Recent 3D concrete printing technology advancements have rapidly progressed in the construction industry. To meet the layer-by-layer stacking requirements, there is a need to develop high-performance cement-based composites compatible with 3D printers. Using fly ash as a substitute for cement reduces the environmental impact of 3D-printed traditional concrete mixes involving ordinary Portland cement (OPC). This study aims to determine the feasibility of adapting high-volume fly ash (HVFA) mortar to 3D printing by modifying the rheology of the mix with mineral additives. Three different additives were used to achieve this goal: sepiolite, slaked lime, and unslaked lime. The mix design includes 70% fly ash by weight and 5% slaked or unslaked lime by the weight of the binder. Additionally, 0.3% of the binder weight of sepiolite was added as a rheological modifier. Rheological parameters were assessed, including time-dependent evolution of yield stress, viscosity, thixotropy, and structural build-up with resting time. A preliminary printability and shape retention assessment of printed samples was conducted using a lab-scale robotic arm with a pipe as an extruder. Experimental results indicated that adding either slaked or sepiolite increased the static yield stress, dynamic yield stress, and thixotropy. The results show that the rheology of HVFA can be improved by using lime and sepiolite, and the quaternary mix design can be an alternative sustainable solution in the 3D printing of building materials.