Exploring the role of metakaolin in binary and ternary blended 3D printable mortars: Deep insights into printability

This study investigates the utilisation of metakaolin and GGBS as partial replacements for OPC, examining their impact on printability, mechanical properties, microstructure and hydration kinetics in binary and ternary mortars. The printability of the mixes was thoroughly assessed through measurements of flowability, extrudability, open time, yield stress, shape retention, and buildability. It is observed that shape retention improved by 46% in the OPC–GGBS–metakaolin mix and 56.25% in the OPC–metakaolin mix, while the OPC–GGBS mix showed minimal improvement. Mechanical properties were evaluated, including water absorption, porosity, compressive, and flexural strength. Printed specimens exhibited anisotropic strength, with the lowest on the ZX plane and highest on the YZ plane. The 70% OPC, 20% GGBS, and 10% metakaolin mix demonstrated superior printability and mechanical performance, supported by optimal particle packing via the modified Andreasen and Andersen model. Furthermore, the denser microstructure observed in SEM micrographs, the decrease in Ca/Si ratio from EDX measurements, the emergence of additional C–S–H and C–A–S–H peaks in X-ray diffraction, the pronounced Si–O–Si/Al stretching bands in FTIR and the increased WH% (with consumption of CH) in TGA collectively confirm that GGBS and metakaolin significantly contributed to the secondary hydration reactions.