Technical Questions

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The purpose of calcium sulfate (gypsum)

Q. What is the purpose of calcium sulfate (gypsum)?


A. For modern cements, the rate of stiffening of the cement paste should be controlled within limits to make the cement a useful product. If the reactions were too rapid, causing flash set, the concrete would stiffen too quickly. If too slow, the delay in acquiring strength would be objectionable. The initial rate of reaction should therefore be controlled. This is done by controlling the amount of calcium sulfate (gypsum [CaSO4·2H2O] or gypsum with anhydrite [CaSO4]) added to the cement as it is being ground. Cement chemists believe that for each combination of raw materials and fineness to which the cement is ground, there is some optimum quantity of calcium sulfate that gives the best results for the product when used at a given temperature, that is, the greatest strength, the least shrinkage on drying, and the least swelling on wetting. This optimum changes with use of chemical admixtures. Manufacturers should know the requirements of their own material. A maximum limit is put on calcium sulfate (in terms of SO3), in current prescriptive specifications for portland cement (ASTM C150) and blended hydraulic cements (ASTM C595). ASTM C1157 is a nonprescriptive, purely performance-based specification for portland cement and blended cements with no limits on composition.


References: SP-1(02); ACI 225R-19; E3-13; ASTM C150; ASTM C595; ASTM C1157

Topics in Concrete: Cementitious Material; Concrete Fundamentals

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