Study of the hydration products of the system CaO---Al2O3---SO3---SiO2 with varying CaO mole ratio

The hydrated phases of the system CaO---Al₂O₃---SO₃---SiO₂ were studied, and the effect of the presence of varying amounts of CaO on the hydrated phases was also investigated. These phases were assessed by the aid of differential thermal analysis (DTA) and X-ray diffractometry. They hydration kinetics of the pastes under study were followed by the determination of the chemically combined water.

Results revealed that 1 mole of CaO in excess reacts with the available SiO₂ and initiates the formation of sulphoaluminate hydrates. As the CaO is increased, the amount of such hydraulic phases is increased and, accordingly, the rate of hydration enhanced. The addition of 2 moles of CaO results in the formation of appreciable amounts of ettringite among the sulphoaluminate hydrates, but some anhydrous forms still exist. On the other hand, 4 moles give better formation of the hydraulic phases and some excess Ca(OH)₂ is also detected.