Characterisation of iron inclusion during the formation of calcium sulfoaluminate phase

The iron distribution among the sulfoaluminate clinker phases and its ability to enter the calcium sulfoaluminate lattice in solid solution can have a significant influence on manufacturing process and reactivity of calcium sulfoaluminate (CSA) cements. X-ray diffraction (XRD) analysis, Mössbauer spectroscopy, scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analysis system (EDAX) and infrared spectroscopy were used to identify the mineralogical conditions of iron inclusion during the formation of calcium sulfoaluminate (C4A3S) phase from different mixtures in the CaO–Al2O3–Fe2O3–SO3 system. The mixtures, heated in a laboratory electric oven, contained stoichiometric amounts of reagent grade CaCO3, Al2O3, Fe2O3 and CaSO4·2H2O for the synthesis of Ca4Al(6 − 2x)Fe2xSO16, where x, comprised between 0 and 3, is the mole number of Al2O3 substituted by Fe2O3. With x increasing from 0 to 1.5, both the iron content of C4A3S phase and the amounts of side components such as C2F and CS increased. For x values included in the range of 1.5–3.0, at temperatures higher than 1200 °C, melting phenomena were observed and, instead of the C4A3S solid solution, ferritic phases and anhydrite were formed.