Effect of microwave heating on interlayer bonding and buildability of geopolymer 3D concrete printing

The emerging concept of 3D concrete printing (3DCP) by the laminate stacking of layers without formwork has major issues of poor interlayer bond adhesion and inadequate buildability of printed layers. This paper explores microwave heating technique as a set-on-demand method to increase the structuration rate of geopolymer concrete for 3DCP applications. The microwave heating was applied just before the placement of layers (i.e. at the nozzle head) and the effect of heating duration on the interlayer bond strength and buildability were assessed. Moreover, the loss of surface and bulk moisture and the bond strength enhancing mechanism were also investigated. The results demonstrated that the optimum microwaving duration of 10 s has increased the interlayer bond strength by 132%. The interlayer temperature and surface moisture loss has shown an increasing trend with the heating duration. Despite the increased surface moisture loss with microwave heating, the observed increment in interlayer bond strength (for up to 10 s of heating) was due to the acceleration in polycondensation reaction between the filaments having adequate stiffness for maximum mechanical interlocking with minimum deformation after stacking. On the other hand, the buildability of printed filaments, as measured from the rate of particle re-flocculation and structuration, has showed a major improvement. Findings from this study propose incorporating set-on demand technique using electromagnetic radiations in construction 3D printing. This initiates a new era of print heads, designed to solve problems faced by current concrete printing industry. (C) 2020 Elsevier Ltd. All rights reserved.