Eco-Friendly 3D-Printed Concrete Innovations

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 9 2024

A recent article published in Materials presented three eco-friendly alternatives for creating artificial aggregates (AAs): organic hemp shives (HSs), pyrolyzed coal (charcoal), and solid waste incinerator bottom slag (BS). The usage of these aggregates was investigated in 3D-printed concrete (3DPC).

Background

Using sustainable building materials has become essential to achieve the 2050 goal of a carbon-neutral building industry. Consequently, 3D-printed concrete (3DPC) is prepared using sustainable mixing materials such as rice husk ash, marble dust, and burnt ashes from municipal solid waste incinerators.

Widely popular cogeneration power plants generate large amounts of waste and bottom slag (BS), the accumulation of which in landfills poses significant waste management challenges. Alternatively, BS can be used as a replacement in mortar and as recycled fine/coarse lightweight aggregate in green concrete. Moreover, granules made from BS can replace all of the natural gravel in concrete.

Among different agricultural organic wastes used in 3DPC manufacturing, hemp is the most popular. It is well known for its insulating properties and environmental friendliness.

Another frequently used organic material in grilling today is charcoal. Its lightweight, insulating, and absorption properties make it attractive in lightweight concrete or concrete bricks as a sand replacement. Thus, this study combined artificial aggregates (AAs) made from organic hemp shives (HSs), charcoal, and BS to produce eco-friendly 3DPC.

Methods

Ordinary Portland cement (OPC; 30 %), hydrated lime (HL; 2 %), and burnt fly ash (BFA; 9 %) were the main binder materials for 3DPC production. Additionally, differing amounts of natural aggregate (ranging from 55 % to 41 %) and sand, along with BS, HS, and charcoal AAs, were used in 3DPC.

Before granulation, HS and charcoal were milled in a cutting mill. Sieving ensured that all AAs were less than 4 mm in size. Since the sugars present in HS could affect the properties of the concrete, a sugar refractometer was used to measure the sucrose content in the investigated organic components.

Agitation granulation was performed to produce AAs with nine different compositions mechanically. The granule diameter was kept under 4 mm for comparison with the natural aggregate-containing 3DPC. Ten of the most-rounded granules from each type of AA were selected and heated at 105 °C until a constant weight was reached.

Subsequently, a thin layer of wax was applied to each granule, and they were weighed using hydrostatic scales. Additionally, the bulk density of the granules was measured through free fall in a one-liter bowl. Next, the strength of different granules was compared in 3DPC composites in the fresh state.

Energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) were used to investigate the microscopic structure of AAs and 3DPC elements. The flow of newly mixed composites was assessed following the standard flow table test. Additionally, the flexural and compressive strength and freeze-thaw resistance of the 3DPC composites were examined by fabricating prism samples.

Results and Discussion

The parameters for mechanical agitation granulation were optimized to obtain granules suitable for 3D printing. Approximately 80 % of the total mass of these granules was achieved with slow water spray and a rotation speed of 35 rounds per minute.

SEM images of the AAs revealed absorption of the moist and dry mass of the binders in both organic materials, forming spherical or round granules. In contrast to charcoal, HS AAs became more brittle after sieving, with weaker bonding to the binder layer. However, loose granulated BS exhibited the most favorable strength in 3DPC.

Widely used burnt oil shale ash and lime exhibited weaker strength than the AAs proposed in this study. Additionally, concrete with BS performed comparably to reference concrete comprising natural aggregates. The reference mix performed poorly in deformation tests compared to the 3DPC compositions containing BS and HS granules, which is attributed to the high stability of BS and the fibrous nature of HS.

Regardless of the relative strength, these results highlight the benefit of granulating materials to obtain particles of similar dimensions, making them appropriate for 3D printing. Moreover, the processed organic aggregates made 3DPC more stable (with smaller deformations) than non-granulated organic aggregates.

Specimens without granulated organic AAs exhibited inferior performance in the freeze-thaw resistance test, with only 2.2-2.7 % accession. The deformation graphic indicated that the expansion regulator could control deformations in concrete only when the organic components were granulated. Otherwise, the regulator slowed the reactivity of organic materials in a concrete mix.

Conclusion

Overall, the researchers successfully demonstrated the potential of mechanically-produced AAs in manufacturing low-carbon 3DPC with enhanced properties. Specifically, the lightweight aggregates of HS, charcoal, and BS could constitute up to 14 wt.% in concrete without compromising performance.

The comparison between 3DPC comprising unprocessed and granulated HS, charcoal, and BS after 28 days of curing indicated the high performance of the latter. However, analysis of the granulation process indicated that organic materials like HS need to be safeguarded from the negative effects of humid environments.

Journal Reference

Butkutė, K., Vaitkevičius, V., & Adomaitytė, F. (2024). Eco-Friendly 3D-Printed Concrete Made with Waste and Organic Artificial Aggregates. Materials, 17(13), 3290. DOI: 10.3390/ma17133290, https://www.mdpi.com/1996-1944/17/13/3290

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