Immobilization of Cr(VI) by hydrated Portland cement pastes with and without calcium sulfate.

This work aims to illustrate the impact of high concentrations of Cr(VI) (based on Na2CrO4) 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 CrO4-U phase, an analogue of SO4-U phase (3CaO·Al2O3·CaSO4·0.5Na2SO4·15H2O). The growth of CrO4-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 CrO42- and SO42- 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 CrO4-U phase was converted quickly to CrO4-ettringite, which generated a slight increase in Cr(VI) concentration. The most leaching sector, approximately 89.3% of added Cr (1wt% of cement), was found in the pH range 11.8-10.5 due to the dissolution of secondary CrO4-ettringite. It could also be shown that the C-S-H had little chemical binding for Cr(VI).