Surface moisture governs inter-stripe pore-band formation and mechanical anisotropy in 3D-printed geopolymer concrete

Inter-stripe interfaces in extrusion-based 3D concrete printing are a dominant source of pore defects, strength loss, and mechanical anisotropy. Although the printing interval is widely used to describe interface quality, it is not an intrinsic descriptor because the substrate condition evolves with ambient drying and early-age reaction. This study identifies surface moisture content as a transferable governing state variable and establishes a process–structure–property relationship in which surface moisture influences the formation of interfacial pores, which in turn governs the mechanical degradation and anisotropy of 3D-printed geopolymer concrete. Double-filament specimens were produced with controlled printing intervals (0, 0.5, 1.0, and 3.0 h) under a regulated environment. The surface moisture of the first filament was measured (wt%) immediately before depositing the adjacent filament. Seven-day compressive and splitting tensile strengths were tested along multiple loading directions to quantify anisotropy. X-ray computed tomography (X-CT) was used to reconstruct the pore network and extract 3D porosity, pore size composition, pore morphology, and pore orientation statistics. Across-interface 2D areal-porosity profiles were parameterized by a Gaussian form to quantify the severity and spatial extent of the inter-stripe porous band. Surface moisture decreased monotonically with printing interval, accompanied by increased 3D porosity and development of a continuous interfacial porous band. Strengths showed strong moisture sensitivity and increasing anisotropy, which is consistent with preferential pore alignment and interface-localized defects. A reduced-order moisture-based framework and a two-regime mechanism with a critical moisture threshold are proposed to support transferable process control.