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.