Containment mechanism of trivalent chromium in tricalcium silicate

Trivalent chromium admixtures in ordinary Portland cement (OPC) have a pronounced effect on the structure of the hydration products of tricalcium silicate (C3S). (I) Cr-III compounds, added as dopants to C3S, simulate chromium waste forms that might be stabilized in a Portland cement matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were used to identify the microstructural changes accompanying the addition of Cr-III solutions to C3S. Energy dispersive spectroscopy (EDS) was used to probe the distribution of chromium in the stabilized hazardous waste forms. Fourier transform infrared (FTIR) and Si-29 nuclear magnetic resonance (NMR) spectroscopies were employed to provide structural information about the crystalline and amorphous products of C3S hydration. In monolithic waste forms containing trivalent chromium, chromium was found to be stabilized both through the formation of insoluble calcium chromium hydroxide nitrate complexes, and localized chemical incorporation of chromium in the stable calcium silicate hydrate (C-S-H) phase. (C) 1998 Elsevier Science B.V.