Development and characterization of eco‐friendly fire‐resistant 3D ‐printable concrete utilizing recycled nickel slag

This study investigates the utilization of industrial nickel slag waste to develop eco‐friendly fire‐resistant 3D concrete printing (3DCP). The benefits of waste nickel slag are bifold: it serves as a partial replacement, reducing carbon‐intensive cement and enhances fire resistance. A varying dosage of nickel slag was introduced as a cement replacement in printable mixes, and the rheological characteristics, mechanical behavior, and fire resistance were systematically evaluated. The addition of nickel slag reduced water demand due to its low water absorption, enhancing initial flowability. However, its angular morphology and rough surface texture increased plastic viscosity and dynamic yield strength, hindering extrusion and pumping. Notably, a 15% nickel slag content exhibited the highest thixotropy index, indicating enhanced structural build‐up and the potential for improved buildability. Improved interparticle and interfacial bonding in nickel slag mixes increased compressive strength in longitudinal ( X ) and lateral ( Y ) directions by 27% and 65%, respectively. This enhancement is attributed to improved particle packing, mechanical interlocking, and matrix densification. Printed specimens with nickel slag showed enhanced residual compressive strength and lower mass loss after exposure to elevated temperatures. The enhanced fire resistance can be attributed to the improved microstructure and presence of thermally stable magnesium and iron phases in the matrix. In addition to the superior fire performance of 3DCP, the addition of nickel slag improved sustainability by reducing energy consumption and carbon emissions by approximately 17%. These findings highlight that nickel slag can be used as a sustainable cement replacement material for 3DCP applications, enhancing the fire resistance and sustainability of concrete structures.