Effects of sulfate supply and P/F impurities in phosphogypsum on the hydration, microstructure, mechanical properties of ordinary portland cement

Phosphogypsum (PG) is a promising sulfate source for preparing Ordinary Portland Cement (OPC); However, its soluble P/F-bearing impurities can affect hydration and reduce cement performance. To distinguish between the effects of sulfates and P/F impurities on cement performance, natural gypsum, Na 3 PO 4 , NaF and raw PG were used to establish single-factor reference systems and analyzed through experimental characterization combined with thermodynamic calculations and molecular dynamics simulations. The results indicated that sulfate mainly regulated aluminate hydration and promoted ettringite(AFt) formation. Meanwhile, PO 4 3- preferentially reacted with Ca 2+ to form stable Ca-P phases, reducing the Ca 2+ activity and suppressing the early nucleation and growth of Calcium Silicate Hydrate (C-S-H). At the early stages, F - formed CaF 2 and subsequently enriched the C-S-H interface. In raw PG, sulfate supply occurred simultaneously with impurity-induced precipitation/interfacial interactions, resulting in AFt enrichment but insufficient C-S-H continuity, pore-structure deterioration, and strength degradation. Furthermore, multiscale analyses indicated that P/F species underwent redistribution, involving precipitation, interfacial enrichment, and competitive interactions with other ionic species, during hydration. These findings establish a cross-scale mechanism that links the key components of PG to hydrate evolution and cement macroscopic performance, providing a theoretical basis for the controlled utilization of PG in cementitious materials.