Thermodynamic modeling of Portland cement hydration containing phosphogypsum impurities

ABSTRACT

Phosphogypsum (PG) is one of the main residues from the production of phosphate fertilizers. It is mostly composed of calcium sulphate dihydrate (CaSO₄·2H₂O) and contains impurities such as sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄). Due to the environmental risk associated with its generation and chemical composition, one management strategy is to use it as a supplementary source of sulfate in cement production. However, the impurities in FG represent an obstacle, since there is still no clarity on its long-term behavior in cement hydration. Therefore, the effects of PG impurities on cement hydration were analyzed using thermodynamic modeling. CP I, CP II and CP V cement pastes with up to 5% H₂SO₄ and H₃PO₄ were simulated. The GEM-Selektor geochemical modeling program based on Gibbs energy minimization was used to analyze the phase composition over 1000 days. The analysis shows that the increase in H₂SO₄ affected the formation of ettringite, monosulfate and portlandite, while the addition of H₃PO₄ delayed the formation of portlandite and C-S-H. This effect is due to the reaction of phosphate ions with calcium to form calcium phosphate, especially in CP II. Therefore, thermodynamic modeling can help with the long-term experimental technical limitations of cementitious materials containing PG.

Keywords:
Phosphogypsum; impurities; cement; hydration; thermodynamic modeling