Technical Questions

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The cause of false set

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 C403ASTM C191ASTM C266ASTM C953

Topics in Concrete: Concrete Fundamentals

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