I have specified gypsum plaster for internal walls of a building and white cement for the external walls. Is there any consequences if the white cement is accidentally mixed with small amounts of gypsum?
Gypsum is a mineral and is hydrated calcium sulphate in chemical form. Gypsum plays a very important role in controlling the rate of hardening of the cement. During the cement manufacturing process, upon the cooling of clinker, a small amount of gypsum is introduced during the final grinding process.
Gypsum is added to control the “setting of cement”. If not added, the cement will set immediately after mixing of water leaving no time for concrete placing. Also it helps in reduction of energy required to grinding the material in mfg process of cement.
Hi Alex,
yes, there are consequences if the cement plaster is mixed with a small amount of gypsum: You may get a desaster because the cement plaster is likely to show very short setting behaviour and thereby teh workability will be very poor. We name this setting behaviour - caused by too much gypsum - "false setting".
Andrea
Gypsum plays a key role in cement and it works as a set retarder. Without gypsum, a rapid hydration of calcium aluminates to form calcium aluminate hydrate could make cement setting immediately. One more point i would like to emphasize that gypsum also can contribute effectively to help gaining in longterm strength. For those seasons, a small amount of gypsum is used in cement production.
If white cement used with additional amount of gypsum then it may increases setting behavior of particular plaster in which it used. It also leads to some extent of cracks in the particular application
Dear Alex
In cement, gypsum amount is controlled to 2 to 4 % of cement to control flash set. Extra amounts may be harmful.
If you have high amounts of C3A in your cement, then it may react with the extra sulfates from gypsum to form Ettringite. Ettringite can expand in volume ad my cause cracking in plaster.
I totally agree the point that gypsum can cause some cracks but i wonder how gysum can accelerate setting time?
Dear all, there are 3 phase Gypsum mineralogy, first phase calls Gypsum or CaSO4.2H2O, second phase Hemihydrate or CaSO4. 0.5H2O and third phase Anhydrate or CaSO4 (have no H2O). The Gypsum which accelerate setting time of mortar cement, if the phase of gypsum is Anhydrate. In X-Ray Diffraction method, as we can see the profile.
In a case White portland Cement, function of added Gypsum same as Grey portland cement, to retard setting time. Generally, setting time white cement faster (40-60 minutes) than grey cement.
I have run into many cases recycling gypsum (the second phase ;hemihydrate) for various application fields. An interesting finding is that gypsum decelerates setting time though.
As for the point that Anhydrate accelerate setting time of mortar, just because, in my opinion, CaSO4 having no water occupies immediately water in mortar and then the early hydration probably start.
Relative setting time of gypsum phase mineral alone, Anhydrate faster set 2-20 minutes, Hemyhidrate moderate set 20-60 minutes, Gypsum slowly set 60-240 minutes. Charracteristic phase depend on presence purity how much combine take place.
But, in portland cement; grey and white, have need Gipsum Hemyhidrate around 2-6% enough, to keep setting time steady.
Dear Alex
For indoor plaster, a mixture of gypsum and lime is usually recommended to control the quick setting of gypsum and furnish a workable mix.
For outdoor plaster, a mixture of cement+gypsum+lime is preferred to control the setting time of the cement (decrease) and provide more durable plaster.
I agree with Qasrawi that if C3A is normal in your cement, then there is no harm.
For indoor application, gypsum combine with white cement, fine limestone and glue such as PVA, become mixed plastize condition to easy handworking.
Thats right, C3A around 6-9% in white cement is optimally proportion reacting with the hemyhidrate gypsum to make good product.
Dear Alex,
Calcium sulfate bearing materials have dual effects on Portland cement depeding on the amount of the aluminate phase. at optimum dosage ( 4 or 5%) they can regulate the setting by delaying the hydration of C3A (precipitation of ettrigite) and provide great strength development. Too much of calcium sulfate within the system leads to the delay formation of ettringite causing thus the expansion that manifests through cracks.....in conclusion gypsum should be use at its optimum value to avoid sulfate attack and other negative effects related to DEF....
Gypsum is a mineral content usually added in the manufacturing process of cement which influence the setting properties of cement. Excess amount will make the cement disintegrate, which affects its binding property
I think there is no problem if we put artificial gypsum on white cement (no white clinker
if you add extra amount of gypsum in the cement it will affect the setting time(reduce the expected setting time) and if there is an excess amount in cement it will make D-Crack by forming an ettringite.
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