Effect of Fe3+ on the precipitation of synthetic calcium silicate hydrates (C-S-H) under controlled conditions has been evaluated. Using extremely basic either initial calcium nitrate or sodium silicate solutions (pH > 11), incipient formation of ferrihydrite in both solutions was observed. In this case both C-S-H and iron-containing siliceous hydrogarnet are formed. For pH < 11, ferrihydrite was absent. Only C-S-H is formed. The presence of magnetically dilute Fe3+ in the C-S-H was shown by EPR and the atomic Fe/Si ratio of the C-S-H phase was shown to be between 0.01% and 0.1% by measuring and modelling the 29Si spin-lattice relaxation in NMR experiments. A uniform distribution of Fe3+ when C-S-H precipitates in the absence of initial ferrihydrite is obtained. When initial ferrihydrite is present, a non-uniform distribution of Fe3+ is found and the concentration of Fe3+ in the C-S-H is merely 3% of the overall iron concentration, suggesting the initial formation of ferrihydrite restricts the incorporation of iron into C-S-H.