Mitigation of lack of fusion in 3D printed limestone calcined clay cement concrete induced by effective microorganisms

Limestone calcined clay cement (LC 3 ) and other supplementary cementitious materials reduce the carbon footprint by replacing clinker/cement content in 3D printed concrete (3DPC) without jeopardising the rheology and mechanical performance of structural concrete. This paper presents the utilisation of effective microorganisms (EM) in LC 3 fibre-reinforced printed concrete (FRPC) to improve interfacial bond. Weak interfaces between layers remain a considerable challenge to the 3DPC construction technology, causing anisotropic hardened mechanical strength and stiffness, and acting as pathways for ingress of deleterious substances that drive deterioration. An experimental campaign was performed on EM-enhanced LC 3 -FRPC and reference LC 3 -FRPC, comprising of three destructive tests: compression, direct tension, and flexure in two orthogonal directions, i.e. parallel and perpendicular to the printed layers, to investigate interfacial strengthening and reduced anisotropy. Analytical investigations were performed to detect microstructural mechanisms of strengthening including scanning electron microscopy (SEM) augmented by energy dispersive X-ray spectroscopy and X-ray computed tomography (X-CT). EM-enhanced strengths of the tensile and flexural specimens resulted in average interfacial bond strength values of 1.45 MPa and 6.27 MPa, respectively, which were 26.1 % (tension) and 33.7 % (flexure) higher than those of non-EM specimens. The level of anisotropy was lowered in compression, tension, and flexure by 0.8 %, 14.8 % and 15.9 %, respectively, compared to the mixture without EM. These are ascribed to the interaction between EM and calcium hydroxide, and catalysed action of lactic acid bacteria, leading to the production of excess calcite precipitation. Conclusively, the material in the interlayer region of EM-enhanced specimens exhibited lower porosity and smaller pore size than non-EM specimens. • A mixing strategy for 3D printed LC 3 and EM concrete is developed for the first time. • EM incorporated in LC 3 printed concrete enhances interfacial bond and reduces anisotropy. • Microstructural analysis confirms hydration products strengthening EM-enhanced interfacial bond.