Effect of Cr (III) on hydration, microstructure of magnesium phosphate cement, and leaching toxicity evaluation

The pollution caused by chromium and its compounds has caused severe harm to the environment, and waste stabilization/solidification containing these contaminants by magnesium phosphate cement (MPC) is one of the best ways to address this problem. If the mechanism between Cr³⁺ and MPC can be understood, it will significantly improve the latter's solidification performance with respect to ameliorating Cr pollution. In this paper, the compressive strength, microstructure, pH in the process of sample hydration, and leaching toxicity of solidified forms were studied by adding various amounts of Cr³⁺ into MPC. The setting time of MPC decreased at first and then increased as the Cr³⁺ concentration increased. The added Cr³⁺ reacted with the phosphate ions to form mineral phases, which changed the MPC matrix structure. The matrix's compressive strength was higher when the M/P ratio (MgO/KH₂PO₄ mass ratio) was smaller. When the concentration of Cr³⁺ was constant, and the M/P ratio was low (< 4:1), the matrix's compressive strength increased as the M/P ratio increased. The presence of Cr³⁺ changed the system's pH and affected the hydration products' morphology; this trend strengthened as the Cr³⁺ concentration increased. The highest leaching concentration of Cr³⁺ was 0.255 mg/L, and the concentration decreased as the M/P ratio decreased. During solidification, the appropriate proportion of MPC can be selected according to the concentration of Cr³⁺ to achieve better solidification performance.