Purified phosphogypsum as a cement set retarder: Mechanical performance and phase evolution quantified by rietveld refinement

This study investigates the use of phosphogypsum, a by-product of phosphoric acid production, as a cement set retarder through clinker cement modification. This work highlights the superiority of purified phosphogypsum over untreated forms, demonstrating its potential as a high-performance and sustainable cement retarder with properties approaching those of commercial gypsum. Phosphogypsum was treated with 50 wt% sulfuric acid at a phosphogypsum-to-acid mass ratio of 0.5, at 60 °C for 2 h under stirring, yielding purified phosphogypsum (PG.P) with a CaSO 4 .2H 2 O purity of 98%. A comparative study with commercial gypsum was conducted to evaluate the effects on cement mechanical and microstructural behaviour. Compressive strength and deformation properties were assessed for water-to-cement ratios of 0.25–0.6 and hydration times of 1, 7, 14, 28 and 90 days. Rietveld refinement of X-ray diffraction data provided mass fraction percentages of all phases, showing rapid hydration with early formation of C–S–H and ettringite after 1 day, alongside portlandite and minor aluminate products. Unhydrated alite and belite were still present, reflecting different hydration rates. After 28 days, continued clinker consumption increased C–S–H as the dominant phase, with higher mass fractions of portlandite and carbonate, while ettringite remained stable and katoite stayed minor. The results indicate that clinker cement modified with purified phosphogypsum achieves a compressive strength of 17 MPa after 90 days (W/C=0.35), alongside a Young's modulus of 1.4 GPa, comparable to commercial gypsum, confirming its effectiveness as a substitute without compromising mechanical or microstructural performance.