Copper Slag Replaces River Sand in 3D Printed Concrete, Study Finds

Construction is one of the world’s largest consumers of natural resources, and few materials illustrate this better than river sand. For decades, the fine aggregate that gives concrete its body has been dredged from riverbeds at a pace that far exceeds natural replenishment, driving sand scarcity, ecological damage and, in some regions, illegal mining. At the same time, the copper industry generates enormous quantities of slag, a glassy by-product of smelting that is typically stockpiled in landfills. A new study from researchers at the CSIR-Structural Engineering Research Centre in Chennai, India, published in Environmental Science and Pollution Research, suggests that these two problems may share a single solution: replacing river sand entirely with copper slag in concrete formulated for 3D printing.

The research, led by Atchaya Sowmiyan under the supervision of Parukutty Sanker Ambily, with Senthil Kumar Kaliyavaradhan as corresponding author, tackles one of the most demanding corners of modern construction technology. Three-dimensional printable concrete, often abbreviated 3DPC, is not ordinary concrete. It must flow smoothly through a nozzle, hold its shape the instant it emerges, and support the weight of successive layers without slumping or deforming. Balancing these competing demands is difficult even with conventional materials, and introducing an industrial by-product as a full replacement for sand risks upsetting the delicate rheological equilibrium that makes extrusion-based printing possible.

To test whether copper slag could meet the challenge, the team formulated three mixes. The reference mix, designated RS-BM, used river sand as the fine aggregate. Two further mixes replaced the sand completely with copper slag, one on a volume-for-volume basis, designated CS-VR, and the other on a weight-for-weight basis, designated CS-WR. Because copper slag is denser than river sand, the distinction between volume and weight replacement matters considerably: it changes the total volume of paste available to coat and lubricate the particles, which in turn alters flow, cohesion and the ability of the fresh material to bond layer upon layer.

The researchers evaluated the fresh, or unhardened, properties of each mix across the full suite of criteria that govern printability. These included rheology, the science of how the material flows under stress; flowability, a measure of how readily the mix spreads; extrudability, its ability to pass continuously through the printing nozzle; open time, the window during which the mix remains printable after mixing; and buildability, its capacity to sustain stacked layers without collapse. Each of these properties must fall within a narrow band. A mix that flows too easily will produce weak, slumping layers, while one that is too stiff will clog the printer and leave gaps between filaments.

The results were encouraging for the volume-replacement mix in particular. CS-VR maintained desirable flow and slump characteristics, and its printability and stability were consistent with those of the river sand reference. Most strikingly, the copper slag mix printed by volume replacement exhibited the longest open time of the three formulations and superior buildability, with minimal shape deformation as layers accumulated. In practical terms, a longer open time gives operators a wider safety margin between batching and printing, reducing waste and allowing larger or slower prints, while high buildability permits taller structures before the material at the base becomes overloaded.

The rheological analysis added a quantitative layer to these observations. The team fitted their flow measurements to mathematical models describing how stress relates to shear rate in the fresh material, and found that the Modified Bingham model best represented the dynamic yield stress, the threshold stress that must be exceeded before the concrete begins to flow. Dynamic yield stress is a central parameter in 3D printing: it must be low enough for smooth extrusion yet high enough for the extruded filament to retain its shape. The CS-WR mix, in which copper slag replaced sand by weight, consistently showed the highest dynamic yield stress values of the three mixes, indicating robust printability and strong structural integrity in the fresh state.

Viscosity measurements across a range of shear rates revealed another important characteristic: all three mixes displayed shear-thinning behaviour, meaning their viscosity decreased as the rate of shear increased. This is precisely the property a printable concrete needs. Inside the nozzle and pump, where shear rates are high, the material flows readily; once deposited on the print bed and left at rest, it thickens and holds its form. Interestingly, the river sand reference mix exhibited higher initial viscosity than either copper slag formulation, suggesting that the glassy, smooth-textured slag particles may reduce internal friction in the fresh paste, easing the material’s journey through the printing system.

Beyond the laboratory performance figures, the environmental logic of the substitution is compelling. Copper slag accumulates as a waste stream in smelting regions, occupying land and posing potential leaching concerns, while river sand extraction has become a global sustainability crisis, with construction demand depleting waterways and coastlines. A concrete that consumes slag instead of sand addresses both burdens simultaneously. The authors conclude that copper slag-based mixes, in both their volume-replacement and weight-replacement forms, offer potential benefits over river sand mixes for 3D printable concrete in terms of material performance as well as environmental impact, positioning the by-product not as a compromise but as an upgrade.

The study also fits into a broader movement within digital fabrication research, in which groups worldwide have explored recycled brick powder, waste glass, construction and demolition debris and other secondary streams as aggregates for printed concrete. What distinguishes the present work is the achievement of full replacement, rather than partial substitution, while preserving the fresh-state properties that printing demands. The work was carried out at the CSIR-Structural Engineering Research Centre in Chennai and funded by the Council of Scientific and Industrial Research, New Delhi, with the first author completing the investigation as an M.Tech thesis project.

For the construction industry, the findings arrive at a moment when 3D printing is moving from experimental pavilions to housing projects, and when regulators in sand-stressed countries are tightening restrictions on riverbed mining. If copper slag can reliably replace river sand in printable mixes, printers could draw on an abundant industrial residue to build walls, homes and infrastructure with a smaller ecological footprint. The Chennai team’s results, demonstrating stable extrusion, extended open time, high buildability and favourable rheology with minimal deformation, provide the fresh-property evidence base that engineers need before such mixes can advance to structural trials and, ultimately, to printed buildings that turn smelter waste into shelter.

Subject of Research: Use of copper slag as a fine aggregate replacement for river sand in 3D printable concrete

Article Title: Effect of copper slag as fine aggregate on the fresh properties of 3D printable concrete

Article References: Sowmiyan, A., Ambily, P. S., Kaliyavaradhan, S. K., Ayyanarsamy, V., & Shekar, D. (2026). Effect of copper slag as fine aggregate on the fresh properties of 3D printable concrete. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38239-0

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38239-0

Keywords: 3D printable concrete, copper slag, river sand, rheology, buildability, extrudability, open time, dynamic yield stress, shear-thinning, sustainable construction, fine aggregate, industrial by-products