Gypsum as set retarder in Cement

Gypsum used in cement manufacturing process is generally of natural origin, but many a times by-product gypsum from different origin is used. By-product gypsum contains different type of impurities, which affects the setting and hardening property of resultant cement. Hence suitability of gypsum based on its reactivity as a set retarder is one of the key factor while selecting the gypsum. In order to establish reactivity and suitability of gypsum, a comparative evaluation study has been carried out on four different varieties i.e. mineral gypsum, chemical gypsum, marine gypsum & Phosphogypsum.   

Tricalcium aluminate (C3A) present in cement reacts with water very rapidly forming various phases and leads to very fast setting, known as flash setting in cement if it does not contain gypsum or the cement is under dosed. The reaction of C3A / Ferrite + gypsum leads to the formation of ettringite & mono-sulfate and setting is controlled by the formation of ettringite which envelopes the reactive C3A grain . As a result temporarily, it causes the retardation of rapid hydration of this reactive phase. Sulfate ions contributed to the solution by dissolution of gypsum have a retarding effect on the aluminates but an accelerating effect on the hydration of silicate phases

 [AlO4]- + 3 [SO4]2- + 6 [Ca]2+ + H2O -----> C6AS3H32 (ettringite)

 [AlO4]- + [SO4]2- + 4 [Ca]2+ + H2O --------> C4ASH12 (mono-sulfate)

Mineral gypsum is available from natural reserve and its addition during grinding of clinker depends on the purity. Many a times its availability is insufficient and it becomes essential to use the gypsum from other source. Marine gypsum is recovered as a by-product during the production of common salt, when sea water is subjected to solar evaporation. The contamination of chlorine in marine gypsum is one of the main constraint to consume in large quantity

Chemical gypsum here refereed to is a by-product from Chemical Industries manufacturing the dye intermediates called H-acid, which  generate large quantity of gypsum sludge, and after drying, it is used as chemical gypsum. In the process, the neutralization is done by soda ash and hence alkali impurities may be higher in it.

Phosphogypsum is a by-product generated during manufacture of phosphoric acid using rock phosphate and sulfuric acid as raw materials through two processes viz. Di-hydrate (single step) and hemihydrate dihydrate (two step) process. This variety usually is contaminated with P2O5 and fluoride, and possesses detrimental effect as a set retarder. Soluble fluorides and phosphates are primarily responsible for set retardation of cement. Phosphatic impurities with at least one acidic hydrogen show more retardation in setting time. The fluorine compounds which possess alkaline unstable linkage have been reported to be most deleterious to setting

P2O5 & fluoride impurities are known to react with Ca+2 in solution when cement is mixed with water and the reaction products (calcium phosphates, calcium fluorides, etc.) precipitate on the anhydrous cement grain causes the slow down of reaction between cement particles & water. Presence of soluble part of these two impurities are the prime cause for delayed setting time & poor early strength development. It has also been observed that fluoride contribute more in delayed setting time than P2O5 . Phosphogypsum obtained from hemihydrate-dihydrate process contains generally lower impurity level than from dihydrate process

Gypsum on heating gives the following transformation

The first two changes are reversible, where as the last one is irreversible. It is established that up to 200 0C raise in temperature and subsequent sufficient cooling in air , the re-conversion of hemihydrate to gypsum was found to be around 30-40 % for the three types of gypsum, viz. marine, mineral and chemical 

The chemical analyses of four different variety of gypsum is given below  

Solubility of different gypsum in saturated aqueous solution

PSD of ground gypsum samples

Effect of gypsum on cement by preparing four different batches of Ordinary Portland Cement (OPC) using same quality clinker, maintaing SO3 level of cement 2 +/- 0.05 % and maintaining Blaine value ~ 3100- 3150 cm2/gm. The cement samples were tested for normal consistency (NC), setting time (IST & FST) & strength at different ages up to 28 days. 

The rate of reaction between C3A and gypsum depends on the availability of dissolved in water. More the solubility, more will be the reactivity of the gypsum. It is revealed from study that the solubility of chemical gypsum is higher than all other variety, which enhances its property as set retarder. It is revealed from literature that the presence of alkali in gypsum increases the solubility of gypsum. During H-acid production neutralization of excess sulfuric acid is done with limestone & soda ash and hence chemical gypsum is mildly contaminated with alkali. It leads to higher solubility of it as compared to other variety. Further, the solubility data indicate that the solubility of hemehydrate is more than the di-hydrate irrespective of the type of gypsum

Since the solubility of hemihydrate is more as compared to dihydrate irrespective of type of gypsum, the reactivity of gypsum is dependent on the peak temperature. Though the peak temperature for first conversion is lowest in case of phosphogypsum, the reactivity of chemical gypsum is highest among the all variety. This is because of the fact that phosphogypsum contains some impurity of P2O5 & fluoride which inhibits the reactivity. This is also corroborated with the result of strength gain at early ages & setting time value. The highest strength gain at 28 days in case of phosphogypsum may be due to filling of extra pores by calcium silicate hydrate (CSH gel) phase developed at later stage by reaction of protected reactive phases with water.

Among the four varieties, chemical gypsum is easier to grind.The average particle size of ground chemical gypsum is as low as 17.29 m, where as in case of marine gypsum it is as high as 39.06 m. The residue on 45 m  is 15.51 % for Chemical gypsum , but 42.04% for marine gypsum. The increasing trend of finer particle of different ground variety is in the order of Marine< Mineral < Phospho < Chemical. Hence the dispersion of gypsum particle in OPC is highest in case of chemical gypsum, which contribute significantly to enhance the reactivity of chemical gypsum.

Chemical gypsum exhibits highest reactivity among the four varieties. OPC prepared using Chemical gypsum shows highest early strength gain also setting time value within limit, establishes it as most suitable. Because of the presence of soluble P2O5 & fluoride content in Phosphogypsum the early strength gain lowest & setting time maximum. The conversion of di-hydrate to hemihydrate occurs at lower temperature in case of phosphogypsum. The particle size of gypsum in OPC with chemical gypsum is very fine.