Mechanical Anisotropy Evolution of 3D Printed Alkali-Activated Materials with Different GGBFS-FA Combinations (2022-01)¶

The mechanical anisotropy of 3D-printing concrete is a crucial factor limiting the application of this technology. This study focuses on the evolutions of mechanical anisotropy and interlayer pore structure in the 3D-printed alkali-activated materials (3DPAAMs) with different precursors combinations of grounded granulated blast-furnace slag (GGBFS) and fly ash (FA), aiming to elucidate the relationship of mechanical anisotropy evolution and precursor selections. The pores generated by the printing process were quantitatively characterized with X-ray computed tomography (X-CT) tests using image processing methods. Results show that the mechanical anisotropy coefficient (IMechanical) of 3DPAAMs mitigates with the curing age since the formed reaction products fill the pores generated by the printing process. With the curing age extension from 3 to 28 days, the IMechanical of GGBFS-based 3DPAAMs decreased 51.1%, 60.9%, and 71.1% for compressive, flexural and split tensile strengths, respectively. Over-high FA incorporation (50 and 75%) yields lower mechanical strengths and aggravated anisotropy. Compared with GGBFS-based 3DPAAMs, the addition of 75% FA increases the IMechanical at 28 days by 274%, 236% and 274% for compressive, flexural and split tensile strengths, respectively. The low activation reactivity of FA and the higher thixotropy of FA-incorporated mixtures coarsen the interlayer pore structure, contributing to a higher mechanical anisotropy.