Probing into the Triggering Effects of Zinc Presence on the Mineral Formation, Hydration Evolution, and Mechanical Properties of Fluorine-Bearing Clinker

The foreign impurity ions, especially fluorine and zinc, are introduced simultaneously when hazardous waste fluorine-containing sludge is utilized as a partial alternative raw material to prepare low-carbon cement clinker; however, the complex influences of the presence of fluorine and zinc on the properties of silicate cement clinker remain unclear, and related studies on the underlying mechanisms are rare. The findings revealed that the presence of ZnO triggers synergistic mineralization effects on the formation of clinker minerals, resulting in quite low free lime levels and high immobilization rates of F and Zn ions. It also assists the formation of brownmillerite and alite but inhibits that of tricalcium aluminate and belite. The copresence of ZnO and CaF₂ stabilizes R-type alite, and the microstructure of the final clinker changes, with new threadlike defects appearing on the surface of mineral phases. F ions tend to distribute mainly in the silicate phases, whereas Zn ions tend to distribute mainly in the molten liquid phases. Most F^- in CaF₂ potentially enters the alite by replacing O^2- and binding with Ca²⁺, while most Zn²⁺ potentially enters the brownmillerite by replacing Fe³⁺ and binding with O²⁺. The copresence of CaF₂ and ZnO accelerates the initial hydration reaction of the prepared cement, prolongs the induction period, and delays the acceleration of hydration. Besides, while the single presence of CaF₂ suppresses the strength development of cement before 3 days, the copresence of CaF₂ and ZnO sharply overwhelms the development before 7 days, but significantly elevates the strength at 28 days (the highest being 120.6 MPa compared to the Blank at 90.5 MPa).