Micro/Nano additives in 3D printing concrete: Opportunities, challenges, and potential outlook in construction applications

3D concrete printing has attracted burgeoning interest in the construction industry for its ability to offer cost-effectiveness, architectural design flexibility, efficient use of energy and materials, as well as significant time savings in the construction process. However, conventional cement-geopolymer-based materials cannot be used directly for printing due to their lack of printability. This review explores the utilization of inorganic micro/nanomaterials to modify the rheological and mechanical performance of fresh-state and post-hardening cementitious composites, respectively, offering an in-depth analysis of the mechanisms underpinning their effects. This paper discusses a wide range of inorganic micro/nanomaterials, including carbon-based nanomaterials, silicon-based nanomaterials, metallic oxide nanomaterials, nano-calcium carbonate particles, and other micro/nano-materials. Those materials have been utilized in 3D printing concrete or show considerable potential for future applications in this field. Furthermore, this work provides insights into the multiple applications that arise from the synergistic combination of 3D printing construction techniques with the distinctive properties of different nanomaterials.