Mechanisms of chromium containment in tricalcium silicate

The containment of chromium wastes in tricalcium silicate (C{dollar}sb3{dollar}S) has been investigated. Tricalcium silicate is the main constituent of Portland cement responsible for the strength and stability of hardened Portland cement paste. Hexavalent and trivalent chromium compounds, added as dopants to C{dollar}sb3{dollar}S, simulate chromium waste forms that might be stabilized in a cement matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were used to identify the microstructural changes accompanying the addition of Cr(III) and Cr(VI) solutions to C{dollar}sb3{dollar}S. Energy dispersive spectroscopy (EDS) was used to probe the distribution of chromium in the phases within the hazardous waste forms. Elucidation of the molecular structure of the reaction products and their concentrations were accomplished with x-ray diffraction (XRD), infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies.; In monolithic waste forms containing hexavalent chromium, CaCrO{dollar}sb4{dollar}.2H{dollar}sb2{dollar}O was found to form within a few minutes of the hydration reaction. With increasing concentration of Ca{dollar}sp{lcub}2+{rcub}{dollar} in the pore solution, {dollar}rm Casb2CrOsb5.3Hsb2O{dollar} became the stable species. The chromium in this phase was found to be very mobile in a standard acetic acid leaching test. Small quantities of Cr(VI) were found to be chemically incorporated in the stable calcium silicate hydrate (C-S-H) phase. When added in the trivalent form, chromium has been found to be primarily contained in C{dollar}sb3{dollar}S pastes as calcium chromium hydroxide complexes. The nature of the complex formed depended on the ambient conditions within the reaction chamber. The concentration of the chromium complex was also found to increase with both age and the chromium concentration in the initial chromium/C{dollar}sb3{dollar}S mixture. Some chromium was also found to be chemically incorporated in the C-S-H phase, independent of the prevailing conditions of hydration and, to a lesser extent, the initial chromium concentration in the mixture. Trivalent chromium has been found to be totally stabilized in C{dollar}sb3{dollar}S paste and immobile in the standard acetic acid leaching test.; Mechanistic models describing the containment of Cr(III) and Cr(VI) in C{dollar}sb3{dollar}S throughout the hydration period have been proposed. These are based on direct evidence from the various microstructural, microchemical and molecular characterization techniques employed.