Enhancing Inter-Layer Bonding Strength of 3D Printed Ternary Geopolymer Using Calcium-Carbonate-Whiskers Spray (2024-05)¶

Weak interlayer bonding, especially induced by a long period of printing interruption, is one of the major concerns for 3D printing concrete. The effect of CaCO3 whiskers suspension (CWS) spray and printing time interval on the interlayer bonding of 3D printing fly ash-granulated blast furnace slag-steel slag based geopolymer was investigated with splitting tensile strength and direct shear strength tests. The underlying mechanisms were explored based on the characterization of micromorphology, porosity, surface roughness and reaction kinetics of interlayer area. It was revealed that the CWS spray at appropriate concentrations can significantly improve the interlayer bonding of geopolymer printed at extended printing intervals, with 0.05 g/mL of CWS increasing the shear strength up to 71% at 60 min and 0.10 g/mL of CWS increasing the splitting tensile strength by 40% at 40 min. The CWS at proper concentrations could replenish the interlayer moisture, fill the interlayer voids and provide nucleation sites for geopolymerization and hydration products, i.e., N(C)-A-S-H and C-S-H gels. However, excessive CaCO3 whiskers or water content was harmful to the interlayer bonding. As the reaction process and microstructure of 3D printed ternary geopolymer were synergistically impacted by the printing time interval, residual water content, compensated moisture and CaCO3 whisker reinforcement, the mix of CWS spray should be carefully designed to optimize its effectiveness on the interlayer strengthening.