Basaltic Rock Dust as a Sustainable Supplementary Cementitious Material for 3D-Printed Concrete: From Mix Optimisation to Printing Performance

The growing demand for sustainable binder systems in 3D-printed concrete (3DPC), together with the declining availability of fly ash (FA), has increased the need for alternative supplementary cementitious materials. This study investigates the feasibility of using basaltic rock dust (BRD) as a high-volume replacement of FA in 3DPC through a two-stage experimental programme involving mix optimisation followed by evaluation under printing conditions. Five mortar mixes incorporating BRD at FA replacement levels of 0%, 25%, 50%, 75%, and 100% by mass were first evaluated to identify the optimum replacement level within the investigated range, after which the optimum mix was comprehensively characterised in terms of its fresh properties, rheological behaviour, hydration characteristics, mechanical performance, anisotropy, and microstructure. The results showed that 75% FA replacement by BRD achieved the optimum overall performance among the investigated BRD-containing mixes, based on the combined consideration of high BRD utilisation, flowability, hydration behaviour, and compressive strength. Although its 28-day compressive strength under conventionally cast conditions remained slightly lower than that of the control mix, under printing conditions, the C75 mix increased the flow diameter from 165 mm to 176 mm and the maximum printable layers from 14 to 16 while reducing the shape retention spread diameter from 96 mm to 89 mm. Compared with the control mix, the BRD-incorporated mix also increased the compressive strength by 10.9% in the Y- direction and 22.9% in the Z-direction, respectively, while reducing the compressive anisotropy from 20.97% to 12.39%. Although the flexural strength decreased from 7.66 to 5.02 MPa in the Y-direction and from 7.29 to 5.19 MPa in the Z-direction, the directional dependence decreased from 4.83% to 3.34%. SEM observations revealed a more compact cementitious matrix with fewer visible large pores in C75 than in the control mix. These findings demonstrate that BRD has strong potential as a high-volume replacement for FA in 3D-printed concrete, reducing reliance on fly ash while achieving favourable printability, compressive performance, and structural uniformity.