Chemical stabilization of chromate in blast furnace slag mixed cementitious materials

Cement waste form (CWF) technology is among the leading approaches to disposing of metals and liquid low-level nuclear waste in the United States. One such material, saltstone, includes slag, fly ash and Portland cement to enhance the immobilization of contaminants (e.g., Cr, ⁹⁹Tc) in alkaline liquid wastes. To evaluate the stability of such redox sensitive contaminants in saltstone, the effects of slag as a source of reductant on Cr immobilization was evaluated in aged (<300d) saltstone monoliths. Specifically, we investigated the effects of artificial cement pore waters on the Cr release and the spatially resolved Cr chemical state analysis using synchrotron based microfocused X-ray microprobe analysis. The microprobe analysis indicated the heterogeneous distribution of insoluble Cr(III)-species in saltstone. Although at most of 20% Cr_total was leached at the top few (2-3) millimeter depth, the release of Cr(VI) was small (<5%) at 5-30mm with slight changes, indirectly suggesting that Cr is likely present as insoluble Cr(III) species throughout the depths. The study suggests that this saltstone formulation can effectively retain/immobilize Cr under the oxic field condition after ≤300d of aging time.