Miscibility in the CaSO4-2H2O-CaSeO4-2H2Osystem:: Implications for the crystallisation and dehydration behaviour

SeO42- ions solid ions can substitute for sulphate in the gypsum structure. In this work crystals of different Ca(SO4, SeO4). 2H(2)O solid solutions were precipitated by mixing a CaCl2 solution with solutions containing different ratios of Na2SO4 and Na2SeO4. The compositions of the precipitates were analysed by EDS and the cell parameters were determined by X-ray powder diffraction. Moreover, a comparative study on dehydration behaviour of selenate rich and sulfate rich Ca(SO4,SeO4) (.) 2H(2)O solid solutions was carried out by thermogravimetry. The experimental results show that the Ca(SO4,SeO4) - 2H(2)O solid solution presents a symmetric miscibility gap for compositions ranging from X-CaSO4 - (2H2O) = 0.23 to X-CaSO4 (- 2H2O) = 0.77. By considering a regular solution model a Guggenheim parameter a(0)=2.238 was calculated. The solid phase activity coefficients obtained with this parameter were used to calculate a Lippmann diagram for the system Ca(SO4,SeO4) (.) 2H(2)O -H2O (c) 2005 Elsevier B.V. All rights reserved.