Is 3D concrete printing low carbon? Getting it right

This paper presents selected findings from an ongoing research investigation aiming a development of low carbon binder materials for 3D concrete printing. 3D concrete printing has potential to become a sustainable construction method owning to reduced demand for materials, labour and time savings compared to traditional concrete construction methods. However, the contemporary 3D concrete printing applications are actually not low carbon given the binder mixes use high amount of cement (up to 60% of the total mix) as a means of achieving satisfactory thixotropic characteristics that enable printing. The present paper shows that the contemporary concrete binder mixes are usually up to 30% high in embodied carbon compared to regular concrete mixes with the same compressive strength. The second half of the paper focuses on establishing binder mixes with natural materials as a partial replacement to cement in 3D printing applications. The paper shows that the use metakaolin, which contains high amounts of aluminium and silicon oxides (35-45% and 50- 60%, respectively), is a suitable cement replacement material for 3D printing binder materials. Since aluminates (i.e. aluminium oxides) compounds ensure rapid hydration compared to regular cement/concrete with silicates, the appropriateness of metakaolin in 3D printing is further justified. Results from a theoretical analysis focusing on the chemical reactions involving metakaolin in binder mixes are shown in the present paper. The results also show that the proposed new binder materials is up to 40% low embodied carbon compared to cement-alone binder mixes.