Eco-friendly 3D printed concrete innovations

A recent article published in Materials presented three environmentally friendly alternatives for the creation of artificial aggregates (AAs), including organic hemp shives (HSs), pyrolyzed coal (charcoal), and slag from solid waste incinerators (SWIPs). The use of the aggregates prepared from these was investigated in 3D printed concrete (3DPC).

Eco-friendly 3D printed concrete innovations
Study: Environmentally friendly 3D printed concrete innovations. Image credits: sergey kolesnikov /Shutterstock.com

Background

The use of sustainable building materials has become necessary to achieve the target of a carbon-neutral construction sector by 2050. Therefore, 3DPC is prepared using sustainable mixed materials such as rice husk ash, marble dust and municipal waste incineration plant ash.

Commonly used thermal power plants generate large amounts of waste and BS, the accumulation of which in landfills poses significant challenges for waste management. BS is also applicable as a replacement in mortar and recycled fine/coarse lightweight aggregate in green concrete. In addition, aggregates made from BS can replace all natural gravel in concrete.

Of the various organic agricultural wastes used in the production of 3DPC, hemp is the most popular. It is known for its insulating properties and environmental friendliness.

Another widely used organic material in grilling nowadays is charcoal. Its lightweight, insulating and absorbent properties are attractive in lightweight concrete or concrete blocks as a replacement of sand. Therefore, this study combined AAs made from organic HSs and charcoal, and BS to produce environmentally friendly 3DPC.

Methods

The main binders for 3DPC production include ordinary Portland cement (OPC; 30%), hydrated lime (HL; 2%) and burnt fly ash (BFA; 9%). In addition, various amounts of natural aggregate (55% to 41%) and sand were used in 3DPC, in addition to BS, HS and charcoal AAs.

For granulation, HS and charcoal were ground in a cutting mill. The size of all AAs was guaranteed to be smaller than 4 mm by sieving. In addition, sugars in HS could influence the properties of the concrete. Therefore, a sugar refractometer was used to measure the sucrose content in the investigated organic components.

Agitation granulation was performed to mechanically produce AAs with nine different compositions. The grain diameter was guaranteed to be below 4 mm for comparison with the natural aggregate containing 3DPC. Of each type of AA, 10 most rounded grains were selected and heated at 105 °C until a constant weight was reached.

Then, a thin layer of wax was applied to each granule and weighed using hydrostatic balances. In addition, the bulk density of the granules was measured by free fall in a one-liter bowl. Then, 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 evaluated according to the standard flow table test. In addition, the flexural and compressive strength and freeze-thaw resistance of the 3DPC composites were investigated by fabricating prism samples.

Results and discussion

The mechanical agitation granulation parameters were optimized to obtain granules acceptable for 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.

The SEM images of AAs showed absorption of the wet and dry mass of the binders in both organic materials, forming spherical or round grains. Unlike charcoal, HS AAs became more brittle after sieving with weaker bonding to the binder layer. However, loosely granulated BS showed the most favorable strength of 3DPC.

Commonly used burnt oil shale ash and lime showed weaker strength than the AAs proposed in this study. Moreover, the concrete with BS had the same performance as the reference concrete with natural aggregates. Furthermore, the reference mix performed poorly in the deformation tests compared to the 3DPC compositions with BS and HS aggregates. This was attributed to the high stability of BS and the fibrous nature of HS.

Regardless of the relative strength, these results show the advantage of granulation of materials to obtain particles of similar sizes, making them suitable for 3D printing. Furthermore, the processed organic aggregates made 3DPC more stable (smaller deformations) than the non-granulated organic aggregates.

In addition, the samples without granulated organic AAs showed inferior accession (only 2.2-2.7%) in freeze-thaw resistance test. The deformation graph showed that the expansion regulator could only regulate deformations in concrete when the organic components were granulated. Otherwise, the regulator slows down the reactivity of organic materials in a concrete mixture.

Conclusion

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

The comparison between 3DPC consisting of raw 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 such as HS need to be protected from the negative effects of humid environments.

Journal reference

Butkutė, K., Vaitkevičius, V., & Adomaitytė, F. (2024). Environmentally friendly 3D printed concrete made from 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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