Set-on-Demand Geopolymer for Concrete 3D Printing Using Ternary Activator

In concrete 3D printing, the fresh concrete’s rheology required during the pumping stage contradicts the rheological requirement during the building stage. Therefore, admixtures such as retarders and accelerators are added before the pumping and building stages, respectively, to alleviate the contradictory rheological behaviour. But these admixtures are designed for Portland cement. As geopolymers do not contain any cement the additives fail to alleviate the contradictory rheological requirement. Therefore, traditional geopolymers have lower pumpability and buildability than cementitious mixes, restricting geopolymers’ application in large-scale construction. Thus, this study investigates the controlled activation of geopolymer mix using ternary activators to ensure a long open time for smooth pumping and rapid yield strength development after extrusion for enhanced buildability. The base mix contains sodium hydroxide and sodium carbonate as activators, whereas different types of sodium silicate solutions were used for print head activation. It was found that the base mix exhibited static yield strength of less than 0.5 kPa at the end of 90 min and low viscosities ensuring high pumpability. Meanwhile, after print head activation, the static yield strength development rate of the best mix increased from 5 Pa/min to 500 Pa/min. The best mix determined was activated with a combination of sodium silicate solution and silica fume at the print head.