Gargamel
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Is there a way to slow down the setting of gypsum?
I dont mean just gypsum but also other salts that create hydrates with water or maybe other solvents.
Usually the material sucks up water, or other solvents, and heat of hydration is set free.
is there a way to slow down this proces, preferable by heating or cooling down the materials?
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vmelkon
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I'm sure temperature effects that reaction like in the general case of lowering temperature by 10 degree C reduces rate by half.
I know that in the case of cement, they sometimes keep it cool to slow things down. I saw it on a show called "Massive machines" or something a long
time ago. They were mixing in ice with the cement. Another thing they do is keep mixing it so that it does not set. I don't know why that works. That
is why cement trucks have those rolling tanks.
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Fulmen
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Temperature is one way. Another could be retarding (or regular plasticizing) additives used in concretes. These work by creating a barrier layer that
reduces reaction rates, it should work for any hydration reaction.
Fun fact: Regular cane sugar is a potent retarder for cement, but I believe it reduces strength as well. In higher doses it completely "kills" the
cement. I have heard of one incident with a cement truck stuck on a stranded ferry, they ended up chartering a taxi boat to get 20 kilos of sugar to
save the truck.
We're not banging rocks together here. We know how to put a man back together.
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Xenoid
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Just came across this in Mellor, with regard to gypsum:
"Admixture of alum or borax with the plaster, reduces the rate of setting, while common salt accelerates it. Alum makes a plaster which once set is
much harder; the mixture of plaster of Paris with alum is known as Keene's cement."
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Gargamel
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Well in fact temperature was the one thing I was thinking of.
Dilluting/mixing the salt with anything else was not my intention.
Cooling down the whole thing will slow it down - is that correct?
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Fulmen
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Correct. Starting with cold raw materials should slow the reaction down substantially, same goes for cooling the part as it cures.
We're not banging rocks together here. We know how to put a man back together.
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Gargamel
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Thanks 
[Edited on 22-4-2015 by Gargamel]
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