Immobilization of chromium in cement matrices

Abstract

Portland cement and blended cements containing blast furnace slag afford both physical and chemical immobilization of chromium. To separate physical and chemical effects, the pore fluid contained in set, hydrated cements has been expressed and analyzed. In Portland cement spiked with 5000 ppm Cr(III), pore fluid levels are 0.1–1 ppm, whereas in well-cured slag blends, they decrease to <0.01 ppm. Both cement types give chemical immobilization, but slag cements give the better performance. Slag-containing cements are the most effective at removing Cr(VI) from the pore fluid, probably by reducing Cr(VI) to Cr(III). Electron microscopy coupled with energy dispersive X-ray analysis shows that Cr(III) can be substituted for Al in most of the calcium aluminate hydrate phases. In synthetic preparations, substitution is complete resulting in CaCr phases that are isostructural to calcium aluminate phases. Three Cr analogues of calcium aluminates were synthesized: Ca 2Cr(OH) 7 · 3H 2O, Ca 2Cr 2O 5 · 6H 2O and Ca 2Cr 2O 5 · 8H 2O, as well as solid solutions, e.g., Cr substituted hydrogarnet 3CaO · (Al 2O 3/Cr 2O 3) · 6H 2O. There is no real evidence that Cr is taken up by C-S-H gel.