Hardened Performance of 3D-Printed Geopolymer Mortars: A Review of Mechanical Properties, Durability, Sustainability, and Practical Implementation

3D-printed geopolymer mortars (3DPGPMs) are emerging as low-carbon construction materials that combine digital fabrication with alkali-activated binder technology. However, their hardened performance remains difficult to assess because it is controlled not only by geopolymer chemistry but also by printing parameters, rheological evolution, curing conditions, interlayer bonding, pore structure, and loading direction. This review critically examines the current literature on extrusion-based 3DPGPMs, with emphasis on mechanical properties, durability, sustainability, standardization, and practical implementation. The reviewed studies show that precursor type, activator system, aggregate/binder ratio, additives, printing conditions, and curing regime strongly influence compressive, tensile, flexural, interlayer bond, and anisotropic mechanical responses. Durability performance is also governed by the coupled effects of matrix chemistry and printing-induced features, including interlayer voids, directional pore networks, weak interfaces, and transport pathways that may affect shrinkage, water absorption, chloride penetration, carbonation, acid and sulphate resistance, freeze–thaw response, and elevated-temperature behavior. From a sustainability perspective, the environmental benefits of 3DPGPMs are conditional and depend on activator production, precursor availability, curing demand, transport distance, life-cycle assessment boundaries, and field-scale implementation conditions. The review identifies that the main knowledge gap is the limited availability of integrated datasets linking fresh-state rheology, interlayer quality, multi-scale porosity, mechanical anisotropy, durability indicators, and structural-scale validation. Future research should therefore prioritize standardized reporting, performance-based acceptance criteria, long-term exposure testing, field-scale validation, and predictive material–process–durability models. Overall, this review provides a hardened-performance-oriented synthesis to support the development of reliable, durable, and sustainable 3DPGPMs for construction applications.