Roles Of Protogenetic Anhydrite In Portland Cement

In recent years, the content of protogenetic sulfate in cement clinker and admixturebecomes higher and higher. However, in order to avoid negative effects resulted bysulfate, the SO3content in cement clinker and admixture is limited by almost allcountries, which makes the cement clinker with high sulfate and pozzolanic sludge withhigh sulfate difficult for the production of cement. In situations such as this, if thesulfate in cement clinker and admixture can substitute for the gypsum added as cementretarder, and there is no need to limit the SO3content in cement clinker and admixture,it just needs to limit the total SO3content of cement. Sulfate minerals in cement clinkerare mainly alkaline metal sulfate, double salt, sulphoaluminate and anhydrite. Sulfateminerals in admixtures are mainly anhydrite. In so many types of protogenetic sulfate,the anhydrite existed in cement clinker and admixture has the most similar performancewith dihydrate gypsum. Therefore, whether protogenetic anhydrite can play the role ofdihydrate gypsum in cement is a precondition for the reasonable use of protogeneticanhydrite and other types of protogenetic sulfate in cement production.Firstly, the types and content of protogenetic anhydrite in cement clinker andadmixtures were analyzed. The results show that protogenetic anhydrite is mainlyexisted in coal combustion waste. The dissolution rate of protogenetic anhydrite islower than that of dihydrate gypsum, but the dissolubility of the former is higher thanthat of the latter. By comparing dissolution property of calcined gypsum produced indifferent technology, it found that calcined gypsums by using natural dihydrate gypsumcalcined at850℃temperature for2hours and at1200℃for0.5hours can be used tosimulate protogenetic anhydrite.Influence of protogenetic anhydrite on physical-mechanical properties of Portlandcement by mixing cement clinker with various contents of protogenetic anhydrite wasresearched. The results show that protogenetic anhydrite does not much affect waterrequirement of standard consistency of Portland cement, but significantly affect thesetting time of Portland cement. The effect of protogenetic anhydrite on settingretarding of cement is stronger than natural anhydrite, but slightly weaker thandihydrate gypsum. Protogenetic anhydrite can remarkably improve the early strength ofcement, and improvement effect is a median between natural anhydrite dihydrategypsum. Protogenetic anhydrite can reduce drying-shrinkage of cement, and the effect of protogenetic anhydrite on compensating shrinkage of cement is almost the same asthat of dihydrate gypsum, but is better than that of natural anhydrite. The research abouthydration process shows that protogenetic anhydrite releases sulphate ion which canform ettringite in the cement solution, and ettringite inhibits the quick hydration of C₃A,so that retards cement hydration. The formation of ettringite improves the early strengthof the cement and reduces dry-shrinkage of cement. Compared with dihydrate gypsum,the dissolution rate of protogenetic anhydrite is too slow to result in the lower sulphateion concentration in the early time of hydration. In addition, the formation of ettringiteis less and later. So the effect of protogenetic anhydrite on inhibition hydration of C₃Ais weaker than dihydrate gypsum.The cements with different C₃A and alkali content were obtained by adding C₃Aand solid natrium hydroxide in cement. On that basis, the influence of cementcomposition on the role of protogenetic anhydrite in Portland cement was studied. Theresults show that the effect of protogenetic anhydrite on cement hydration andhardening is closely related to cement composition. When the C₃A and alkali content incement is higher, the setting retarding effect of protogenetic anhydrite is reduced. Andfor high C₃A （about12%） or high alkali （about1.7%） cement, it can’t effectivelyprolong the setting time of cement by adding a certain amount of protogenetic anhydrite.Compared with dihydrate gypsum, protogenetic anhydrite in cement with high C₃A andhigh alkali can’t effectively improve the early strength of cement. In cement includinghigh C₃A and high alkali content, the dissolution rate of protogenetic anhydrite is lowerthan the consumption rate of sulphate ion, so that sulphate ion concentration is lowerand the formation of AFt becomes difficulty, that cause the setting time of cementshortened, and the early strength reduced.The changing rule of the role of protogenetic anhydrite in Portland cement underdifferent curing temperature （including20℃,65℃and90℃） and curing humidity（including dry-curing, seal-curing and water-curing）. The results show that curingtemperature will affect the optimum dosage of protogenetic anhydrite in cement.Cement produced by protogenetic anhydrite under high temperature curing in early-agestill has better volume stability in the long age, but the risk of formation of delayedettringite is higher than that of cement produced by dihydrate gypsum. The influence ofcuring humidity on physical-mechanical properties of protogenetic anhydrite issimilarly to that of dihydrate gypsum cement. But the dissolution and hydration ofprotogenetic anhydrite under dry-curing condition become much slower, so that there is still some unreacted anhydrite in cement paste at28-days age, which increases the riskof formation of delayed ettringite.