Set-on-demand of alkali-activated slag mixture using twin-pipe pumping

Abstract

To achieve the set-on-demand purpose of concrete for digital fabrication where the opposing requirements for pumpability and buildability exist during the process, an inline mixing strategy, named twin-pipe pumping, has been developed utilizing a static mixer to blend two separate streams to accelerate setting right before extrusion. The system was initially developed utilizing two streams including a Portland cement-based mixture (without accelerator) and a limestone powder-based mixture (with an accelerator but without cement). However, the high content of cement may result in higher material costs along with lower sustainability, given the fact that the production of Portland cement contributes to around 8% of global CO2 emissions. This study aims to develop an alkali-activated slag mixture to increase the sustainability in digital fabrication, using twin-pipe pumping. The constituent mixtures consisting of a slag-based (without alkali) and a limestone powder-based (with varying alkali content) mix were formulated and their rheological behavior was measured for good pumpability. Phase transition from liquid to solid phase, mechanical strength, and pore structure was evaluated. Twin-pipe pumping was used in a printing trial. Results showed that the alkali-activated slag mixture achieved fast phase transition for set-on-demand purposes by combining the alkali in the limestone-based mixture with the slag in the slag-based mixture. Increasing the NaOH concentration or incorporating nano clay improved the structural build-up for high buildability.