From cold joints to engineered transition zones: automated inline electrostatic SCM deposition in extrusion-based 3D concrete printing

Weak interlayer bonding in extrusion-based 3D concrete printing (3DCP) is a persistent structural limitation arising from sequential layer deposition, which creates porous “cold joint” zones susceptible to cracking and anisotropic failure. This study presents a novel automated electrostatic dry-powder deposition system that applies supplementary cementitious materials (SCMs) onto each extruded layer without moisture addition, process interruption or mix-design modification. A corona-discharge spray gun mounted 60 mm behind the extrusion nozzle delivered Class F fly ash (FA) or ground granulated blast-furnace slag (GGBS) at a calibrated dosage of 100 ± 10 g/m2 across all printed layers. Performance was assessed through flexural and compressive strength at 7, 28 and 56 days, ultrasonic pulse velocity (UPV), 56-day drying shrinkage, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (EDS). The study is positioned as a comparative evaluation of relative interlayer improvement under fixed mix and printing conditions. FA-treated specimens achieved the highest long-term gains, with 13.8% and 41.1% increases in flexural and compressive strength, respectively, and a 30.4% rise in UPV at 56 days, accompanied by a 33.7% increase in drying shrinkage. GGBS treatment yielded moderate mechanical improvements alongside a 25.7% reduction in drying shrinkage. EDS analysis indicated spatially localized reductions in Ca/Si at the treated interfaces relative to the bulk matrix, consistent with localized interfacial modification. Electrostatic dry-powder deposition therefore offers a practical, scalable route to improving interlayer zone quality and converting passive cold joints into engineered transition zones in automated 3DCP.