Tetracalcium alumino-ferrite phase (C4AF) prepared from pure starting materials was employed for composing various mixes prepared of C4AF phase, CaSO4·2H2O, Ca(OH)2 and CaCO3. The effect of replacing calcium sulphate (gypsum) by calcium carbonate as a set retarder on the hydration behaviour of ferrite phase was studied. The mixes were hydrated for various periods and the hydration products were investigated using the appropriate techniques. The kinetics of hydration was studied by measuring the chemically-combined water as well as the combined lime contents. The mineralogical constitution was studied by using XRD, and DTA. The microstructure of some represented hydrated samples was investigated by scanning electron microscopy. Some interesting conclusions have been drawn. It was found that calcium carbonate reacts with tetracalcium alumino-ferrite phase (C4AF) in the presence of hydrolime [Ca(OH)2] to form carboferrite compounds which may coat the aluminate grains as ettringite does and this may probably regulate the setting time.