Effect of Cr on the Mineral Structure and Composition of Cement Clinker and Its Solidification Behavior

In order to reveal the solidification behavior of Cr in the cement clinker mineral phase, ²⁹Si magic-angle spinning nuclear magnetic resonance, X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray spectroscopy techniques were used to analyze the morphology and composition of the cement clinker mineral phase doped with Cr. The results showed that the addition of Cr did not change the chemical environment of ²⁹Si in the clinker mineral phase, and it was still an isolated silicon-oxygen tetrahedron. Cr affected the orientation of the silicon-oxygen tetrahedron and the coordination number of calcium, leading to the formation of defects in the crystal structure of the clinker mineral phase, by replacing Ca²⁺ into the mineral phase lattice to form a new mineral phase Ca₃Cr₂(SiO₄)₃. Cr acted as a stabilizer for the formation of β-C₂S in the clinker calcination. As the amount of Cr increased, the relative content of C₃S decreased and the relative content of C₂S increased. Further, Cr easily dissolved in C₂S, while it was not found in C₃S. This study is conducive to further research on the mechanism of heavy metal solidification in cement clinker. Furthermore, it is important to evaluate the environmental risk of heavy metals in the process of sludge disposal through cement kiln and promote the utilization of sludge resources and the sustainable development of the cement industry.