Architecture：Gypsum and Gypsum Products.pdfVIP

449 11 Gypsum and Gypsum Products 1.0 INTRODUCTION The use of gypsum to control setting in Portland cement ac- counts for considerable quantities of the use of this material.[1] Control of the reaction rate of tricalcium aluminate (C3A), the constituent of cement that reacts most rapidly with water, is most commonly achieved through the addition of gypsum (sometimes hemihydrate is also used) to commercial portland cement.[2] 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. 449 450 Chapter 11 - Gypsum and Gypsum Products Gypsum has useful fire-resistant properties due to its water of crystallization (20.9 %). Gypsum plaster is widely used as an insulating material for protecting columns and beams of wooden materials from the high temperatures that develop during a fire. It is apparent that rapid methods of analysis for gypsum, hemihy- drate, and anhydrite are of significant practical interest. Therm