3D printing can lead to a technological breakthrough in the construction sector. However, the sustainability aspect of 3D printing mortar can be disputable, as 3D printable mortar contains a high amount of ordinary Portland cement (OPC). The sustainability can be increased by replacing OPC with an Fe-rich slag, which originates from the metallurgical industry and is nowadays used for low-value applications. A mortar composition consisting mainly of slag and a small amount of OPC is called a hybrid mortar and is alkali-activated to ensure that the slag is participating in the binder formation. In this study, the amount of OPC is decreased significantly, down to 6 wt% and the slag content is increased up to 28 wt% over total solid content. This work investigated the effect of several components in the hybrid mixture on the early-age stiffness development, late-age shrinkage, creep and mechanical strength and is compared to a commercial OPC-based 3D printable mortar. The components, which are important to obtain a 3D printable mixture, comprise OPC, Si fume, fine limestone, superplasticizer and carbon fibres. This study shows that the additions significantly influence the stiffness and mechanical strength development of the hybrid. The shrinkage and creep behaviour of the hybrid was considerably lower compared to the benchmark material.