The hydration products of Cr-VI-doped tricalcium silicate (C3S)* have been investigated. C3S is the main constituent of Portland cement responsible for the strength and stability of hardened Portland cement paste. Chromium trioxide (CrO3), added as a dopant to C3S, simulates hexavalent chromium waste that may be stabilized in ordinary Portland cement. X-ray diffraction was used to monitor the development of the hydration reaction products from the early stages to the late reaction stages. Leaching studies were carried out to evaluate the stability of the Cr-VI-containing phases in the hydrated C3S matrix.In monolithic waste forms containing hexavalent chromium, CaCrO4 . 2H(2)O was found to form within a few minutes of the hydration reaction. With increasing concentration of Ca2+ in the pore solution, Ca2CrO5 . 3H(2)O became the stable species. The chromium in this phase was found to be very mobile in a standard acetic acid leaching test.