With the pandemic came the necessary but waste-producing wave of single-use masks, especially in healthcare settings.
When you add them all up, we use over 1500 billion face masks globally per year, meaning that three million face masks are being thrown away every minute.
What if we could build this waste into something valuable?
Single-use medical masks are made of polypropylene or polyester fabric: fibres that, it turns out, could be re-purposed in the concrete industry.
Microfibres can sometimes be added to cement concrete to strengthen it. The fibres help by absorbing or dissipating fracture energy, reducing micro-cracking within the concrete. But microfibre is expensive.
Enter a Washington State University, US, research team, which has shown that incorporating old masks into a cement mixture can create a stronger, more durable concrete, and help curtail our mask waste problem.
To test this concept, the team removed the cotton loops and metal from masks, cut them up, and then reduced them into fibres ranging from five to 30 millimetres in length, with a diameter of 20-40 micrometres.
Some of the microfibres were mixed into a solution of graphene oxide (GO), which provides an ultrathin layer that strongly adheres to the fibre surfaces to aid in the waste-mask mix (WMM) adhering to the cement mix.
These were then incorporated into Portland cement, the most used type of cement in the world, which consists of just concrete, mortar and grout. However, it can be prone to cracking, especially through expansion and shrinkage with moisture.
To test whether waste-mask microfibres enhanced concrete, both compressive and splitting tensile strength tests were performed on hardened five-by-ten-centimetre cylinders, made with or without GO. The cylinders were hardened for either seven days or 28 days, to also test whether hardening time improves concrete performance.
Compressive strength was tested by applying diametric compressive pressure to the opposing flat bases of the concrete cylinder, while splitting tensile strength was tested by applying pressure along the entire length of the cylinder.
The promising results found that splitting tensile strength increased by 33 per cent with the WMM, and 47 per cent with the WMM+GO combination, compared to regular concrete mix.
Interestingly, the compressive strength was slightly weaker in both the WMM (-9 per cent) and WMM+GO (-3 per cent) concrete samples, indicating some tweaking of the mix is required. All the concrete cylinders that hardened for 28 days performed better overall (over 50 per cent) than the cylinders hardened for only 7 days, showing longer hardening times are essential.
Cement making is a carbon-intensive process and is responsible for as much as 8 per cent of global carbon emissions.
This technology has promise to provide a win-win-win-win situation - it would make construction stronger and cheaper, re-purpose a waste product, and help reduce environmental impacts by reducing the number of new materials required to make concrete.
"These waste masks actually could be a valuable commodity if you process them properly," says study corresponding author Professor Xianming Shi, of Washington State University's Department of Civil and Environmental Engineering. "This work showcases one technology to divert the used masks from the waste stream to a high-value application."
The researchers are hoping to extend this technology to include other polymer materials, such as discarded clothing.
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