Email Address is required Invalid Email Address
In today’s market, it is imperative to be knowledgeable and have an edge over the competition. ACI members have it…they are engaged, informed, and stay up to date by taking advantage of benefits that ACI membership provides them.
Read more about membership
Learn More
Become an ACI Member
Founded in 1904 and headquartered in Farmington Hills, Michigan, USA, the American Concrete Institute is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensus-based standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete design, construction, and materials, who share a commitment to pursuing the best use of concrete.
Staff Directory
ACI World Headquarters 38800 Country Club Dr. Farmington Hills, MI 48331-3439 USA Phone: 1.248.848.3800 Fax: 1.248.848.3701
ACI Middle East Regional Office Second Floor, Office #207 The Offices 2 Building, One Central Dubai World Trade Center Complex Dubai, UAE Phone: +971.4.516.3208 & 3209
ACI Resource Center Southern California Midwest Mid Atlantic
Feedback via Email Phone: 1.248.848.3800
ACI Global Home Middle East Region Portal Western Europe Region Portal
Home > Tools > Frequently Asked Questions
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
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
Search other resources on the ACI website for answers to technical questions