Researchers from RMIT University have developed a new eco-friendly 3D printable concrete material that uses recycled glass as an aggregate.
Conventional concrete formulations tend to rely on natural river sand as a primary ingredient, but this resource is slowly becoming a scarce commodity and its use tends to have damaging effects on the environment. By opting for recycled glass particles instead, the RMIT team believes its work could propagate circular economy principles in the construction sector, bridging the benefits of 3D printing and sustainable concrete production.
Interestingly, the team also investigated the effects of the glass particles on the flexural strength and crack propagation characteristics of the concrete. They determined that the novel formulation could actually provide benefits in the way of mechanical properties too, depending on the direction it is 3D printed.
Replacing river sand with recycled glass
To make concrete, you need a mixture of portland cement, water, and several aggregates such as sand and gravel. The aggregates are crucial as they act as a binder for the concrete mix. Without them, water-based cement mixtures don’t hold very well and aren’t suitable for construction applications. Aggregates also act as a filler for the mix, reducing the porosity of the concrete for a stronger product.
One of the most widely used aggregates in construction is natural river sand, which is extracted from river beds and riverbanks. Although it’s readily available on the market, the overuse of river sand can have extremely damaging consequences for local ecosystems that live in and around rivers.
The RMIT team believes that a more sustainable approach to concrete 3D printing lies in the use of recycled glass instead, since glass has a chemical composition and physical properties not too dissimilar to raw sand. According to the United States Environmental Protection Agency, we as humans produce millions of tons of glass waste every year – repurposing a portion of this in the construction sector could lead to significantly less landfill waste.
The effects of glass content on concrete performance
So how did RMIT’s glass-based mixture stack up against conventional concrete? The team prepared five different glass-filled mixes with varying ratios of glass to sand, and 3D printed a set of concrete test specimens using each of the formulations.
The flexural strength of the structures was characterized via a three-point bending test, while X-ray micro-computed tomography and scanning electron microscopy (SEM) were used to evaluate the specimens’ microstructures. This allowed the team to analyze the pore size distribution and crack propagation pathways in the post-test specimens.
The results showed that, firstly, the integration of recycled glass particles increased the porosity of the 3D printed samples. This was more evident with the use of coarse glass particles than fine glass particles. Using X-ray imaging, the researchers were able to show that the primary component of crack propagation was the presence of glass particles in voids.
Despite this, the flexural strength of the 3D printed samples actually increased by 25% to 33% with the addition of glass particles, but only when the beam span was perpendicular to the direction of printing. When the beam span and printing direction were parallel, the glass particles were found to decrease flexural strength by 8% to 20%.
As such, the RMIT team determined that recycled glass could be used as a suitable alternative to river sand for sustainable concrete 3D printing, provided the printing direction was carefully controlled.
Further details of the study can be found in the paper titled ‘3D-printed concrete with recycled glass: Effect of glass gradation on flexural strength and microstructure’.
Earlier this year, scientists based at the Swinburne University of Technology and Hebei University of Technology conducted a similar study, turning construction waste into a sustainable 3D printing concrete. Using recycled concrete aggregate, ceramsite particles, and desert sand, the team was able to formulate a low-cost extrudable building material in three different particle gradings.
Elsewhere, researchers from Tongji University recently published the results of a study exploring the viability of using recycled fine aggregates (RFA) in 3D mortar printing. The study sought to achieve a more accurate picture of the buildability of 3D concrete printing, and how the use of recycled and waste material impacts upon the fabricated structures.
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Featured image shows 3D printed concrete structures created by RMIT. Photo via RMIT University.