The large blocks of gypsum were broken into pieces
of 40–60 g in weight. The broken lumps were then
chiselled to approximate spheres of 30–40 mm in
diameter. Prior to the durability tests, the material was
oven dried at slightly below 40C, which is the temperature
at which some types of gypsum start to ‘calcine’ to
calcium sulphate hemihydrate CaSO4ƽH2O (bassanite)
(Kocman and Foley 1987). An extremely wide range of
durability values was obtained under variable pH conditions
after two 10-min cycles. As indicated in Fig. 1,
the results varied from medium (98%) values