Influence of ambient moisture and temperature on the phase transitions in the CaSO4-H2O system - a contribution to the production of single-phase binders from FGD gypsum

The phase boundaries in the CaSO4-H2O system have been fully characterized for the first time with, respect to sample temperature and ambient moisture up to a water vapour partial pressure of 1 000 mbar by the X-ray powder di ffraction (XRPD) method using a newly developed heating cell. Simple relati onships are given for the transition process from hemihydrate to anhydrite III (reversible) and for gypsum to hemihydrate. At fairly high water vapour partial pressures the phase fields are wide enough for industrial plant ut ilization. This has already been reflected in a new process for producing s ingle-phase CaSO4-hemihydrate.