NTU’s New Recycled Glass Concrete Mix Leads the Way in High-Strength Construction 3D Printing

Researchers at Nanyang Technological University (NTU Singapore) are exploring the use of recycled glass as a replacement for sand in 3D printing concrete, paving the way for environmentally friendly building materials.

Many conventional concrete formulations today contain natural sand as a primary component. Unfortunately, due to urbanization and infrastructure development, sand is slowly becoming a scarce resource, so there is an increasing need to find a suitable alternative.

Glass, on the other hand, is one of the least recycled wastes, although it is fully recyclable without loss of quality. According to the National Environment Agency of Singapore, 74,000 tonnes of glass waste was produced in Singapore in 2021, but only 13% of it was recycled. The rest was either incinerated or dumped in a landfill.

In an effort to harness the untapped potential of glass, the NTU team created a concrete mix made from recycled glass and used it to 3D print a 40cm high concrete bench. The proof of concept successfully demonstrates that their special material formulation can be used to additively manufacture everyday load-bearing objects, boding well for high-strength construction.

The bench printed in 3D. Photo via NTU Singapore.

Why does concrete need aggregates?

Concrete is a mixture of Portland cement, water and aggregates such as sand, gravel and glass. Aggregates act as a binder for the mix and without them, water-based concrete does not hold together and is not strong enough to support significant compressive loads. Aggregates also serve to reduce the porosity of concrete mixes, acting as a filler to further increase strength.

One of the most common aggregates used in the construction industry is natural river sand, which is mined from river beds and banks. Although it may be readily available in the market, the overuse of river sand can lead to pollution, flooding and other extremely damaging consequences for local ecosystems that live in and around rivers.

Glass, while promising, has never been able to match the strength properties of conventional sand, so it is often seen as an inferior alternative in construction 3D printing. According to the NTU researchers, however, no one has been able to 3D print a concrete structure made from recycled glass as strong as the bench until now.

Professor Tan Ming Jen, lead researcher of the study, said: “The main challenge in formulating 3D printable concrete mixes is determining how much of each component to add to achieve a structurally sound structure with minimal defaults. Our team has come up with a workable formula, demonstrating for the first time that glass can indeed be used to 3D print a bench with excellent structural integrity.

Members of the NTU research team include (LR, standing) Lim Jian Hui, Professor Tan Ming Jen, (LR, seated) Andrew Ting and Noel Tan.  Photo via NTU Singapore.
Members of the NTU research team include (LR, standing) Lim Jian Hui, Professor Tan Ming Jen, (LR, seated) Andrew Ting and Noel Tan. Photo via NTU Singapore.

Recycled glass instead of sand

To kick off the project, NTU researchers first established the optimal composition of their 3D-printable recycled glass concrete mix through extensive analysis and testing. The mix included commercial cement, water, additives and crushed recycled glass in a variety of sizes from “medium” to “super fine”.

A four-axis gantry-based 3D printer with a build volume of 1.2 x 1.2 x 1 m was then used to 3D print the L-shaped concrete bench. According to the researchers, the material was highly exrudible, meaning it was fluid enough to flow through the print nozzle without any problems. It would also have exhibited excellent buildability as it did not collapse or even deform in subsequent compressive strength tests.

Andrew Ting, first author of the study, said: “Our research has shown that recycled glass can be used to replace up to 100% of the sand in concrete for 3D printing. The result is a concrete bench whose mechanical strength meets acceptable industrial standards.

The NTU research team now intends to fine-tune the printing algorithm for consistent performance and partner with recycled glass startup Soda Lemon to 3D print even larger structures using their durable concrete mix.

Further details of the study can be found in the article titled “Parametric study of the extrudable region of 3D printable concrete using recycled glass concrete”.

Glass aggregates have been explored in construction 3D printing research in the past, but with less success. Researchers from Technische Universität Berlin (TU Berlin) and Brunel University recently replaced sand in 3D-printable concrete with recycled glass, limestone and plastic fillers. The team succeeded in significantly improving the strength and thermal conductivity of the material.

Elsewhere, researchers at RMIT University have already developed an eco-friendly 3D printable concrete material that uses recycled glass as the aggregate. Through their work, the RMIT team believe they can spread the principles of the circular economy in the construction sector, combining the advantages of 3D printing and sustainable concrete production.

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Featured image shows members of the NTU research team. Photo via NTU Singapore.

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