| Abstract: | Conditions for production of plasters are surveyed in relation to variations in phase composition, surface area and crystallite size. The commercially known α- and β-hemihydrate plasters prepared by autoclaving and dry calcination are not two distinct phases—they differ in crystal habit, surface area, crystallite size and lattice perfection. The terminal β-hemihydrate and γ-CaSO4 produced in vacuo have greater surface areas than samples produced in air. The most active γ-CaSO4 (Class C plaster) samples are finely porous and rapidly rehydrate in atmospheric water vapour with considerable loss of surface area and porosity. ‘Wet’ rehydration (liquid water) or γ-CaSO4 causes more rapid development of the 6.01 Å (10.10) spacing, in accord with the c-direction elongation of the ‘α’-hemihydrate crystals prepared by autoclaving or precipitation from 50% HNO3. The porosity of the γ-CaSO4 diminishes considerably between 200° and 300°, but remains to a smaller extent up to 500° when the γ-CaSO4 has changed to β-CaSO4, anhydrite (Class D plaster). Anhydrite sintered extensively between 500° and 900° before decomposing to lime at temperatures up to 1400° (Estrich Gips plaster). Removal of the SO2 and O2 is facilitated by streams of nitrogen or carbon dioxide, which increases the decomposition rate. Calcination at temperatures below 1200° gives anhydrite and lime crystals of more comparable sizes, and avoids complete ‘dead-burning’ of the lime. |
