The CARBCOMN project proposes a compression-dominant, carbon-negative structural system enabled by extrusion-based 3DCP and mineral carbonation. This paper presents an overview of early outcomes from the initial experimental validation of the proposed framework. This involves the factorial experimental design used to develop formulations composed solely of stainless steel slag derivatives and the first 3D-printing tests. Mix M-0.5_0.25 (a/b = 0.5, w/b = 0.25) achieved the highest compressive strength after carbonation curing at 3% CO₂ concentration, 20 °C temperature, and 60% relative humidity, for 7 days. Preliminary rheological characterization confirmed sufficient yield stress to ensure desired buildability for the proposed trial geometry. The optimum mix was successfully extruded in the 3D-printing trials, maintaining geometric stability during deposition. These findings confirm the feasibility of producing 3D-printable, carbon-negative formulations solely from industrial by-products. The work supports CARBCOMN’s broader objective of enabling circular construction systems through digital fabrication and material innovation.