Set on demand geopolymer using print head mixing for 3D concrete printing

Abstract

The concept of set-on-demand in concrete with an intervention at the print head could be used to accelerate the concrete's yield strength development after placement without significantly affecting the pumpability and extrudability. This study demonstrates the proof-of-concept of the set-on-demand approach in geopolymer concrete, where the initial mixing of reactive components (i.e., activators + binder) is conducted at the print head. The study first analyses the feasibility of the proposed concept by assessing the yield strength development rates of fresh concrete for varying mix design parameters. With a suitable mix design is chosen, a modular in-line dynamic mixing equipment was utilised to assess the effect of various mixing regimes and operating parameters (shearing histories) on the yield strength development rates. The results demonstrated that the in-line mixing process of reactive components for a short duration (i.e. 45 secs) would be sufficient to achieve a yield strength development rate of 800 Pa/s after placement, making it suitable for the print-head mixing process. The rapid setting of geopolymer concrete was utilised to print out-of-plane structures (simply supported beams) as a proof of concept.