Stabilization Mechanism of Hexavalent Chromium Ions in Portland Cement-Based Materials

To improve the resource utilization of chromium-containing solid waste in cement-based materials, the leaching rate, valence state, and microscopic morphology of hexavalent chromium in Portland cement-based materials were studied. The results indicate that the stabilization rate of hexavalent chromium in cement reaches 95% for specimens containing 0.5% potassium chromate by mass after 60 days. However, when the potassium chromate content is increased to 5%, the stabilization rate drops below 90%. Analysis through photoelectron spectroscopy elucidates that chromium mainly exists in two valence states within the cement paste: trivalent chromium, present as Cr(OH)3 and CrOOH, and hexavalent chromium, existing in the forms of CaCrO4 and K2CrO4. After curing for 28 days, the ratio of trivalent chromium to hexavalent chromium is approximately 7:13, with the main hexavalent chromium compound CaCrO4 constituting about 21.4% of the total chromium content. Microscopic phase tests further intricate interactions between calcium ions and hexavalent chromium ions in a low water-cement ratio matrix, resulting in the additional CaCrO4 whose quality steadily increased with age. The rise in ion content leads to a continuous augmentation in the diffraction peaks density of ettringite (AFt). This is attributed to the slightly larger spatial position of CrO42- compared to SO42-, facilitating its partial replacement in the columnar structure lattice of the ettringite hydration product.