Tough, Ductile, and Strong Hard-Soft Cementitious Composite Enabled by Multi-Material Additive Manufacturing.

Monolithic cementitious materials lack fracture resistance and are brittle. Advancements in additive manufacturing techniques with cement-based materials and architected designs have remained limited to the use of a single (cement-based) constituent. This work charts a new pathway for manufacturing and designing tough, ductile, and strong architected cement-based composites by proposing a novel multi-material additive manufacturing (MMAM) technique for the first time, integrated with a coupled experimental-numerical design approach. The new class of architected cementitious composites (ACC) of mortars and elastomeric constituents (silicone and polyurethane) are exemplified, through a layered hard-soft architected design, inspired by the microstructure of a sea sponge (glass sponge Euplectella aspergillum). The MMAM technique alternates extrusion of hard-soft composites, enabling systematic control over the geometry and constituents of resulting architected structures. The results demonstrate layered mortar-silicone composites achieved up to 3.9- and 8.8-fold enhancements in fracture toughness and up to 11.7- and 12.4-fold enhancements in ductility relative to monolithic 3D-printed (3DP) and cast mortars, respectively. A coupled large-deformation phase-field-cohesive-zone (PF-CZM) framework was used to systematically probe soft-layer thickness and bulk soft-material properties. Simulations revealed that combining higher-stiffness soft layers with reduced thickness can yield up to a 24-fold increase in work-of-fracture while recovering the load-bearing capacity of monolithic mortar. Guided by numerical predictions, experiments on thin, stiffer polyurethane interlayers validated the numerical predictions and provided additional experimental evidence that the proposed (mortar-polyurethane) composites achieve 82- and 187-fold higher fracture toughness, and 22.6-fold higher ductility, relative to 3DP and cast monolithic references while recovering the flexural strength to levels statistically comparable to monolithic mortar. These large gains arise from three synergistic mechanisms: crack arrest/deflection, crack bridging, and discontinuous layerwise crack re-nucleation, activated by the layered hard-soft architecture and supported by DIC/AE fracture analyses). The proposed MMAM-enabled fabrication-design-mechanics approach in ACC can unleash entirely new pathways for engineering next-generation damage-resilient and multi-functional concrete structures.