Solidification/stabilization of Cr(VI) with cement - Leachability and XRD analyses

The leachability of Cr(VI) from solidified Cr(VI)-Portland cement mixtures cured for 28 days were investigated. Cr(VI) was solidified with Type I Portland cement at concentrations of 0.5%, 2%, and 5% (based on K2CrO4) by weight of the cement. The addition of Cr(VI) increased the initial and final setting times of cement. X-ray diffraction (XRD) study indicates that Cr(VI) inhibits cement hydration process by reacting with Ca2+ during the hydration of cement, which was also supported by increased setting times. Increasing the Cr(VI) content in the cement reduced the compressive strength of the solidified cement. The leachability of Cr(VI) during the toxicity characteristics leaching procedures (TCLP) test was dependent on the initial Cr(VI) concentration and the leaching time. The treatment efficiency of cement was independent of the initial Cr(VI) concentration. The reaction products and crystalline phases were identified using the XRD. One of the reaction products identified was CaCrO4. An empirical relationship was developed to predict the leaching of Cr(VI). Based on this relationship, the treatable amount should be limited to K2CrO4/cement ratio of 0.2% to meet the TCLP limit of 5 mg/l. (C) 2000 Elsevier Science Inc. All rights reserved.