Hemihydrate phosphogypsum as a waste-derived sulfate source for low-carbon supersulfated cement: Hydration regulation...

Hemihydrate phosphogypsum (HPG) can serve as a waste-derived sulfate source in low-clinker supersulfated cement (SSC), expanding phosphogypsum utilization while reducing demand for natural gypsum. This study examines how HPG content governs the temporal evolution of sulfate supply and how this evolution relates to hydration, pore structure, and long-term strength in SSC co-activated with ferroaluminate clinker (FAC), carbide slag (CS), and sodium silicate (SS). Mixtures containing 5-25 wt% HPG were evaluated for compressive strength. The 10, 15, and 20 wt% HPG mixtures were further analyzed by hydration kinetics, pore-solution chemistry, quantitative phase analysis, and microstructural characterization. Composite-activated SSC developed 10.23-14.36 MPa at 1 d, corresponding to a 205.4-328.7% increase over the CS-activated reference. The 15 and 20 wt% HPG mixtures reached 65.54 and 63.76 MPa at 28 d and continued to increase to 81.56 and 78.79 MPa at 270 d, respectively. At 10 wt% HPG, sulfate depletion after early hydration constrained continued ettringite formation and late-age pore refinement. At 20 wt% HPG, sustained reaction of ground granulated blast-furnace slag (GGBS) generated slightly more ettringite and C-(A)-S-H, but a larger fraction of pores >50 nm remained, diminishing the strength efficiency of the additional hydration products. The 15 wt% HPG mixture provided the most favorable coupling of sulfate supply, hydrate accumulation, pore refinement, and strength, and achieved the lowest 28 d energy- and carbon-intensity indices among the investigated mixtures.