Environmentally benign modification of fluorogypsum with sodium molybdate based on the precipitation-dissolution...

Fluorogypsum (FG, CaSO₄-II) originates during hydrofluoric acid production, has low hydration reactivity and contains high levels of free fluoride ions (F^-), posing risks to soil and water quality. To enable the secure application of FG, this study evaluates sodium molybdate (Na₂MoO₄) as a modifier to enhance FG's hydration reactivity and immobilize F^-. A novel method was employed, in which the FG paste was centrifuged at different time points prior to initial setting. The supernatants were then analyzed for calcium ions (Ca²⁺), molybdate ions (MoO₄^2-), sulfate ions (SO₄^2-), and F^-, enabling real-time tracking of ion behavior and revealing the mechanism of F^- immobilization. Based on these data, we provide a systematic explanation of anhydrite (AH) (CaSO₄) hydration and hardening within a precipitation-dissolution equilibrium framework. Results show that MoO₄^2- consume Ca²⁺ by precipitation transformation, driving the dissolution equilibrium of anhydrite to shift to the right. This induces AH to dissolve until reaching the solubility product constant (Ksp) of dihydrate gypsum (DH) (CaSO₄·2H₂O) without exceeding that of AH, thereby triggering DH crystallization in a positive feedback loop. Meanwhile, the formation of insoluble calcium molybdate provides heterogeneous nucleation sites that accelerate the growth of DH. With 0.7% Na₂MoO₄, the 28-day strength reaches 49.1 MPa, approximately 10 times that of untreated FG. Under extreme pH conditions and high temperature, this approach suppresses over 90% of F^- leaching by means of chemical fixation and physical encapsulation. This modification offers a solution for stabilizing hazardous FG and converting it into safe outdoor building materials.