Ultra-low dosages of novel graphene types enhance the rheological properties and buildability of 3D printed binders

The use of graphene as a high-performance concrete additive is attractive; but, its cost and concerns about production scalability and dispersion efficiency in concrete are impediments to widespread use. This study explores the impact of ultra-low dosages (<= 0.02 % by mass of binder) of two novel graphene types-fractal graphene (FG) and reactive graphene (RG)-produced through a cost-effective, environmentally friendly, and scalable process, on the rheological properties of 3D-printable concrete. Both FG and RG significantly enhance the dynamic and static yield stresses and viscoelastic properties of the binder, with RG-modified mixtures exhibiting slightly more pronounced enhancements due to the presence of functional groups. Temporal evolution of static yield stress (tau s) and storage modulus (G') reveal aspects relating to structural build-up facilitated by the graphene particulates (structuration parameter from tau s-t, and rate of structural build-up, and residual structural factor from G'-t relations), that are important in extrusion and shape stability. Experimental buildability tests on hollow cylinders reveal that the selected ultra-low graphene dosages more than double the achievable build heights at 30, 60, and 90 min of mixing. This enhancement is further corroborated by an analytical model for plastic collapse, which incorporates plastic yield stress derived from green compression testing. Finally, this paper introduces an approach wherein the storage modulus and its evolution-determined through oscillatory rheology experiments-serve as versatile indicators of key rheological properties essential for material delivery, extrusion, and layer build-up in concrete 3D printing. This methodology holds promise for paving the way toward a standardized rheological test for 3D-printable binders.