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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.
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Home > Tools > Frequently Asked Questions
Q. What is the cause of false set? A. False set is a form of premature stiffening of the paste or concrete, which occurs within 1 to 5 min after mixing. False set can be eliminated by continuous mixing or by reworking and may not be noticed on jobs supplied by truck mixers or with central-mixed concrete that is agitated during delivery to the site. The most common cause is the presence of gypsum that has partially dehydrated to form plaster of paris (CaSO4 ⋅ 1/2 H2O), which results if the temperature in the grinding mill is too high. Later, when water is added to the cement, the plaster of paris immediately starts to hydrate to gypsum and stiffen. Reworking breaks this stiffness and allows normal hydration. To keep the temperature below the dehydration point of gypsum, the clinker should be cooled before grinding. References: SP-1(02); ACI Physical Testing of Cement Training Video; ASTM C403; ASTM C191; ASTM C266; ASTM C953 Topics in Concrete: Concrete Fundamentals
Q. What is the cause of false set?
A. False set is a form of premature stiffening of the paste or concrete, which occurs within 1 to 5 min after mixing. False set can be eliminated by continuous mixing or by reworking and may not be noticed on jobs supplied by truck mixers or with central-mixed concrete that is agitated during delivery to the site. The most common cause is the presence of gypsum that has partially dehydrated to form plaster of paris (CaSO4 ⋅ 1/2 H2O), which results if the temperature in the grinding mill is too high. Later, when water is added to the cement, the plaster of paris immediately starts to hydrate to gypsum and stiffen. Reworking breaks this stiffness and allows normal hydration. To keep the temperature below the dehydration point of gypsum, the clinker should be cooled before grinding.
References: SP-1(02); ACI Physical Testing of Cement Training Video; ASTM C403; ASTM C191; ASTM C266; ASTM C953
Topics in Concrete: Concrete Fundamentals
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