The Impact of Post-Printing Hydration in NaCl Solution on the Properties of Binder Jet 3D-Printed Calcium Sulfate and Its Converted Hydroxyapatite

Binder jet 3D printing of calcium sulfate-based materials combined with phase transformation offers a versatile route for fabricating customized bone grafts; however, controlling the transformation process remains a key challenge. This study investigates the effect of post-printing hydration in sodium chloride (NaCl) solutions on the phase transformation, dimension, and compressive properties of binder jet-printed calcium sulfate (3DPCaS) toward hydroxyapatite (3DPHA) formation. The as-printed 3DPCaS primarily consisted of bassanite with minor gypsum, which progressively transformed into gypsum upon immersion in NaCl solutions of varying concentrations (1–5 M) and durations (2–30 min). Increased immersion time and moderate NaCl concentrations (2–4 M) promoted gypsum formation without inducing dimensional instability. Subsequent transformation in phosphate solution produced 3DPHA with high hydroxyapatite (HA) purity, reaching 100% conversion. Microstructural analysis revealed recrystallized, plate-like gypsum crystals that served as favorable templates for HA nucleation. The resulting 3DPHA exhibited enhanced specific modulus (up to 274.9 MPa.m³/kg) and specific strength (up to 7.5 MPa.m³/kg). The optimal condition, immersion in 4 M NaCl solution for 30 min, achieved a balance between complete HA transformation, mechanical enhancement, and dimensional stability. Controlled ionic hydration thus represents a simple, low-cost, and effective strategy for improving properties of 3DPHA bone grafts.