Dosage-Dependent Regulation of β-Hemihydrate Phosphogypsum by Calcium Sulfate Whiskers: Workability, Dry–Wet Strength and Pore Structure

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The use of β-hemihydrate phosphogypsum (β-HPG) in building materials is limited by insufficient strength and water sensitivity. This study evaluated calcium sulfate whiskers (CSW) as a modifier at 0–4 wt% relative to β-HPG mass under a fixed water-to-gypsum ratio of 0.58. Workability, dry flexural and compressive strengths, wet compressive strength, and water resistance were assessed alongside isothermal calorimetry, X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. The 2 wt% mixture showed the highest mean dry flexural and compressive strengths among the tested formulations, reaching 5.95 and 22.0 MPa, respectively, approximately 49% and 23% higher than the control. Wet compressive strength after 24 h immersion increased from 7.4 to approximately 8.5 MPa at this dosage. The corresponding paste spread diameter was 155 mm. Higher dosages further reduced mercury-accessible porosity and water absorption without exceeding these mean strengths. The higher wet strengths were accompanied by lower softening-coefficient estimates. CSW advanced the hydration heat-flow peak, while dihydrate gypsum remained the dominant crystalline phase. This integrated assessment distinguishes pore-volume reduction from mechanical improvement and absolute wet strength from relative strength retention. The findings provide an experimental basis for selecting CSW-modified phosphogypsum formulations according to strength, workability, and moisture-exposure requirements.