Powder-bed 3D printing of geopolymers offers significant potential for sustainable construction; however, the particle-fluid interactions governing the mechanical performance and dimensional accuracy remain poorly understood. Unlike extrusion-based systems governed by paste rheology, powder-bed systems are controlled by particle packing, activator penetration, and saturation. These fundamentally different material-process currently lack a quantitative optimization framework. This study systematically investigates key material and process parameters, including particle packing density (PD), specific surface area (SSA), layer height (LH), feed height (FH), and activator deposition waveform. Three powder-bed gradations - gap graded (1:3 and 1:4.5), and well graded (1:3) were printed using selective binder activation process. Wet packing density, contact angle, and surface roughness were used for characterisation. One of the key innovations of this work is the introduction of the ratio of activator flow density to packing density (FD/PD) as quantitative metric. It is first proposed for geopolymer-based powder-bed 3D printing to characterize the particle-fluid interactions and identify optimal saturation levels. In addition, the FD/SSA ratio is used to describe the relationship between activator availability and particle surface area during strength development. Increasing the FD/SSA ratio from 44.4 × 10⁻⁵ to 79.6 × 10⁻⁵ g/mm² (with constant SSA) improved 28-day compressive strength from approximately 2 MPa to 20 MPa, a trend contrary to conventional cast geopolymers, where increased activator content typically reduces strength. An FD/PD ratio of approximately 0.86 was identified as the critical saturation threshold beyond which sufficient activator penetration ensures effective geopolymerization. Microstructural observations confirmed that adequate saturation promotes the formation of continuous potassium aluminosilicate hydrate (K-A-S-H) networks bridging adjacent particles, thereby enhancing mechanical performance. These findings provide fundamental insights and practical guidelines for optimizing geopolymer-based powder-bed 3D printing, with direct relevance to application-oriented construction components such as façade panels, prefabricated elements, and customized structural components.