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

Cement-less ultra-high performance concrete (UHPC) could be a sustainable option for 3D concrete printing (3DCP) but controlling its rheology and rapid setting are major challenges. This paper reports the first-ever development of a novel alkali-activated UHPC (A-UHPC) for 3DCP by combining ground granulated blast-furnace slag and silica fume activated with a potassium-based activator. The potential of borax decahydrate (Na2B4O7∙10H2O), sodium gluconate (NaC6H11O7), and zinc nitrate hexahydrate (Zn(NO3)2∙0.6H2O) as retarders to regulate setting, and wollastonite as rheology-modifier and micro-reinforcement were investigated. A-UHPC mix with 3 wt% borax and 2 wt% wollastonite achieved suitable rheological properties and delayed setting for 3DCP. Printed A-UHPC made from this mix showed excellent printability, 7-day compressive strength of 127.7 to 146.0 MPa, and flexural strength of 10.2 to 20.3 MPa, depending on curing regime and loading direction. Environmental impact and cost assessments confirmed that the developed printed A-UHPC is a sustainable alternative to printed OPC-based UHPC, achieving 29% reduction in global warming potential and 37% reduction in embodied energy, while maintaining comparable cost and mechanical performance.