Anhydrite Solubility And Its Influence On The Formation Of Ettringite

It has been uncertain that influences of anhydrite in circulating fluidized bed combustion(CFBC)fly ash on cement hydration.This research investigated the influence of calcination conditions on the solubility of anhydrite,the reactions between anhydrite and tricalcium aluminate(C3A)and the simulated CFBC fly ash was used for verification.The anhydrite calcined at 820? for 3 min showed significant lattice distortion and highest solubility.After 15min calcination the produced anhydrite showed perfect crystal structure with dense surface and a significant decrease in solubility.With the increase in water temperature the solubility of anhydrite decreased,however,the solubility of gypsum had no change.The relative supersaturation of calcium sulfate increased,but the concentration of calcium sulfate decreased in 0.1mol/LNa2SO4 solution.The calcination conditions of anhydrite influenced the formation and mophaology of ettringite.3 min calcined anhydrite had been beneficial to the formation of ettringite until 3 day curing ages,and rod-like shaped ettringite formed by dissolution-precipitation.The formation of ettringite delayed in the early curing ages when the calcination temperature and time of anhydrite increased to 15min,and bundles of ettringite formed by solid reaction.The major hydration product was monosulphate when anhydrite and tricalcium aluminate reacted at 180? steam curing for 12h.After being cured for 3 days at room temperature a large amount of large hexagonal column shaped ettringite formed,which resulted in the crack of samples.Both high calcination temperature and long calcination time lowered down the water requirement for standard consistency of simulated fly ash.The formation of ettringite changed little but the pozzolanic reactions of simulated fly ash were accelerated when the samples were steamed cured at 65?.Stream curing at 180? led to sample crack due to the delayed formation of ettringite.Carbonization led to the shrinkage of 90 day compressive strengths of samples.