Abstract
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.
Highlights
As of the end of June 2018, the amount of municipal and industrial sludge in China reached90 million tons, with a utilization rate of only 30%, and random landfills have seriously polluted the environment
In the 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR) study, the chemical environment of 29 Si is represented by Qn, where n (0–4) is the number of oxygen atoms shared by each silicon–oxygen tetrahedron and the adjacent silicon–oxygen tetrahedral
While Q0 represents an island-shaped [SiO4 ] tetrahedron with a chemical shift in the range of −68 to −79 ppm, Q1 represents the silicon–oxygen tetrahedron connected with one silicon–oxygen tetrahedron, that is, the silicon–oxygen tetrahedron is at the end of the straight chain in the dimer or hypermer, and the chemical shift is in the range of −76~−82 ppm
Summary
As of the end of June 2018, the amount of municipal and industrial sludge in China reached. By utilizing its advantages such as high temperature in the kiln, long residence time of materials, and stable incineration status, dangerous substances such as heavy metal ions in the sludge can be solidified in the mineral phase of cement clinker [1,2,3]. Heavy metal ions will form an enriched phase due to the crystal structure defect of the cement clinker mineral phase itself, and will be present in the empty interstitial spaces of the mineral phase [4,5,6,7,8]. The research on the influence of heavy metal ions on the calcination degree of clinker and the mineral phase structure of clinker has gained attention. The aim is to provide a new theoretical basis for the high efficiency and safety of solid waste disposal in cement kilns
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