The solubility of selenate-AFt (3CaO•Al2O3•3CaSeO4•37.5H2O) and selenate-AFm (3CaO•Al2O3-CaSeO4•xH2O)

The Se(VI)-analogues of ettingite and monosulfate, selenate-AFt (3CaO(.)Al(2)O(3)-3CaSeO(4)(.)37.5H(2)O), and selenate-AFm (3CaO(.)Al(2)lO(3)(.)CaSeO(4)(.)xH(2)O) were synthesised and characterised by bulk chemical analysis and X-ray diffraction. Their solubility products were determined from a series of batch and resuspension experiments conducted at 25 degreesC. For selenate-AFt suspensions, the pH varied between 11.37 and 11.61, and a solubility product, log K-so=61.29 +/- 0.60 (1=0 M), was determined for the reaction 3CaO(.)Al(2)O(3)(.)CaSeO(4)(.)37.5H(2)O + 12 H+ double left right arrow 6Ca(2+) 2A1(3+) 3SeO(4)(2)(-) + 43.5H(2)O. Selenate-AFm synthesis resulted in the uptake ofNa, which was leached during equilibration and resuspension. For the pH range of 11.75 to 11.90, a solubility product, log K-so =73.40 +/- 0.22 (1=0 M), was determined for the reaction 3CaO(.)Al(2)O(3)(.)CaSeO(4)(.)xH(2)O + 12 H+ double left right arrow 4Ca(2+) 2Al(3+) SeO42-(x+6)H2O. Thermodynamic modelling suggested that both selenate-AFt and selenate-AFm are stable in the cementitious matrix; and that in a cement limited in sulfate, selenate concentration may be limited by selenate-AFm to below the millimolar range above pH 12. (C) 2003 Published by Elsevier Ltd.