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.

Regards,

Raj Sahu

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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

Abstract

"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."

http://www.sciencedirect.com/science/article/pii/0008884677901156

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

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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?

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Re: Lime-treated Gypsum

norew:

 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.

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