Stability Of α-calcium Sulfate Hemihydrate In Potassium Chloride Solutions Under Atmospheric Pressure

a-calcium sulfate hemihydrate(α-HH) has been applied in comprehensive areas due to its superior workability and high strength.It could be prepared through salt solution method under atmospheric pressure.Considering the uncertainties of the phase-transition among CaSO₄ phases and the stability ofα-HH in salt solutions,it is need to investigate phase-transitions of CaSO₄ phases andα-HH's stablility in some representative salt solutions.The solubility and phase-transitions of calcium sulfate dihydrate(DH),α-HH, calcium sulfate anhydrite(AH) in KC1 solutions(0.0-10.0%) were studied under conditions of atmospheric pressure and(80-105)℃.The gypsum in a fixed ratio to the solution was added into KC1 solutions at an expected temperature with different concentration.The purpose was to measure the solubility of three gypsum phases in KC1 solutions,to sketch out the path forα-HH's recrystallization,hydration or dehydration, the theoretical and actual phase-transition diagrams.Finally,the stability ofα-HH in KC1 solutions could be presented.In 0.0-10.0wt%KC1 solutions,the maximal solubility for DH at 85,90,95,100℃was (0.67,0.71,0.66,0.69) g CaSO₄l00ml respectively,the maximal solubility forα-HH at 90,95,100℃were(0.68,0.63,0.59) g CaSO4/100ml correspondingly,and the maximal solubility for AH at 85,90,95,100℃was(0.65,0.97,0.85,0.91) g CaSO₄/100ml respectively.At the same temperature,the solubility of DH increased with KC1 concentration,the solubility ofα-HH and AH increased with KC1 concentration,after passing a maximum,the solubility would decline.This may be related with the combined effect of temperature and KCl concentration.It was found that the stability ofα-HH in KC1 solution relied heavily on KC1 concentration and temperature.The phase-transition for calcium sulfate is successfully outlined as a function of KC1 concentration and temperature.DH is stable in regionâ… and metastable in regionâ…¢.AH is stable in regionsâ…¢andâ…£,coexisted with potassium pentacalcium sulfate monohydrate(goergerite).Only in regionâ…¡,α-HH is metastable.A narrow operating window forα-HH's recrystallization has been determined.This window is limited by KC1 concentration and reaction temperature,even the reaction time.This window is outlined as 0.0-3.0wt%concentration for KC1 solution and 89-105℃for reaction temperature,and in general the reaction time should be controlled within certain time,for example,5-6h.Meanwhile,there are discrepancies between the actual phase-transition diagram gained byα-HH's reaction in KC1 aqueous solution and the theoretical phase-transition diagram obtained by solubility measurement.In the theoretical diagram,there are two regions forα-HH to be metastable,but in higher concentration region it would dehydrate to AH in a short time.