Salem - 24-7-2012 at 13:34
Dear Experts
I would like to get your technical support to solve a a problem we face in cement.
It might be of value to state that i am not a chemist, i have good experience in concrete technology.
The problem is that when our cement is being used in concrete, it exhibit a loss of workability/loss of workability often after one hour. We suspect
that Gypsum might be dehydrated.
Would you please guide me the process to solve this problem??
Your kind response is highly appreciated.
Thanks & regards
salem
Zan Divine - 24-7-2012 at 15:41
Doesn't your industry have entire journals and many books written about it? I would have thought you'd start there.
However....
No less an authority than Wikipedia states.. "In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but
typically 5%) of calcium sulfate (usually gypsum or anhydrite) is added to the clinker and the mixture is finely ground to form the finished cement
powder"
Apparently anhydrous CaSO4 is OK in cement.
Isn't this a question to put to whoever supplies your cement, though? And if it's yours, how have you diverged from your accepted procedure?
[Edited on 24-7-2012 by Zan Divine]
cyanureeves - 24-7-2012 at 16:53
reminds me of when i worked with concrete and had to pour it and spread it within an hour before the cement truck got too hot and ruined the mix.i
never understood why they just couldn't keep adding water but look everywhere concrete is worked and you will see all hands from all crews merge on
one site and spread it quick.
Zan Divine - 24-7-2012 at 17:16
There's a set maximum amount of water that can be (eventually, it proceeds for a long time) reacted with concrete or cement. If you exceed that amount
you get weak cement.
Twospoons - 24-7-2012 at 19:01
Sounds like you need to add a retarder.
Snipped from Wikipedia article on concrete:
"Retarders slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid."
I suggest you read the whole article. http://en.wikipedia.org/wiki/Concrete
Cement sets by hydration and crystallisation, not by drying. Its a chemical reaction, rather than a solvent + binder system.