Evolution of extrusion, buildability, and mechanical properties of phosphogypsum-based 3D printing materials under admixture regulation

Phosphogypsum (PG), a bulk industrial by-product of phosphate fertilizer production, poses risks of soil and groundwater contamination because of extensive stockpiling. Previous studies demonstrated that, owing to its relatively high early strength, dimensional stability, and low density, pretreated PG has considerable potential for construction-scale extrusion-based 3D printing. This study systematically investigated the extrusion behavior, buildability, mechanical properties, and microstructural evolution of PG-based materials under admixture regulation using two representative routes: rapid-setting integrated mixing–stirring–extrusion and premixed extrusion. Three water reducers (PCE, MS, and NS) were evaluated for the rapid-setting route, while three retarders (CA, SQ, and SG) were examined for the premixed route. Setting time and rheological tests characterized fresh-state behavior; compressive and flexural strength tests evaluated mechanical performance; and XRD, TG–DTG, N₂ adsorption, SEM, and image analysis examined hydration products, microstructure, and printability. In the rapid-setting route, PCE, MS, and NS exhibited generally comparable performance, with PCE providing a favorable overall balance of extrusion continuity, structural stability, and strength. Greater differences occurred among the retarder systems. SQ provided a suitable balance between workable time and post-deposition structural build-up, whereas CA and SG exhibited different limitations in shape retention or extrusion stability. Overall, PG was applicable to both routes, although premixed extrusion required more precise regulation because of its intrinsically rapid setting. The results support route-dependent admixture selection and provide guidance for process optimization and engineering applications.