IMMOBILIZATION AND FIXATION OF MOLYBDENUM(VI) BY PORTLAND-CEMENT

The immobilisation of molybdenum (VI) in Portland cement has been studied. Analytical methods are reviewed. As a precursor to studies using commercial cements, the cement constituent phases Ca3Al2O6 and Ca3SiO5 and mixtures of Ca3Al2O6 and gypsum (CaSO4.2H2O) have been synthesised and subsequently hydrated with an aqueous solution containing initially 2000 ppm Mo. The aqueous Mo contents decrease slowly over the first 12 days before attaining a steady-state concentration, ca. 40-50 ppm. Its main speciation is as molybdate, MoO42-. Commercial Portland cement was also studied; chemically soluble Mo was determined following pore fluid extraction. The 28-day Mo concentration, in the pore fluid, was in the range 50-80 ppm. The insoluble Mo in Portland cement is distributed mainly between powellite, CaMoO4, and a calcium aluminate sulphate hydrate with a very small amount in cement gel, a hydrated calcium silicate. The synthesis and characterisation data are reported for Ca3Al2O6.CaMoO4.10-14H2O (Mo-AFm). Solubility data are reported for synthetic powellite and Mo-AFm at 25-degrees-C.