Extrusion-based 3D concrete printing (3DCP) typically relies on high Portland cement content to meet strength requirements, which necessitates sustainable alternative binders for 3DCP. This study investigates the use of calcined waste clay as a sustainable alternative in printable mortars, focusing on a novel inline calcination through short duration, on-demand activation. This technology enables on-site calcination of waste clay derived from excavations, benefiting the diversion of waste clay to value-added resources, while developing sustainable printable mortar. A comprehensive experimental program was conducted to assess the performance of waste kaolinite clay calcined for different durations (0, 3, 9, 27, and 60 min) to assess the feasibility and performance of short duration calcination process. Raman spectroscopy and thermogravimetric analysis (TGA) revealed progressive dehydroxylation and structural transformation of the clay with increasing residence time, substantial reduction in structurally bound hydroxyl groups after 9 min and pronounced amorphization observed after 27 min. All OPC-blended calcined clay mixes achieved approximately 30% higher compressive strength than OPC-blended uncalcined clay mixes, with strength activity indices (SAI) of 0.97–0.98 across all residence times. In printable mixes, calcined clay replacement levels of 20% and 40% resulted in compressive strength enhancement of 31% and 13% for the short calcination duration of 3 min, and 37% and 12% at 60 min, respectively, compared to equivalent uncalcined clay replacements. Importantly, printable mortar with 20% calcined clay and a short calcination duration of 3 min showed perpendicular directional compressive strength above 25 MPa, indicating suitability for many construction applications. These findings demonstrate that waste clay activated through short thermal treatment achieves mechanical performance comparable to conventionally calcined clay, supporting the potential implementation of on-site, on-demand calcination for low-carbon 3DCP.