Experimental and thermodynamic insights into activator-dependent hydration and structure–property evolution of multiphase phosphogypsum

Multiphase phosphogypsum (MPPG) contains rapidly hydrating hemihydrate and slowly reacting anhydrite, causing asynchronous hydration and incomplete structural development. This study compared Na 2 SO 4 , K 2 SO 4 , calcium sulfate whisker (CSW), and Ca(OH) 2 under fixed laboratory conditions. TG–DTG/XRF analysis estimated 23.6 wt% gypsum, 14.1 wt% hemihydrate, and 54.4 wt% anhydrite. Na 2 SO 4 and K 2 SO 4 shortened setting and increased gypsum supersaturation, whereas Ca(OH) 2 delayed early hydration despite a favorable equilibrium precipitation potential, indicating kinetic or interfacial control. Na 2 SO 4 gave the most favorable tested response, increasing 28 d compressive strength by 26.6%, reducing porosity and water absorption to 33.8% and 10.1%, and raising the softening coefficient to 0.368. CSW provided moderate reinforcement, while K 2 SO 4 produced K 2 SO 4 -rich surface deposits. All softening coefficients remained below 0.40, indicating persistent water sensitivity. These results provide a laboratory-scale basis for dosage optimization and durability assessment.