Unveiling substitution preference of chromium ions in sulphoaluminate cement clinker phases

Abstract

Understanding the doping behaviors of impurity ions in mineralogical compositions is of great significance for optimizing the quality of sulphoaluminate cement (SAC) clinker as well as its performance in engineering utilizations. The key issue is determining the exact existing state of these impurity ions. Here, combined with experiments and density functional theoretical (DFT) simulations, we systematically studied the substitution preference of chromium in the three main composed phases of SAC clinker, i. e, C4A3S̅, C2S and C4AF. The formation energies indicate that Cr ions prefer to enter C4AF by substituting Fe ions followed by incorporating into C4A3S̅ with replacement of S ions. The results from X-ray diffractions of single phase show the successful incorporations of Cr ions into both individual mineral, C4AF and C4A3S̅. The dominant content of C4A3S̅ in SAC clinker is well illustrated from Rietveld refinement. Further proof of the doping preference is from the difference in relative amounts of Cr(III) and Cr(VI) in SAC clinker samples with lower dosages. More in-depth analyses from both electron contributions and bond order-bond length distributions show that the substitution preference follows the “electronic structure matching” principle, which ensures the effective substitution by forming stronger bonds between guest ions and oxygen atoms. The doping preference provides a possibility of controlling the valence state of Cr ions by tuning the mineralogical compositions of SAC clinker, thus is essential in guiding the synthesis of SAC-based materials by utilizing Cr-bearing solid waste.