Understanding time-dependent rheological behaviour is essential for extrusion-based 3D concrete printing, as it governs structuration and early-age strength development. However, limited data exists for cement-free mortars based on non-activated industrial slags, whose behaviour differs fundamentally from conventional cementitious systems. This work explores time-dependent rheological behaviour of fully cement-free concrete mixes tailored for extrusion-based 3D printing. The studied formulations are based on stainless steel slags as both binder and fine aggregates, with no hydraulic or alkali activation but mineral carbonation. Industrial additives are incorporated to optimize flowability, while maintaining low water-to-binder ratios to achieve a pore structure that facilitates CO2 diffusion. Strength development is achieved exclusively through post-printing mineral carbonation under controlled temperature and relative humidity conditions in a carbonation closet. To understand the rheological behaviour crucial to 3D printing, flow table tests were used to monitor open time for extrudability, while green strength evolution was assessed using a slow penetration test. Both tests were performed at 15-min intervals up to 60 min, and were used to assess the critical print height that can be achieved for the investigated mix. Results show a progressive reduction in flow value and a non-linear increase in green strength, depicting structuration governed by particle-particle interactions and moisture redistribution. These findings inform process control for 3D printing of ultra-low-carbon, cement-free concrete recipes that enable CO2 sequestration while valorising industrial by-products.