Rapid early age strength development of in-line activated geopolymer for concrete 3D printing

Abstract

The in-line activation using print head mixing technology involves two parts pumping and mixing at the print head. This technology can be applied in geopolymer concrete, where the base mix (precursors, sand and water) and alkaline activators can be pumped as two separate parts and mixed at the print head for activation. While such methods provide prolonged open time, the early age strength development is low since water from the base mix dilutes the activator solution during print head mixing. Besides, choosing an activator solution with high alkalinity to combat the dilution effect reduces the immediate static yield strength development after print head mixing, which is essential for rapid building. Therefore, this study investigates the hydrated lime as an additive in the base mix to overcome the challenge to attain high 1-day compressive strength along with immediate static yield strength development. The effect of hydrated lime dosage on the pumpability (i.e. evolution of viscosity with time), static yield strength development after print head mixing (buildability) and hardened properties were assessed. The optimum hydrated lime dosage of 1 wt% of the precursors exhibited 1-day compressive strength of 20 MPa and static yield strength of 38.8 kPa after 5 min from print head mixing with minimal changes in pumpability of the base mix for up to 6 h. Moreover, the effect of rapid static yield strength development of the printed layers on the interlayer bond strength was assessed with varying cycle times between 5 s and 40 min. It was found that the interlayer bond strength reduced by 63% when the cycle time was increased from 5 s to 40 min, however, the reduction can be decreased to 15% by surface wetting of the previous layers.