This study investigated the use of class C fly ash (FA) as a precursor for alkali-activated mortar (AAM) for 3D-printed concrete (3DPC). AAMs with different water-to-FA (W/FA), alkaline activator-to-FA (Alk/FA), and sodium silicate-to-sodium hydroxide (SS/SH) ratios were examined to develop mixtures that can be tailored for different structural applications of 3DPC. The fresh properties, including extrudability and buildability, were evaluated through the open time (OT) and immediate deformation tests, respectively. Different cycle times (CTs) were applied to achieve a strain limit state necessary to maintain the printed shape. The strength of AAMs in different directions at different CTs was examined. Scanning electron microscopy (SEM) was carried out on AAM specimens having different CTs for a better understanding of the bond area. OTs ranging from 2.5 min to 31 min and axial strains ranging from 0.17% to 11.2% were achieved depending on the proportions of the AAMs and CT, which offers flexibility in optimizing the speed of printing and strength of concrete for different projects. The 3DPC specimens displayed anisotropic behavior compared with full-height specimens, where the compressive strength of full-height specimens was higher by 0.2% to 18% and 0.9% to 28% than 3DPC specimens when tested parallel and normal to the printing directions, respectively. SEM images and line scan indicated an approximately even intensity of the element concentration at the interfacial zones of AAMs having short CTs, which explained the relatively high compressive strength of those specimens. For AAMs having long CTs, there was a significant change in the intensity of the element concentration at the interfacial bond zone, and voids were observed resulting in low compressive strength of those specimens.
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