One-Part vs. Two-Part Alkali-Activated GGBFS Mortars: Influence of Water-to-Binder Ratio on Extrusion-Based 3D Concrete Printing

3D concrete printing (3DCP) offers a transformative approach to industrialized construction and housing solutions. However, its reliance on high Portland cement (PC) mixes raises sustainability concerns. This study explores alkali-activated materials (AAM) as low-carbon alternatives, leveraging an industrial by-product (ground granulated blast furnace slag—GGBFS) to enhance environmental performance of 3DCP mortars. The novelty of this study lies in demonstrating that, despite equivalent activation parameters, one-part (“just-add-water”) and two-part AAM systems exhibit substantially different fresh-state, rheological, and printing behavior in extrusion-based 3DCP—an activation-method-dependent divergence that, to the best of the authors’ knowledge, has not been previously reported. Rheological behavior (dynamic yield stress, plastic viscosity, and structural build-up), flow table spread evolution, hydration kinetics (via isothermal calorimetry), and mechanical performance (cast and extruded specimens of selected mixes) were systematically evaluated. Results reveal that one-part mixes exhibit greater flowability when compared to two-part mixes (140–239 mm vs. 135–210 mm), inferior dynamic yield stress (35.94–93.61 Pa vs. 52.91–152.30 Pa), and slower structural build-up, while maintaining comparable compressive and flexural strengths as the equivalent two-part mortars. These findings demonstrate the potential viability of AAM for extrusion-based 3DCP and highlight the critical influence of activation method and water content on extrudability and performance.