Thermodynamic Stability and Dissolution of Calcium Fluoride of Phosphogypsum Solidified in Cement-Based Cementing Materials in Bicarbonate Solution

Abstract

Cement-based cementitious materials present an alkaline environment and mainly exist in the form of Ca(OH)₂. Phosphogypsum, as a retarder of cementitious materials, is often added to cement instead of gypsum, and the fluorine by-product of phosphogypsum is also fixed in the form of CaF₂. These cement-based materials often serve in environments containing carbonate groundwater, which changes the stability of solidified CaF₂. This paper studied the solid and liquid changes in Ca(OH)₂-CaF₂, Ca(OH)₂-CaF₂-HCO₃, CaF₂-HCO₃-OH systems, and therefore studied the solubility of the by-product CaF₂ of phosphogypsum in the groundwater containing HCO₃^- anion after curing in the cement-based cementitious system through experiments and thermodynamic equation simulation. The results showed that the dissolved Ca²⁺ of Ca(OH)₂ led to the dissolution of CaF₂, which led to a significant reduction in the F^- anion concentration. However, with the increase of HCO₃^- concentration in groundwater, the F^- anion concentration of Ca(OH)₂-CaF₂-HCO₃ system increased, and the main product formed in the solid phase was calcium carbonate; The dissolution of F^- anions in CaF₂-HCO₃-OH system also increased with the increase of HCO₃^- concentration. Therefore, the influence of HCO₃^- in groundwater should be considered when using phosphogypsum instead of gypsum as a retarder in cement-based cementitious materials.