Metakaolin-slag-based 3D-printed one-part geopolymer concrete with calcium-carbonated-recycled plastic eco-aggregate: Rheology, mechanical behaviour, and microstructure

The conservation of natural aggregates, valorisation of industrial waste, and development of sustainable low-carbon construction materials, together with the integration of construction automation, are increasingly critical for mitigating the environmental impacts of conventional construction materials and addressing housing shortages. This study investigates the rheological behaviour, mechanical performance, and microstructural characteristics of 3D-printed one-part geopolymer concrete synthesised from a blended metakaolin-ground granulated blast-furnace slag binder. The fine aggregate of the mix was replaced with calcium-carbonated recycled waste plastics of Classes 1 to 7, herein called RESIN8 Core (R8C) up to 20% by volume, while an aggregate-to-binder ratio of 1.9 was employed to limit binder demand and enhance structural integrity. The inclusion of R8C had a negligible effect on setting time but increased dynamic and static yield stresses, re-flocculation, and structuration rate. The 28-day density ranged from 1885 to 2067 kg / m 3 , while the compressive strength of the control mixture (35–38 MPa) decreased by 18.6%–25% at 5%–10% R8C and by 37.7% at 15%–20%. Specimens containing up to 10% R8C achieved splitting tensile strength comparable to the control, whereas direct tensile strength and secant modulus decreased with increasing R8C content. X-ray diffraction and energy-dispersive spectroscopy confirmed the formation of an amorphous C–A–S–H gel with good interfacial bonding between R8C and the geopolymer matrix. Overall, the results demonstrate that incorporating up to 10% R8C is feasible for the 3D printing of lightweight structural geopolymer concrete, providing a viable pathway toward environmentally friendly and sustainable digital construction. • 3D-printed one-part geopolymer concrete was produced using a blended metakaolin–GGBS binder and high aggregate content. • Calcium-carbonated recycled waste plastic (RESIN8 Core) replaced fine aggregate up to 20 vol%. • RESIN8 Core increased yield stress and structuration rate, while having a negligible effect on setting time. • Up to 10 vol.% RESIN8 Core maintained mechanical performance with acceptable strength reduction. • Incorporation of RESIN8 Core enables lightweight, low-carbon concrete for sustainable digital construction.