Effect of additives on the properties of α-hemihydrate

The effects of Na-succinate and succinic acid on the properties of the α-hemihydrate synthesised from the fluegas desulphurised (FGD) gypsum of the Mae Moh power plant, northern Thailand, by the classical vapour process, were studied in comparison with natural gypsum. It was found that the additives strongly affected the content and microstructure of the α-hemihydrate, and consequently the properties of its dihydrate. The physical and mechanical properties of the hydrated plaster were investigated in conformity with dental stone (type III). The additives, in an appropriate concentration, favoured the increase in α-hemihydrate phase but they tended to reduce the aspect ratio (c/a axis) of both the α-hemihydrate and its dihydrate crystals. Hence the morphology of the α-hemihydrate changed from long and prismatic towards a short and thick rod, and that of the dihydrate, from a long and thin plate to a small and thick plate. In comparison, the effect of the additives on the FGD gypsum was more drastic, and between the two additives, the effect of succinic acid was stronger. Alpha-plaster containing > 99 wt.% α-hemihydrate and dental stone having compressive strength of 28·75 MPa at a water/ plaster ratio of 0·32 were successfully produced from the FGD gypsum.