Page 474 - Handbook of Thermal Analysis of Construction Materials

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11

Gypsum and Gypsum

Products

1.0 INTRODUCTION

The use of gypsum to control setting in Portland cement ac-
[1]
counts for considerable quantities of the use of this material. Control
of the reaction rate of tricalcium aluminate (C A), the constituent of cement
3
that reacts most rapidly with water, is most commonly achieved through the
addition of gypsum (sometimes hemihydrate is also used) to commercial
[2]
portland cement. This material is normally added to the cement clinker (in
amounts of approximately 2 to 3%) before grinding. The cement manufac-
turers usually specify a sulfur trioxide content of about 36%. Excess sulfate
in the form of hemihydrate can cause flash set in portland cement.
Calcined gypsum (e.g., plaster of Paris formed by grinding and
heating gypsum to about 150–190ºC) has many uses especially in the
construction industry. Gypsum wallboard is widely used in the North
American housing industry. At temperatures greater than 190ºC, the
soluble anhydrite forms followed by the formation of an insoluble anhy-
drite. Dehydration of the gypsum often results in the formation of a mixture
of hemihydrate and anhydrite. All plasters eventually revert to gypsum on
setting, the rate of transformation being dependent on the conditions of
calcination.

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