3. Results and discussion
3.1. Investigation of behavior of raw meal upon thermal treatment
The HT-XRD plot in Fig. 2(a) shows the changes in phase composition during the thermal treatment of raw meal. The shift in quartz diffraction is caused by the transformation of low-quartz (α) to high-quartz (β) at the temperature of 575 °C. This reversible displacive (athermal) transition is deeply discussed in the following literature [73], [74] and [75]. The diffraction lines of strontium carbonate disappear within the temperature range from 775 to 850 °C. In the same temperature interval the maximum intensity of diffraction lines of strontium oxide was reached. The amount of SrO then decreases with the formation of tristrontium aluminate (Sr3Al2O5). With increasing temperature, tristrontium aluminate reacts with alumina to hexagonal strontium aluminate. The features of Sr3A and SrO disappear at the temperature of 1125 °C