A novel additive mortar leveraging internal curing for enhancing interlayer bonding of cementitious composite for 3D printing

A major flaw of 3D concrete printing (3DCP) is its weak interlayer bonding. Consequently mechanical, bond behaviors and durability are compromised. The shrinkage because of excess surface water in the extrusion process and its evaporation promotes the reduction of interlayer bonding. In this study a novel additive mortar based on calcium sulphoaluminate (CSA) cement, cellulose fiber and limestone filler was proposed for application between layers, which permits extension of printing time interval and also caters for breaks in construction printing. The mortar was applied on substrate filaments whereas the overlaying filaments were printed at 60 min, 90 min and 120 min print time intervals corresponding to initial and final setting times of the 3D printing concrete. Its effect was verified through accurate measurements of interlayer tensile and shear bond strength on cross bonded samples. Microstructural investigation of interlayer structures was through SEM to cater for both EDS and XRD to study hydration products. Results suggest that the proposed additive mortar holds water and utilizes it for internal curing to attain overall enhancement in early age hydration, produces expansive ettringite to counter shrinkage and generates additional mechanical bonding between printed layers by fiber and fine aggregate interlocking. Interlayer tensile strength was boosted greater than 1.91 MPa for 60 min print time interval. The composite could possibly be utilized between each layer of a typical 3DCP operation to improve filament deposit and stacking process, to reduce voids and longitudinal flaws and to enhance durability because it is very easy to manufacture and apply while its constituent materials are plentiful, cheap, safe and environmentally friendly. (C) 2020 Elsevier Ltd. All rights reserved.