The Role of Nickel in the Polymorphs, Microstructure, and Hydration Evolutions of Fluorite-Bearing Tricalcium Silicate

Cement kiln coprocessing technology is a key measure for achieving low-carbon cement production. The present research proves that wasted fluorine-containing sludge is a promising alternative for raw meals. However, fluorine and nickel in the sludge can uncontrollably change the hydration properties of the produced cement. Therefore, further discussion is required on the detailed role of nickel on the polymorphs, microstructure, and hydration evolutions of fluorite-bearing tricalcium silicate. The results revealed that Ni₂O₃ triggered synergistic mineralization effects with CaF₂, leading to quite low f-CaO contents and a high immobilization rate of F and Ni ions. Ni₂O₃ doped with 1.0 wt % CaF₂ increased the cell symmetry of C₃S to M3 type while that with 2.0 wt % CaF₂ reduced its symmetry. The diffraction positions for C₃S polymorphism shifted to a larger angle with more doping of CaF₂ and Ni₂O₃, and its microstructure was rebuilt with new thread-like defects that appeared on the C₃S surface. In C₃S cell regions, F^- in CaF₂ possibly replaced the O^2- of the Ca-O bond and connected with Ca²⁺, and Ni³⁺ in Ni₂O₃ was most likely to replace Ca²⁺ of the Ca-O bond and connected with the O^2-. Ni₂O₃ accelerated the initial hydration of CaF₂-bearing C₃S, advanced the acceleration period, while severely inhibiting the later hydration, and distinctly reduced the hydration rates during the entire process. The lower doping addition of CaF₂ and Ni₂O₃ expedited the release of Ca ions and reduced the release of Si ions, while a higher doping addition presented the opposite effects. The findings are expected to offer a prospect for controlling cement hydration by doping selected impurities into cement clinker phases during cement kiln coprocessing of multisource solid wastes.