Exploration of the Open Time and Compressive Strength of Alkali-Activated Mixtures for 3D-Printed Concrete

This research explores the use of class C fly ash (FA) as the primary component in the development of alkali-activated mortar (AAM) for the purpose of 3D-printed concrete (3DPC). Sodium silicate (SS) and sodium hydroxide (SH) were employed as the alkali activators. To tailor mixtures for 3DPC, the study examined three AAMs with different SS/SH ratios, specifically 0.5, 1.0, and 2.0. The extrudability of the AAMs was evaluated through the open time test. The compressive strength of AAMs in various directions with different cycle times (CTs) of 1.0, 2.5, and 5.0 min was examined. The findings indicated that the open times of the AAMs ranged from 3.5 to 24 min. These open times depend on the ratio of SS/SH, allowing for the customization of printing speed to suit diverse applications. The 3DPC samples showed anisotropic phenomena compared to those full-height samples. Full-height samples displayed 1.6–12% higher compressive strength than 3DPC samples in the X-direction and 3.1–17% higher in the Z-direction with respect to the AAM layers, respectively. Regardless the tested direction and CTs, 3DPC samples with an SS/SH ratio of 2.0 demonstrated the highest compressive strength in contrast to those with SS/SH ratios of 0.5 and 1.0. The compressive strengths of 3DPC samples varied from 25.2 to 37.6 MPa as SS/SH ratios increased from 0.5 to 2.0.