The Effects Of Inorganic Additives On The Hydration Performance Of α-hemihydrate Gypsum And Its Action Mechanism

The effects of inorganic additives on the hydration performance of α-hemihydrategypsum were studied. The setting time, strength and expansion of gypsum paste weredetermined by general methods and the mechanisms were also discussed based on theexperimental results such as the chemical analysis , SEM observation, the measure of therate of heat evolution and ζ-potential.The results indicate that the cations are the predominant factors affecting thehydration process if they cannot react with Ca²⁺,SO₄^2- to form deposition or the complex,while the anions have little influence. These phenomena are explained as follows thatduring the hydration of α-hemihydrate gypsum, cations have significance influence on theζpotential of α-hemihydrate gypsum, thereby the dispersed state and the dissolving ratesof α-hemihydrate gypsum are changed. But anions have slight influence because it isdifficult that anions enter the double-layer. The result showed that the one-valence cationsaccelerate the hydration process of α-hemihydrate gypsum greatly because they improvethe dispersed degree of α-hemihydrate gypsum particles due to the remarkable effect ofincreasing theζpotential; the effects of bivalent and trivalent cations on ζpotentialdecrease in turn, compared with monovalent cations, the effects on both the disperseddegree of α-hemihydrate gypsum particles and the setting time of paste are little.With the exception of aluminium sulphate, the inorganic additives that were adoptedin the experiment have about the same effects on strength under the circumstance of thesame adding amount. The introduce of the addictives brings on the salt effect, increases thevalue of K/Ksp, enlarges the size of dihydrate gypsum crystal, and decreases the length-to-diameter ratio of dihydrate gypsum crystal, so the strength is affected at a certain extent.The change of strength is not obvious because the needless morphology of dihydratedoesn't change. In the samples containing aluminum sulphate, some ettringite crystalparticles are formed, which do not match with dihydrate gypsum crystals and increase thedefects in the hardened plaster. The strength decreases sharply with increase of aluminumsulphate amount.The experimental results show that when the kinds of inorganic additives in the α-hemihydrate gypsum paste are different, both the shape and size of dihydrate gypsumcrystals that are formed during the hydration are different. Some inorganic additives thatresult in bulky dihydrate gypsum crystals will decrease the expansion of plaster. Both theshape and size of dihydrate gypsum crystals are the primary factors that affect the extent ofexpansion.