Dehydration of gypsum studied by positron annihilation lifetime technique

Positrons have been used to monitor the possible phase changes that occur during isothermal annealing of an artificially grown monocrystal of gypsum CaSO4 . 2H(4)O at 90 degrees C and 100 degrees C, respectively. The crystal as received did not show any contribution of positronium formation in cavities. It showed two lifetimes of 173 ps (31%) and 397 ps (69%) respectively. A long lifetime component of 2.4 ns became apparent when the sample was heated. The water loss rate for the 90 degrees C series was about a factor 3 smaller than for the 100 degrees C series. It is expected that during dehydration cavities will be present of the size corresponding with the volume of a water molecule (approximate to 0.03 nm(3)). However, the observed lifetime is much larger than expected for ortho-positronium trapped in cavities of this size. Moreover, the observed intensity is very small. Size reduction by relaxation of the "water"-vacancies on one hand and clustering of vacancies on the other hand may explain the occurrence of the relative long lifetime observed during the dehydration. Changes of the intensity of the long lifetime component indicate the onset of phase changes of the gypsum.