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Recycled glass could reduce concrete’s reliance on sand

Words:
Stephen Cousins

As well as having high embodied carbon, concrete is also draining the world's supplies of fine-grained sand. Now researchers have shown that recycled glass could provide a suitable replacement for 3D-printed buildings

A 40cm-tall bench was 3D printed as a proof of concept.
A 40cm-tall bench was 3D printed as a proof of concept. Credit: NTU Singapore

Concrete’s damage to the environment isn’t limited to high embodied carbon. A requirement for fine-grained sand as a primary component in the mix has depleted natural resources and decimated natural river systems and habitats worldwide.

But researchers from Nanyang Technological University in Singapore (NTU Singapore) have come up with a more ecologically sound approach, replacing the sand component in 3D printed concrete with recycled crushed glass.

A 40cm-tall proof-of-concept bench made from the material has passed compression and strength tests and showed 'excellent' buildability and extrudability.

The same mixture could be used to 3D-print buildings, say researchers, as the recycled glass is a 100 per cent substitute for sand, although high-rise buildings would still require steel reinforcement bars.

Glass has the benefit of being completely recyclable with no reduction in quality, yet in Singapore only 13 per cent of the 74,000 tonnes of glass waste produced in 2021 was recycled, the rest was either incinerated or dumped in landfill.

Meanwhile, heavy reliance on natural sand in concrete, extracted from river beds and banks, has encouraged unregulated sand mining in the developing world, causing irreparable damage to water systems and habitats in countries such as India and China.

According to Tan Ming Jen, professor and principal investigator at NTU Singapore, the extent to which recycled glass can stem a reliance on natural sand in concrete depends on the scale of global ambition.

'It’s a question of how much glass we recycle from demolition, recycling waste windscreens, drink bottles, etc,' says Tan. 'Our insatiable need for sand is the result of rapid urbanisation. Besides buildings, there is a need for roads and dams etc, so we will need many different solutions at the same time.'

  • Recycled glass was crushed into different sizes to create the concrete mix.
    Recycled glass was crushed into different sizes to create the concrete mix. Credit: NTU Singapore
  • Researchers at NTU Singapore claim the composite concrete they developed could be used to print buildings.
    Researchers at NTU Singapore claim the composite concrete they developed could be used to print buildings. Credit: NTU Singapore
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The composite concrete developed by researchers comprised ordinary Portland cement, fly ash and other constituents, plus recycled glass crushed into three different size classes.

Printing was carried out in a single session using a four-axis robot fitted with a nozzle, with the flow rate adjusted to match the hardening properties of the concrete.

According to Tan, the main challenge in formulating a viable concrete mixture is working out how much of each ingredient to add to achieve a sound structure with minimal defects.

Around three-quarters of glass is silicon dioxide, or silica, which comes from sand, which explains its effectiveness as a replacement for the material.

Researchers are currently studying the potential of using other waste material in concrete printing and other ways to cut embodied carbon apart from using recycled content. The study was published in a paper in the Journal of Building Engineering.

 

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