Phosphogypsum Additive as Shrinkage-Reducing Agent in Ordinary Portland Cement-Based Mortar

Phosphogypsum, a by-product of phosphate fertilizer production, has shown promising potential as a shrinkage-reducing additive in ordinary Portland cement-based mortar. The incorporation of low PG dosages (≤6%) enhances early hydration, slightly increases the hydration peak temperature, and promotes the formation of additional ettringite and bound-water-rich hydrates, contributing to improved early performance. PG also substantially reduces drying shrinkage—from 0.0397 mm/m (reference) to −0.1600 mm/m at 15% PG—through ettringite-induced expansion and pore refinement, demonstrating its effectiveness as a shrinkage-reducing agent. Thermal analysis (XRD/TG–DTA) confirms that PG modifies hydration chemistry by increasing low-temperature dehydration while reducing portlandite and carbonate phase formation due to clinker dilution. As a result, Ca(OH)2 content decreases from 11.89 wt% for the reference mix to 8.55 wt% at 15% PG. However, higher PG levels (>9%) negatively affect strength: at 15% PG, flexural and compressive strength decrease by 38% and 47%, respectively, due to clinker dilution, excess ettringite, and unreacted gypsum. All compositions maintained durability levels corresponding to roughly 300–450 freeze–thaw cycles. Overall, PG effectively reduces shrinkage and alters hydration behavior while offering environmental benefits through industrial waste valorization. Nevertheless, high replacement levels compromise mechanical performance, underscoring the importance of optimizing PG dosage to balance shrinkage control, strength, and sustainability.