Raj Sahu
198 posts
TimePosted 23/08/2011 09:53:03
Raj Sahu says

Re: Lime-treated Gypsum

Hello Norew,

Your used gypsum is mineral gypsum or chemical gypsum or industrial byproduct phosphogypsum.Chemical gypsum contain impurities like free phosphoric acid,phosphates, flourides and organic matter.Lime treatment is used to neutralize the impurities to some extent to get rid of its negative effects on cement hydration and setting time.Mineral gypsum is free from such impurities and doesn't need any  chemical treatment.I do hope this will help.


Raj Sahu


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Ted Krapkat
537 posts
TimePosted 24/08/2011 03:29:15

Re: Lime-treated Gypsum

Thanks Raj,

Ahaaa, phosphogypsum! I've never heard it called lime-treated gypsum before. :)

Here is an interesting paper regarding the use of phosphogypsum as a kiln raw material rather than a cement additive, to get around the impurities problem;-

"Utilization of phosphogypsum in portland cement industry"
P.K. Mehtaa and James R. Bradya, Department of Civil Engineering, University of California at Berkeley Berkeley, CA 94720, USA


"Disadvantages associated with the presence of impurities in phosphogypsum can be overcome when, instead of adding it directly to portland cement as a set-retarding additive, it is added to the raw mix before clinkering. Due to the mineralizing action of phosphogypsum, the clinker can be made at a substantially reduced temperature. Cements made by grinding a clinker containing 2 percent SO3 derived from phosphogypsum did not need any further retardation and possessed high early strength characteristics."



...and here's a paper which suggests another treatment method for removing phosphate and fluoride impurities from phosphogypsum for cement use;-

 "Treating waste phosphogypsum for cement and plaster manufacture"
Manjit Singh, EST Division UA, Central Building Research Institute, Roorkee 247 667, India

"In the investigation reported in this paper, treatment of phosphogypsum with aqueous citric acid solution was attempted to purify phosphogypsum and improve its quality to make it fit for manufacture of cement and gypsum plaster for the first time. The treatment of gypsum converts phosphatic and fluoride impurities into water-removable citrates, aluminates and ferrates. The findings of chemical and physical tests and differential thermal analysis of the phosphogypsum with and without citric acid treatment established improvement of the treatment for purifying phosphogypsum. The purified phosphogypsum was found to have lesser amount of impurities of phosphates, fluorides and organic matter than the impure material. The Portland and Portland slag cements produced with purified phosphogypsum were found to have strength properties similar to those produced from mineral gypsum, whereas gypsum plaster produced conformed to the relevant Indian Standards."


99 posts
TimePosted 24/08/2011 08:22:46
norew says

Re: Lime-treated Gypsum

Thanks Ted.

I just thought our use of this type of gypsum might well be associated with our present problem with our delayed setting time for our Portland Cement.

We received a customer complaint that after 3 days the structure remains plastic/rubber like and we failed in the testing of their molded cylinders. I checked the C3A content of the cement, some batches showed only 7% and and SO3=1.9, are these values related to the problem?


84 posts
TimePosted 25/08/2011 11:16:36
Silastman says

Re: Lime-treated Gypsum


 I checked the C3A content of the cement, some batches showed only 7% and and SO3=1.9, are these values related to the problem? 

Hi colleagues! Greeting from Russia. 

I'm study this problem. This problem is not connect with content in cement C3A or SO3. This problem connect with content in phosphogypsum free  phosphoric acid. Ion PO4 block clinker grains, making on it's surface insolubility film of Ca3(PO4)2. This firm brake up processes of hydration of cement.   Deep  neutralization of   phosphogypsum by lime in water suspension impossible. We make it by higher pressure phosphogypsum with lime & some useful additives on the base of aluminium or ferrum oxides.