Effect of soluble phosphate, fluoride, and pH in Brazilian phosphogypsum used as setting retarder on Portland cement hydration

Phosphogypsum (PG) is a residue from the fertilizer industry used by Portland cement companies as a source of calcium sulfate to control the setting time during the hydration (or hardening process). However, certain impurities can alter the cement hydration kinetics. Thus, this paper aims to analyze the soluble phosphate, fluoride, and pH effects of Brazilian phosphogypsum used as a setting retarder on the hydration of Portland cement. Different sources of calcium sulfate were analyzed: natural gypsum (NG), alkaline phosphogypsum (PGA), and acid phosphogypsum (PGB). The materials were characterized physically and chemically, and Portland cement pastes were assessed by isothermal calorimetry, DRX, and TGA to identify the main hydrated products. Complementary compressive strength was evaluated in mortars. Furthermore, chemically soluble fluoride and phosphate contents were added to cement with NG, whose hydration was monitored by isothermal calorimetry. Portland cement with acid PG, soluble phosphates and fluorides presented hydration delays. However, soluble phosphates showed a more significant delay effect on cement than soluble fluorides. Cement setting times were extended with P2O5 soluble from 0.33% to 1.32%. Also, there is no relevant difference between the hydration products and compressive strength of alkaline PG cement and NG cement. Thus, the results suggest that low phosphate – fluoride and alkaline PGs may substitute NG in Portland cement.