10 (OH) 2 ) with 0.5 < x < 1.35 appeared after 180 days of equilibration. The progressive development of characteristic nontronite reflections (at 33 and 41° 2-theta) from the initial N-F-S-H matrix suggests that N-F-S-H acts as a poorly crystalline precursor and evolves toward nontronite. The results provide new insight into the stability of Fe-bearing solid phases in alkaline environments and have implications for the use of Fe-rich materials in civil engineering.