Review of 3D-Printed Cementitious and Geopolymer Permanent Formwork for Concrete Structures

Three-dimensional concrete printing (3DCP) has emerged as a promising technology for developing stay-in-place cementitious and geopolymer permanent formwork systems. This review critically synthesizes current knowledge on printable materials, printability requirements, structural performance, interfacial bonding, durability, sustainability, and microstructural characteristics. Existing studies demonstrate that 3D-printed permanent formworks can contribute structurally by enhancing the axial, eccentric, and flexural performance of composite members through confinement, composite action, and effective load transfer between the printed shell and cast concrete core. Interfacial bonding is identified as a key factor governing structural performance, with layer height, interface geometry, surface roughness, and casting interval significantly affecting tensile and shear bond strengths. Microstructural studies further highlight the roles of pore characteristics, hydration development, and mechanical interlocking in interface formation. Although cementitious systems dominate current research, geopolymer permanent formworks remain comparatively underexplored despite their sustainability potential. Moreover, durability studies, life-cycle assessments, and direct comparisons between cementitious and geopolymer systems remain scarce. This review identifies key research gaps and outlines future directions toward performance-based design methodologies and the broader structural implementation of 3D-printed permanent formwork systems.