Effects of Phosphogypsum Substitution on Early-Age Macro- and Microproperties of GGBFS-Based Alkali-Activated Binders

Sodium sulfate ( Na 2 SO 4 ), a naturally derived activator, can be used to activate ground granulated blast-furnace slag (GGBFS) to prepare low-carbon cementitious materials. However, the extremely slow development of early strength remains a key limitation. To address this limitation, this study investigated the effect of partially replacing GGBFS (up to 15%) with phosphogypsum (PG), an industrial by-product, under seawater mixing conditions. Results showed that incorporating 5%–10% PG further optimized the hydration process in pastes by supplying additional Ca 2 + and SO 4 2 − ions, improving the early mechanical properties of the binder. This was attributed to the promotion of ettringite nucleation, the dissolution of aluminum phase, and the formation of key hydration products, including ettringite, calcium aluminosilicate hydrate [C-(A)-S-H] gel, and hydrotalcite. Moreover, PG induces an Al(IV)→Al(VI) phase transition in the hydration products, promoting the growth of larger ettringite crystals and thereby improving structural stability. Although excessive PG content (15%) hindered early strength development, it provided notable strength compensation at later ages. These findings provide a sustainable strategy for enhancing the performance of Na 2 SO 4 -activated GGBFS in marine and coastal construction.