Mix design optimization and sustainability considerations for 3D‐printed ultra‐high‐performance concrete

Extrusion-based 3D printing of ultra-high-performance concrete (3DP-UHPC) combines the geometric freedom of additive manufacturing with high mechanical performance. But its implementation remains constrained by a narrow processing range, anisotropy, cost, and binder-related environmental burdens. This review critically synthesizes advances in mixture design and sustainability, focusing on how particle packing, water-to-binder ratio, supplementary cementitious materials (SCMs), fibers, nanomaterials, and chemical admixtures govern rheology, extrusion stability, structural build-up, interlayer bonding, and hardened performance. The evidence shows that printability cannot be optimized through isolated constituent adjustments. Superplasticizers must reduce pumping resistance without delaying structural rebuilding and viscosity-modifying agents improve cohesion without impairing extrusion. Fiber dosage and dispersion must enhance crack bridging without promoting blockage or anisotropy and deposition time and surface condition must preserve interlayer integrity. SCMs, low-clinker binders, industrial by-products, recycled fibers, and alternative aggregates can reduce environmental impact, although their benefits must be verified against printability, strength, shrinkage, durability, print-failure rate, and life-cycle performance. Standardized reporting, data-driven mixture proportioning, in-process monitoring, life-cycle assessment, and performance-based acceptance criteria are identified as priorities for translating 3DP-UHPC from laboratory development to reliable structural deployment.