Immobilization of hexavalent chromium in cement mortar: leaching properties and microstructures.

Stabilization and solidification (s/s) of heavy metals by cementitious materials are one of the effective methods in hazardous waste management. In cement alkaline environment, Cr(VI) compounds appear in the form of chromate anion (CrO), which is highly soluble; it makes the implication of the s/s method challenging. Therefore, it is important to study the amount of chromium leaching from cementitious materials. The effects of Cr(VI) concentration and water-to-cement (w/c) ratio on the level of leaching of chromium from cement mortar (CM) were investigated in this study. Results indicated w/c not significantly affect the leaching of chromium in the age of 28-day but in the 90-day-old samples indicated a reduction in leaching of chromium from mortar with increasing w/c. Results from toxicity characteristic leaching procedure (TCLP) tests indicated that the efficiency of Cr(VI) stabilization was reduced with greater chromium content but was enhanced with increased w/c. In detail, results showed that only about 0.21% and 0.26% cement weight in TCLP and tank test of Cr(VI) was stabilized in CM, respectively. The results of X-ray diffraction (XRD) and scanning electron microscope (SEM/EDS) tests indicated that increasing the Cr(VI) content leads to changes in the formation of the cement main phases and microstructure of CM.