Mechanical anisotropy evolution of 3D-printed alkali-activated materials with different GGBFS/FA combinations

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 ( I Mechanical ) 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 I Mechanical 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 I Mechanical 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. • Mechanical anisotropy and interlayer pore structure were quantitatively characterized. • The pores generated by the printing process were characterized. • The mechanical anisotropy of 3DPAAMs mitigates with the extension of curing age due to the formation of reaction products. • High-dosage FA incorporation aggravates mechanical anisotropy due to the weak activation reactivity and high thixotropy.