Synergistic effects of the use of metakaolin, sand and water on the properties of cementitious composites for 3D printing

[Display omitted] •A new method similar to printing was proposed to evaluate the extrudability.•New benchmarks for 3DCP development seem to have been obtained.•There was a significant interaction between MK and sand in the voids, increasing fluidity.•The water to dry material ratio was the determining factor for the printability.•Metacaulim modified the viscosity of 3DCP and increased the runtime. 3D concrete printing (3DCP) is a construction technique based on the deposition of successive layers of a cementitious composite without the need for conventional formwork. For the technique to be applied it is necessary that the cementitious composite used allows the passage in continuous flow in a 3D printing nozzle (extrusion capacity) and presents low deformation of the printed layers (buildability). In this perspective, the aim of this study was to understand the influence of Metakaolin (MK), sand and water incorporation on the extrusion capabilities and buildability of cementitious composites for 3D printing. The analysis was designed in a simplex lattice mixture statistical experimental planning. Extrusion ability was evaluated using an experimental flow rate by extruder mill and slump flow. Buildability was analyzed by the maximum number of printed layers, shape retention index, layer thickness variation, and squareness deviation in printed blocks. The squeeze flow test was carried out as a parameter for both printing properties. The water/binder ratio was the most determinant variable in extrusion capacity for its responsibility to fluidize the mixture. Increasing the sand content improves buildability but reduces the application time. On the other hand, MK presented the advantage of controlling viscosity and maintaining good fluidity over time, due to a slower setting. There was a significant interaction between sand and MK in the packing of grains, increasing the properties related to extrusion. It was possible to print with a 30% MK content by adjusting the water. The yield stress was directly related to the flow rate, validating the experimental methodology created. Thus, it was possible to understand the effect of each component, as well as their interactions.