Low carbon dioxide 3D printable concrete: balancing rheology, strength and environmental impact using bentonite and ground granulated blast-furnace slag

To achieve an optimum mix design for three-dimensional concrete printing (3DCP), a life cycle assessment (LCA) analysis was performed based on an experimental database gathered from the literature. Based on the LCA, a sustainability comparison was conducted on the three-dimensionally printed concretes in the database. Then, an extensive experimental programme was conducted to check the performance of the LCA results of 20 concrete mixtures. This programme considered various binder systems, including Portland cement and supplementary cementitious materials (SCMs). Various tests for fresh and hardened properties were conducted. Finally, a sustainability assessment was performed by measuring environmental effects through carbon dioxide emissions (CDE), environmental damage and energy demand. The findings showed that using SCMs can significantly reduce flowability and increase workability. Specifically, a 10% increase in the amount of bentonite caused a 25% decrease in the spreading diameter and a 20% increase in buildability. A key finding was that substituting 30% of Portland cement with SCMs reduced CDE by up to 18% while achieving a compressive strength of 25 MPa for the optimised mix. In contrast, the sustainability and 3D printability criteria conflicted with the LCA and must be balanced to achieve an optimal mix design for 3DCP.