Particle Size Distribution and Extrusion Performance of Mine Tailings-Based Mortars for 3D Concrete Printing: A Case Study from Chile and Japan

The incorporation of mine tailings into 3D concrete printing offers a promising pathway to reduce cement consumption while valorizing large volumes of mining waste. This study investigates the feasibility of replacing 30% of cement with Chilean copper mine tailings through two experimental campaigns conducted in Chile and Japan using different local materials and extrusion systems. Fresh-state behavior was evaluated through flowability testing and rheological characterization of the printable mixtures, while printability and hardened-state performance were assessed through printing trials and mechanical characterization. Mine tailings incorporation generally increased flowability and reduced yield stress and viscosity while maintaining adequate structural build-up for printing. However, one mine tailings-containing mixture exhibited complete extrusion blockage despite presenting flowability comparable to successfully printed mixtures. This discrepancy indicated that the flow-table results and visual workability assessment alone did not predict the extrusion blockage observed in the present case. Analysis of particle size distribution, particle arrangement, and paste film thickness was consistent with a possible interaction between paste availability, granular characteristics, and the confined flow conditions generated by the nozzle contraction. However, because the redesigned mixture involved simultaneous changes in granular composition, water content, and superplasticizer dosage, the contribution of individual factors to extrusion recovery could not be isolated. To address this issue, the mix design was revised by separating particles finer and coarser than 75 µm, treating the coarse fraction as part of the aggregate skeleton. The revised mixture was successfully extruded and printed in the investigated case study. The results highlight the importance of considering particle size distribution together with rheological properties when incorporating unconventional fine materials into 3D printable mortars and provide practical guidance for the use of mine tailings in digital construction applications.