  There is an increasing interest in the fabrication of cement-based materials
via additive manufacturing (AM) techniques. However, the processing-induced
heterogeneities and interfaces represent a major challenge. The role of
processing in creating interfaces and their characteristics requires
understanding of the microstructure of 3D-printed hardened cement paste (hcp).
This work investigates the microstructural features of architectured
cement-based materials, including processing-induced heterogeneous patterns,
interfacial regions (IRs), and pore network distributions with respect to the
architectural pattern. A 3D printer was modified and merged with an extrusion
system and specimens were 3D-printed using a layer-wise direct ink writing
(DIW) process capable of fabrication of lamellar architectures of materials. A
lab-based X-ray microscope (XRM) was used to perform X-ray micro-computed
tomography (micro-CT) evaluations to explore the microstructural
characteristics of 3-day old intact (i.e. not tested) 3D printed and cast
specimens at two levels of magnification: 0.4X and 4X. CT scans of printed
specimen revealed a patterned pore network and several microstructural
features, including: a) macropores, b) micropores at interfacial regions (IRs),
c) accumulation of anhydrous cement particles near macropores, and d)
rearrangement of filaments away from their designed toolpath. In comparison,
microstructural investigation of cast specimen at 4X scan revealed randomly
distributed pores with no connectivity throughout the specimen. The aptitude of
micro-CT as a non-destructive technique for microstructural characterization is
discussed. The role of processing to induce and to pattern heterogeneities such
as IRs in materials is demonstrated and the role of architecture in controlling
such heterogeneities and their directionality through the interface is
discussed.
