Title: Flow Characterization of Three-Dimensional Printable Cementitious Pastes during Extrusion Using Capillary Rheometry
Author(s): Sooraj A. O. Nair and Narayanan Neithalath
Publication: Materials Journal
Appears on pages(s): 123-137
Keywords: capillary rheology; extrusion; slip layer; three-dimensional (3D) printing; wall shear stress
Three-dimensional (3D) printing of cement-based materials is carried out using extrusion, which requires a fundamental understanding of the non-Newtonian flow of pastes through capillaries, which is the focus of this paper. 3D-printable cementitious pastes, qualified using steady-state extrusion pressure, are subjected to multiple-speed extrusion tests under apparent shear rates that correspond to typical printing speeds. The true, non-Newtonian flow curves are obtained by carrying out the relevant end corrections, deconvoluting the apparent shear rate (or velocity) into its true and wall slip components and applying the Weissenberg-Rabinowitsch correction. An exponential relationship is observed between the slip velocity and the wall shear stress, which is used to determine the slip layer thickness. The velocity profiles in the capillary demonstrated the shear-thinning nature of the pastes
and the existence of a plug-flow zone with invariant velocity, while the viscosity profiles showed the near-Newtonian response of the superplasticized paste at higher shear rates. The influence of printing speed, particle concentration, and the presence of superplasticizer on the slip layer thickness is explored. A particle-depleted slip layer could be beneficial in reducing the energy needed
for printing but could have implications in interlayer bonding and durability. The flow characterization approach presented herein can be adopted to optimize the paste material design and printing characteristics for extrusion-based 3D printing.