Printable concrete with multi-source recycled aggregates: particle packing design, rheological behavior, and 3D printing validation

Abstract Concrete 3D printing faces challenges due to the global shortage of natural sand. In this context, construction and demolition waste, scallop shell waste, and recycled PET offer sustainable alternatives. This study develops and validates a concrete mix design methodology incorporating these recycled aggregates, using the Funk-Dinger model to optimize particle size distribution for 3D printing. A comprehensive experimental campaign was conducted to characterize the physical properties of source and blended aggregates, and to assess mixture printability through mortar flow tests, medium-scale 3D printing trials, and rotational rheometry. The results indicate that dense and loose packing fractions are governed not only by particle size distribution but also by aggregate morphology, as quantified by the Sphericity Deviation Index (SDI). Five formulations incorporating binary, ternary, and quaternary natural and recycled aggregate blends were successfully printed at a medium scale. Printable mixtures exhibited static yield strength values in the range of about 1500–2000 Pa, consistent with requirements for extrusion-based printing. In addition, flow-loss rates over 30 min ranged from approximately 3.2 to over 6 mm/min and were found to correlate with aggregate water absorption, granulometric parameters, and superplasticizer dosage. An exploratory, data-driven classification analysis further indicated that static yield strength and SDI are informative indicators for distinguishing printable from non-printable mixtures within the investigated material space. Overall, the findings highlight that, beyond controlling flowability and particle size distribution, accounting for aggregate shape and packing behavior is essential for the development of printable concretes incorporating recycled aggregates. The proposed framework provides a basis for future optimization and extension toward sustainable additive construction.