Untreated phosphogypsum in an acetic-acid-activated geopolymer–cement hybrid: a low-energy, no-calcination route

A hybrid binder coupling acid-activated geopolymerization with Portland-cement hydration was formulated using 45 wt.% untreated phosphogypsum (PG), 15 wt.% metakaolin, and 40 wt.% cement, activated by acetic acid (0–5 wt.%). The optimal composition (4 wt.% acid) achieved a 28-day compressive strength of 19.03 MPa under standard curing conditions (20 °C, relative humidity (RH) > 95%). While the final setting time was 1918 min at ambient temperature, it decreased to 163 min at 40–60 °C, indicating effective acceleration at elevated temperatures. This study demonstrates the feasibility of utilizing 45 wt.% raw PG in hybrid binders without calcination or chemical pre-treatment, providing a direct utilization route with application-relevant strength.