Calcium Sulfate Fillers are made from high purity, naturally occurring gypsum (CaSO4•2H2O). The dihydrate forms of gypsum have a fairly neutral pH and about 20% water of crystallization. Although exact temperatures will vary depending upon vapor pressure, relative humidity, etc., gypsum starts to lose its water of crystallization as it is heated above –50ºC (122ºF).

As water is removed, both pH and specific gravity increase. When the hemihydrate form of calcium sulfate is heated to a temperature range of 150ºC to 180ºC, an unstable form of calcium sulfate soluble anhydrite is formed.

This is a highly active material which will readily absorb moisture from the atmospheres. (We don’t supply the soluble anhydrite form of calcium sulfate).

When calcium sulfate is calcined at temperatures above 400ºC an insoluble anhydrite is formed. It should be noted that this reaction is not reversible. Different calcinations techniques can affect product stability but all of our anhydrous fillers can be successfully used in polymer-based systems.