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 Ni 2 O 3 triggered synergistic mineralization effects with CaF 2, leading to quite low f-CaO contents and a high immobilization rate of F and Ni ions. Ni 2 O 3 doped with 1.0 wt % CaF 2 increased the cell symmetry of C 3 S to M3 type while that with 2.0 wt % CaF 2 reduced its symmetry. The diffraction positions for C 3 S polymorphism shifted to a larger angle with more doping of CaF 2 and Ni 2 O 3, and its microstructure was rebuilt with new thread-like defects that appeared on the C 3 S surface. In C 3 S cell regions, F – in CaF 2 possibly replaced the O 2– of the Ca–O bond and connected with Ca 2+, and Ni 3+ in Ni 2 O 3 was most likely to replace Ca 2+ of the Ca–O bond and connected with the O 2– . Ni 2 O 3 accelerated the initial hydration of CaF 2 -bearing C 3 S, 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 CaF 2 and Ni 2 O 3 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.