Researchers have found that recycled concrete powder (RCP) can replace up to 50 % of Portland cement in 3D-printed mixes—reducing carbon emissions by over half while maintaining printability. The study evaluated two types of RCPs in cementitious pastes using rotational rheometry, isothermal calorimetry, and life cycle assessment (LCA), along with printability tests to assess extrudability and buildability.
Study: Rheological and Environmental Implications of Recycled Concrete Powder as Filler in Concrete 3D Printing. Image Credit: natthawut.2529/Shutterstock.com
Rethinking Cement in 3D Printing
Three-dimensional concrete printing (3DCP) is becoming an increasingly attractive approach for improving construction efficiency and reducing labor and material costs. However, the high cement content typically required in printable mixtures continues to raise environmental concerns.
One potential solution is to incorporate alternative materials, particularly supplementary cementitious materials like recycled aggregates and powders. While recycled aggregates have seen more widespread use in 3DCP, especially as substitutes for sand and coarse aggregates, recycled concrete powders remain less explored despite their potential.
Because RCPs affect both the physical makeup and chemical behavior of cement-based mixes, they can play a significant role in shaping the fresh and hardened properties of the material. This study set out to develop printable cement mixtures that significantly reduce cement content by replacing it with two different types of RCP, then evaluate their performance from both a technical and environmental perspective.
Materials and Methodology
The researchers used two types of Portland cement: a high early-strength cement (CemHES) and a limestone filler cement (CemL). The recycled concrete powders came from two distinct sources—lab-cast concrete specimens (RCP-L) and waste concrete collected from a local ready-mix plant (RCP-C).
Each powder was characterized using a range of physical and chemical tests, including laser granulometry, helium pycnometry, Blaine fineness, BET surface area, and X-ray fluorescence. Water absorption was measured through a constant volume saturation test, while ignition loss was determined via high-temperature furnace testing.
In the mix design, 50 % of the cement was replaced with either RCP-L or RCP-C. Isothermal calorimetry was used to monitor hydration behavior over seven days at 25 °C. Rheological properties—namely static yield stress and plastic viscosity—were assessed with a rotational viscosimeter. Environmental impacts were analyzed through life cycle assessment.
To test printability, mortars containing RCPs were compared with control mixtures. Extrudability was evaluated by printing a 3.72-meter-long filament and observing its flow and shape consistency. Buildability was assessed by printing a cylindrical structure layer-by-layer, monitoring how well it held form over successive deposits.
Key Findings
The properties of the RCPs varied significantly based on their source and recycling process. RCP-L, created under controlled lab conditions, had finer grains and retained more hydrated paste content. RCP-C, derived from washed concrete truck residues, was coarser, absorbed more water, and had a lower surface area.
Although RCP-C exhibited better packing density than RCP-L, both powders were less dense than the cements. When mixed, RCP-C offered slightly higher compaction. However, chemical analysis showed that RCP-L contained more hydrated residues, indicating a greater presence of porous paste material.
Calorimetry revealed that both RCPs accelerated early hydration compared to the reference mix—mainly due to the filler effect—but ultimately resulted in lower total heat output. These hydration trends were reflected in rheological tests: RCP-L pastes showed higher yield stress than both the reference and RCP-C mixes, suggesting better structural stability.
From an environmental perspective, the results were compelling. Replacing 50 % of the cement with RCPs led to an average 51 % reduction in carbon emissions. Printability tests showed some trade-offs: RCP mixes displayed greater variation in filament width and some crumpling at the edges. Still, RCP-C’s lower viscosity improved flow during extrusion, which can be an advantage in certain applications.
Conclusion
This study demonstrates that recycled concrete powders can effectively replace a significant portion of cement in 3D-printed mixtures, offering up to a 62 % reduction in CO2 emissions without undermining print performance.
The research also highlights the importance of the powder’s origin—RCP-L, from lab-prepared concrete, delivered better rheological performance than RCP-C, despite its lower compaction. Importantly, the low water absorption levels of both powders had minimal impact on the paste’s fresh-state behavior.
While promising, the printability tests underline the need for careful calibration of mix properties and print settings when working with high RCP content. Striking the right balance between flow characteristics and structural buildability will be essential to scaling up this approach in practice.
Journal Reference
Cavalcante, T. C., Filho, R. D. T., & Reales, O. A. M. (2025). Rheological and Environmental Implications of Recycled Concrete Powder as Filler in Concrete 3D Printing. Buildings, 15(8), 1280. DOI: 10.3390/buildings15081280, https://www.mdpi.com/2075-5309/15/8/1280
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