Revealing the substitution preference of zinc in ordinary Portland cement clinker phases: A study from experiments and DFT calculations.

Ordinary Portland cement (OPC) clinker mainly consist four minerals, tricalcium silicate (C3S), dicalcium silicate (C2S,), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). To learn the doping behaviors of Zn in OPC clinker, a series of samples were prepared by calcinating the mixtures of CaCO3, SiO2, Al2O3, Fe2O3, and ZnO. Our results from energy-dispersive spectroscopy, X-ray diffraction and density functional theoretical simulations show that a small amount of ZnO enter C3S and C2S by replacing Ca ions while most incorporate into C4AF by substituting Fe atoms, resulting in a decrease of C3A in OPC as dosage increases. Further analyses from partial density of states and distributions of bond order-bond length indicate that the doping preference can be ascribed to the similar electron contributions and small structure distortions between host and guest ions. Unlike the strong Fe‒O bond, the newly formed Zn‒O is much weaker. The weak Zn‒O may be responsible for the limited solubility of Zn in C4AF. These results provide a possibility of increasing solubility of Zn in OPC clinker by increasing the contents of C3A and C4AF, thus will be very meaningful in the synthesis of OPC clinker by utilizing Zn-bearing alternative raw materials.