Towards sustainable 3D concrete printing: influence of waste foundry sand on rheology and printability of OPC–LC2 systems

Concrete demand depletes natural resources and increases CO2 emissions; simultaneously, industrial waste causes land pollution. Using waste foundry sand (FS) and low-carbon limestone calcined clay (LC2) offers a sustainable solution to these challenges. In this study, quartz sand (QS) was partially replaced with 10–30% FS to develop OPC–LC2-based 3D-printed concrete (3DPC). The fresh properties investigated include flow table diameter, static and dynamic yield stresses, and plastic viscosity. For the hardened state, compressive strength was measured. Additionally, X-ray diffraction (XRD), Fourier-transform infra-red spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDX) characterised crystalline phases, functional groups, microstructure, and elemental composition, respectively. The analysis shows that up to 30% FS replacement reduces flowability while maintaining strength and microstructural integrity. Optimised superplasticizer dosage achieved suitable rheology (static yield stress: 2.0 ± 0.5 kPa, dynamic yield stress: 1.05 ± 0.05 kPa, plastic viscosity: 30.0 ± 2.0 Pa·s), ensuring stable extrusion. The 28-day cured samples with 30% FS showed only ∼0.8% lower compressive strength than the control mix. Overall, the findings highlight the potential of these materials for sustainable 3D concrete printing.