Development of 3D-printable alkali-activated UHPC for additive construction

The development of cement-less ultra-high performance concrete (UHPC) offers a sustainable solution to reducing the carbon footprint of this advanced construction material, while the emergence of additive construction technologies highlights the need to tailor fresh properties to achieve 3D concrete printing (3DCP) requirements. In this context, this paper reports the development of an alkali-activated UHPC (A-UHPC) for 3DCP applications. A combination of ground granulated blast furnace slag (GGBS) and silica fume was activated with a potassium-based activator to prepare the alkali-activated binder. Various types of retarders were used to adjust the setting time, while different amounts of wollastonite were used as a rheology-modifying agent and micro-reinforcement. The effects of the type of retarders and wollastonite content on the fresh and hardened properties of the mould-cast A-UHPCs were investigated to identify an optimum mixture for 3DCP. Subsequently, the compressive and flexural strengths of the printed specimens made of the optimum mixture were tested in multiple directions. In addition, the effects of different curing conditions on the hardened properties of the printed specimens were investigated. The results showed that while all cast mixtures achieved compressive strength exceeding 150 MPa, the mixture containing 3 wt.% borax and 2 wt.% wollastonite was identified as the optimum mixture for 3DCP. Regardless of the testing directions, printed specimens exhibited lower compressive strength than cast specimens, whereas their flexural strength was superior to cast specimens in Z and Y directions but lower in X direction. The results also showed that the curing condition had a significant effect on the flexural strengths of both cast and printed specimens made of the optimum mixture. The underlying reasons are discussed.