Reactivity of lime and related oxides. XV. Crystalline forms of sulphated lime

Changes in surface area and crystallite size in the interconversions of the forms of sulphated lime have been related to time and temperature conditions. The specific surface increases when hemihydrate, ‘soluble’‐γ‐ and ‘anhydrite’‐β‐calcium sulphate are formed during the thermal decomposition of gypsum. The activation is initially delayed in each dehydration stage, before the products recrystallise to their normal lattice structures. Subsequently, the specific surface decreases (and the average crystallite size increases) as each product sinters. ‘Wet’ rehydration of γ‐calcium sulphate rapidly replaces the water in the zeolitic hemihydrate structure, without appreciably ageing it. The less mobile ‘dry’ rehydration in water vapour produces hemihydrate extensively aged. Both ‘wet’ and ‘dry’ conversions of hemihydrate to gypsum are completed with considerable loss of surface. The gypsum crystallinity is more rapidly developed during the ‘wet’ rehydration. Effects of additives on the rehydration and ageing of hemihydrate gypsum plasters have been similarly studied. Anhydrite ‘dry’ rehydrates only very slowly, but ages considerably. ‘Wet’ rehydration slowly produces gypsum of low crystallinity; the surface initially increases, before decreasing greatly as the rehydration accelerates. The rehydration rate finally falls as the material continues to age and improve in gypsum crystallinity.