Evaluation of Fresh Properties of High Calcium Content Fly Ash-Based Alkali-Activated 3D-Printed Mortar (2025-03)¶

This study investigated the applicability of using class C fly ash (FA) having high calcium content to produce alkali-activated mortar (AAM) for 3D-printed concrete (3DPC). The fresh properties were examined through different tests including open time (OT), initial setting time (IST), immediate axial strain, and penetration test (PT), as well as the rheological characterization. In addition, the relationship between the OT, static yield stress (τo), and cycle time (CT) was discussed. Effect of water to FA (W/FA), alkaline activators to FA (Alk/FA), and sodium silicate to sodium hydroxide (SS/SH) ratios were studied to offer a wide selection of AAMs for different 3D-printing applications. The findings indicated that the OT for the mortar ranged from 4 to 93 minutes, suggesting suitable extrudability for 3D-printing. Variations were observed in OT, IST, and immediate axial strain based on the W/FA, Alk/FA, and SS/SH ratios. The τo evolution of AAMs linearly increased with rest time (trest) with R2 ranging from 0.91 to 0.99. The static yield stress evolution rate (Athix) ranged from 0.07 to 3.58 kPa/min. Regardless of the trest, the τo values obtained from the rheometer were about 17-53% lower than the τo values obtained from the PT. The lowest estimated CT possible while avoiding a collapse of the 3DPC structure was based on the Athix and shear stress induced from gravity. The study demonstrated that by adjusting the AAM composition, properties can be tailored to suit specific 3D-printing requirements, highlighting the material's adaptability for diverse applications.