Recycled glass to replace river sand in 3D printed concrete

Researchers at RMIT University in Melbourne, Australia, have developed a sustainable 3D printing process of concrete for building structures by replacing 50% of the river’s natural sand with different grades of recycled glass. This study is available in the journal Building and construction materials.

To study: 3D printed concrete with recycled glass: effect of glass gradation on flexural strength and microstructure. Image Credit: Belish / Shutterstock.com

The researchers analyzed the impact of the orientation of the beam-shaped sample relative to the printing direction, the correlation between microstructures and mechanical properties, and the effect of glass particles on resistance to bending and crack propagation.

With the advent of rapid prototyping technology, the traditional construction industry has leaped into a different dimension of high-speed construction, intrinsic design and cost efficiency. 3D Concrete Printing Technology (3DCP) enables the fabrication of concrete structures through layer-to-layer deposition based on extrusion. However, it still inherits the drawbacks of traditional molding-based construction, especially those that are not environmentally friendly..

Portland cement and natural river sand are key components of traditional building techniques. However, they also cause carbon dioxide emissions and the exploitation of limited natural resources.

What is a more sustainable approach for 3DCP?

The answer to this is to replace natural river sand with recycled glass. Hundreds of metric tons of waste glass are generated around the world. Also, the amount of recycled glass is significant in developed countries. Recycling this waste glass leads to fewer landfills and more energy savings. In addition, the chemical compositions and physical properties of glass resemble those of concrete raw materials, such as river sand and cement.

About the study

Researchers investigated the relationship between the gradation of recycled glass and the performance of 3D-printed concrete structures in terms of microstructures and bending properties. They prepared five sample mixes with varying matrix / filament ratios.

Further, classified the recycled glass into two different categories, viz. coarse and fine. Then mixed them at various mixing ratios keeping the overall sand / glass ratio constant. The median grain sizes of coarse glass, fine glass, and river sand are 796 m, 367 m, and 204 m, respectively.

The mechanical properties are investigated on the basis of the three-point bending test. X-ray computed tomography (μCT) is used for microstructural evaluations, to analyze pore size and spatial distribution in prepared specimens and fracture area and crack propagation path in post-test specimens. Scanning electron microscopy (SEM) is used to probe the microstructures of interlayer binding regions and the interface between glass and cement arrays.

3D printed concrete. Image Credit: Zapp2Photo / Shutterstock.com

Binders used for this study included general purpose cement (GPC), ground slag (GS) and silica fume (SF). River sand and recycled glass were obtained locally. A small amount of clay is added to the mixture to improve the constructability of the printed structures. Two different directions of print deposition are incorporated, one along the extent of the beam sample and the other across it.

What does the study show?

The researchers partially replaced river sand with recycled glass and studied the effect of different grades of recycled glass on the bending performance of 3DCP structures and found the following results. The presence of glass particles increases the porosity of the samples.

In addition, the porosity of samples containing coarse glass particles is greater than that of samples containing fine glass; the porosity is highest for a sand / glass ratio of 50:50. In addition, the amount of medium to small pores is almost the same for all samples; the number of large pores increases with an increase in the coarse glass content.

X-ray CT images confirm that the main factor in the propagation of cracks is the presence of glass particles in the voids. Samples containing glass particles show stress softening in the post-peak region of the stress-strain graph, while samples with only sand filaments show brittle fracture. In addition, the flexural strength of the samples depends on the quality of the glass grains and the type of loading. The addition of glass particles increased the flexural strength from 25% to 33% when the scope of the beam is perpendicular to the printing direction, unlike this,

when the extent of the sample was parallel to the printing direction, it decreases the flexural strength from 8% to 20%.

Therefore, a cross-loaded 3DCP with an optimal concentration of coarse glass particles is a suitable alternative to natural river sand and a sustainable approach in the construction of building structures.

Reference:

Liu, J., Li, S., Gunasekara, C., Fox, K., Tran, P., 3D printed concrete with recycled glass: effect of glass gradation on flexural strength and microstructure, Building and construction materials, 314, part B, 2022, 125561, ISSN 0950-0618. https://www.sciencedirect.com/science/article/pii/S0950061821032992?via%3Dihub

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