Performance of 3D-Printed UHPC Optimized by Particle Packing Model Under Standard and Steam Curing (2026-01)¶

This study developed four 3D-printed ultrahigh-performance concrete (UHPC) mixes using Type IL cement with fly ash (FA), and silica fume (SF), including a reference mix (Mix 1), mixes with optimal FA (Mix 2) and SF (Mix 3) content, and a low-clinker mix (Mix 4). Fresh properties (setting time, flowability, and thixotropic behavior), extrudability, and buildability were evaluated. Then, directional compressive strength at parallel (x), lateral (y), and perpendicular (z) was measured under moisture curing at 23°C and steam curing at 50°C and 90°C. The phase evolutions were characterized through thermodynamic modeling and SEM-EDS. The results indicate that Mixes 1 and 2 exhibited superior thixotropic behavior, buildability, and the highest compressive strengths (∼132 MPa at 56 days), 5-13% higher than Mixes 3 and 4. Strength anisotropy was minimal, with the z-direction only 3-5% lower than cast specimens, while the x-direction showed a 12% reduction. Thermodynamic modeling and SEM-EDS revealed higher calcium-silicate-hydrate (C-S-H) formation in Mixes 1 and 2, consistent with their mechanical performance. Notably, steam curing at 90°C promoted carbonate-containing gismondine and cubic calcium-carbon-aluminum-silicate-hydrate (C-C-A-S-H) formation, highlighting the potential of controlled curing to tailor microstructure and enhance UHPC performance. These findings provide practical guidance for optimizing 3D-printed UHPC formulations for structural applications.