DEHYDRATION REACTIONS OF GYPSUM - A NEUTRON AND X-RAY-DIFFRACTION STUDY

The kinetics of the dehydration of gypsum was investigated by powder diffraction methods. Using the incoherent scattering effect of H with the neutron beam, the background intensity as a measure of the water content was checked in the temperature range 295-623 K. The superposed Bragg peaks yielded four major phases: Gypsum, subhydratesCaSO4(H2O)x (1 > x > 0),AIII-CaSO4, AII-CaSO4. For the subhydrates a maximum water content ofx > = 0.74was determined. A different kinetic was found using Guinier X-ray technique with the heated sample prepared on a thin foil. Only with high local H2O steam pressure, produced in the comparable larger sample container of the neutron diffraction experiment, could this high H2O occupation of the subhydrate tunnel structure be found. A topotactic mechanism can describe the phase transitions for this reaction. © 1990 Academic Press, Inc. All rights reserved.