If the calcination process occurs under pressure in a 30% calcium chloride solution or in the presence of more than 1% of sodium succinate; type of gypsum product is called:
Manufacture of high strength (α modified) stone
The gypsum is calcined by boiling it in 30% calcium chloride solution. The chlorides are then washed away or autoclaved in presence of sodium succinate 0.5%. These particles are the densest of all three types. After controlled grinding, these powders have an even higher apparent density and yield a stronger set.
Microscopically, they are cuboidal in shape.
Phillips Science of Dental Materials ed 12 pg 183
Alpha- hemihydate
Beta-hemihydrate
Modified beta hemihydrate
Modified alpha hemihydrate