Some aspects of the hydrothermal calcination of gypsum

Abstract

Lump-size and powdered gypsum samples were calcined at 135, 150 and 180°C. The products were thoroughly investigated by differential thermal analysis and X-ray diffraction, and their chemically-combined water was determined thermogravimetrically. Results indicated that almost all the treated samples contained gypsum, hemihydrate and anhydrite in varying proportions. Gypsum and anhydrite increased, and the hemihydrate decreased as the heating temperature was increased and the grain size decreased. Such a variation in the phase constitution of the treated samples is discussed. The relative intensity of the main gypsum X-ray diffraction characteristic lines changes upon treatment. In the atmospheric calcination the relative intensities of the lines at 3.059, 2.867, 2.679, 2.59, 2.495, 2.45 and 2.40 Å show a pronounced decrease. Autoclaving, on the other hand, causes a detected decrease in the relative intensity of the line at 7.56 Å and an increase in the 4.27, 3.059, 2.59, 2.495 and 2.45 Å lines. These findings postulate that X-ray diffraction data could hardly be used for the quantitative assessment of the gypsum phase in the dehydration products of any calcined material.