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This work aims to illustrate the impact of high concentrations of Cr(VI) (based on Na 2 CrO 4 ) on the hydration assembly and microstructural development of hydrated Portland cement, and the results also present the role of calcium sulfate on the immobilization of Cr(VI) in Portland cement. The results showed that the immobilization of Cr(VI) in hydrated Portland cement was attributed to the formation of CrO 4 -U phase, an analogue of SO 4 -U phase (3CaO·Al 2 O 3 ·CaSO 4 ·0.5Na 2 SO 4 ·15H 2 O). The growth of CrO 4 -U phase on the surface of clinker particles formed a diffusion barrier and hence increased the setting time. Increasing the calcium sulfate dosage impaired the Cr(VI) immobilization due to the competition between CrO 4 2− and SO 4 2− integrated into the U phase. The generalized acid neutralization capacity (GANC) test indicated that the Cr(VI) leaching behavior was a function of the leachate pH value. As the pH decreased to 11.8, the CrO 4 -U phase was converted quickly to CrO 4 -ettringite, which generated a slight increase in Cr(VI) concentration. The most leaching sector, approximately 89.3% of added Cr (1 wt% of cement), was found in the pH range 11.8–10.5 due to the dissolution of secondary CrO 4 -ettringite. It could also be shown that the C–S–H had little chemical binding for Cr(VI). [ABSTRACT FROM AUTHOR]

This study investigated the immobilization potential of alkali-activated slag (AAS) pastes for Cr(VI) by examining compressive strength, toxicity characteristic leaching procedure (TCLP) and generalized acid neutralization capacity tests. Alkaline digestion, total acid digestion, XRD, FTIR and SEM-EDS were carried out to clarify the immobilization mechanism. The AAS pastes gave high compressive strengths, which contributed to the physical encapsulation of Cr. The addition of Cr(VI) induced an increase in compressive strength compared to Cr(VI)-free pastes. The leaching results showed that AAS pastes exhibited effective immobilization for Cr(VI) and that the leachability was strongly dependent on the NaOH dosage, water to slag ratio, initial Cr(VI) content, leachate pH and curing duration. The initial Cr(VI) content up to 1.5 wt% by weight of slag was well immobilized with the total-Cr leachability below the TCLP regulatory limit of 5 mg/L. The digestion results demonstrated that the reduction of chromium from hexavalent to trivalent played a significant role in the immobilization of Cr(VI) in AAS without any additional reductants. The XRD and SEM-EDS results suggested the formation of a clear CrO 4 -U phase as the primary retention phase for unreduced Cr(VI). Therefore, alkali-activated slag binder is effective in the immobilization of Cr(VI)-bearing wastes. [ABSTRACT FROM AUTHOR]