Hi. Gypsum is mainly used for regulating the setting time of cement. Best wishes. Maxime.
Gypsum mitigates flash setting that should happen due to C3A hydration.
Hi. Gypsum is mainly used to dominate the hydration reaction of C3A, leading to a slower setting rate rather than flash setting. Wei
Dear Mr. Alireza,
The setting of cement is primarily due to the reaction of tricalcium aluminate C3A, which is the most reactive of the four clinker minerals present in cement.
L’hydratation des aluminates tricalciques C3A permet de produire de l’aluminate de calcium hydrate 3CaO.Al2O3.6H2O. The increase of C3A can speed up the setting and hardening rate of cement, and the heat of hydration is high at the same time. Thus, the reaction of C3A with water is very rapid and is regulated by adding gypsum during production. Gypsum (composed of crystallized calcium sulfate (CaSO4) is added to Portland cement to regulate the extreme setting reaction which occurs in the presence of water. The gypsum content of cement is expressed in terms of its sulphate (SO3) content. The retardation mechanism of gypsum is: when cement is hydrated, gypsum reacts with C3A quickly to generate calcium sulfoaluminate hydrate which deposits and forms a protection film on the cement particles to hinder the hydration of C3A and delay the setting time of cement. Nevertheless, gypsum also contributes for strength acceleration in the early stages of hydration.
Please consult this two articles.
Work as a retarder ( to control flash set of cement, when water is added)
Remember, the reason you want the cement in concrete to react, and set slowly is to reduce the expansion of the cement, and aggregate due to the
exothermic reaction. working time is important, but more important is to
not undergo excessive expansion, followed by excessive contraction, and the creation of numerous micro fractures in the concrete. Concrete is good at cracking on its own without giving it an extra advantage of cracking sooner by allowing a high heat of hydration ( rapid expansion ), followed by rapid contraction when the concrete stops hydrating at a low
strength value. Manufacturing cements is a combination of art and science, along with employing useful Natural aspects, like pouring large slabs at night, when you expect cloudy overcast weather the following day.
Temperature control is critical to getting a long lasting product.
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