New mix design for 3D printed mortar incorporating recycled fine aggregate

Abstract
3D printing for construction is an emerging technology with significant potential to replace conventional formwork-based methods. However, printable mortars remain unsustainable due to the high consumption of cement and natural sand. This study addresses this limitation by developing a novel mix design that simultaneously incorporates recycled sand (RS) to reduce natural sand consumption and recycled concrete crushed fines (RCCF) as a partial cement subtitutions, establishing a dual-benefit approach for sustainable 3D printing. The experimental procedure began with preliminary printability and strength assessments, followed by rheological investigations and 3D printing trials once performance was confirmed. The results indicate that replacing up to 75% of natural sand with RS and 10% of cement with RCCF significantly minimizes the environmental footprint of the mortar without compromising its fresh or hardened properties. Specifically, the incorporation of RS primarily influences compressive strength while maintaining excellent extrudability, whereas the addition of RCCF has a minor effect on strength while leaving the rheological properties virtually unaffected. By optimizing this simultaneous replacement, the mixture maintained highly satisfactory fresh performance. The optimized mix achieved a spread diameter of 17 cm, yield stress of 157.77 to 160.62 Pa, viscosity of 25.58 to 26.94 Pa·s, and compressive strength between 47.99 and 50 MPa. This study provides a distinct scientific contribution by demonstrating the synergistic viability of dual recycled components, offering a practical approach for developing eco-friendly 3D printing mortars.