HEXAVALENT CHROMIUM IN TRICALCIUM SILICATE - PART I - QUANTITATIVE X-RAY-DIFFRACTION ANALYSIS OF CRYSTALLINE HYDRATION PRODUCTS

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