In-line activation of geopolymer slurry for concrete 3D printing

In-line activation techniques in concrete 3D printing enhance the buildability of fresh concrete without influencing its pumpability, thus enabling fast construction rates with long pumping durations. While the in-line activation of cementitious mixes with set-accelerators is a promising approach, the set-accelerators fail in geopolymer system. Therefore, this study investigates a new approach of attaining on-demand setting in geopolymer by alkali activation of binder slurry at the print head. The primary advantage of the proposed method is that the mix remains non-activated until it reaches the print head, thus facilitating a long pumping duration. It was found that the mix design parameters (i.e., precursors to activator ratio, fly ash to slag ratio etc.) and operating conditions (mixing duration of precursor slurry, stage wise activation etc.,) are critical in attaining the desired fresh concrete properties in geopolymer concrete. This is demonstrated by the fact that the optimum mix design chosen with 20 wt% of fly ash as precursor and the activator dosage of 35 wt% of binder showed a yield strength growth rate from ∼500 Pa before activation to ∼70 kPa in 20 min after activation. Moreover, this study provides a new insight into the potential benefits of polycarboxylate based superplasticiser, which was widely reported as an incompatible admixture for silicate activated geopolymer concrete.