Would you live in a house made with recycled nappies?
Researchers have taken on the twin challenges of nappy waste and housing affordability, and they've even got a house to show for their efforts.
While it might sound a bit on the nose, it turns out nappies can be added to concrete mortar as a partial replacement for sand, according to research published today in Scientific Reports.
PhD student Siswanti Zuraida recognised that used nappies were a huge source of waste that went to landfill or worse, especially in regions lacking recycling infrastructure.
"The recycling process that is available [for nappies] is limited to developed countries because [the technology] is hard to apply and expensive," said Ms Zuraida of the University of Kitakyushu in Japan.
"So it is important to propose a low-cost recycling treatment for diapers in developing countries."
Ms Zuraida also saw the need for cheaper building materials and decided to test whether she could tackle two problems at once.
"Replacement of a part of the sand [in mortar] is an alternative way to reduce the cost of materials," she said.
"It also [means that we see] diaper waste as something valuable, since until now the waste is ending up in incineration or landfill."
Before we go any further, we need to clear something up: yes, the nappies were cleaned before use, according to Ms Zuraida.
"The step is to remove the faeces with water. For urine it only needs to be soaked in a solution of water that contains chemical additives. Then [it was] dried up and shredded."
Putting it to the test
While the idea of adding nappies to concrete wasn't entirely new, Ms Zuraida said her project was the first to put it to the test in housing.
The researchers' aim was to build a low-cost dwelling that complied with Indonesian building standards.
"Those [earlier studies] encouraged me to apply their findings on a macro scale, which is to build the actual housing by using the diapers as part of the building components."
Ms Zuraida and her colleagues first tested what is called the compressive strength of concrete — the integrity when different ratios of nappy were submitted for sand.
Sand is added to cement along with an aggregate such as rocks, and water to make concrete. Sand's role in the mix is to increase density of the mortar, prevent shrinkage, increase volume (more economical), and to increase the surface area coming into contact with the binding agent (cement).
Though ratios change depending on the application, a typical concrete mix is around 60 – 80 per cent sand and gravel.
The researchers found the compressive strength of the concrete declined as more nappy was substituted, meaning different ratios were needed depending on the use of the mix.
Overall, they found they could use 1.73m3 of reclaimed nappies in their building, which had a floor area of 36m2.
Structural and load-bearing components had a maximum capacity of 10 per cent nappy, and non-structural components could take up to 40 per cent nappy.
The found the nappy ratio would need to be reduced in structural columns if more floors were added to the building.
Could we use it in Australia?
Rackel San Nicolas, who researches sustainable construction materials at the University of Melbourne, said there are a lot of regulations in Australia that would stop nappies being used in concrete here on any commercial scale.
However, Dr San Nicolas says, we are already substituting aggregates, and cement itself, for waste products in a bid to make concrete more environmentally friendly.
The manufacture of cement involves converting calcium carbonate to lime, releasing CO2 in the process.
Substituting cement for fly ash, which is a by-product of burning coal, helps cut down on CO2 emissions, according to Dr San Nicolas, who was not involved with the study.
"At the moment on the market, we've got easily up to 50 to 60 per cent [substitution] of cement," she said.
"We are testing and trying to [prove] more applications where we can use no cement at all, where it's only fly ash — cement-free concrete basically."
Old concrete itself can be crushed up and reused as an aggregate too.
And there are other applications where waste products are being used in place of virgin materials, from old tyres in playgrounds to glass in slabs, walls, and road base.
But Dr San Nicolas doesn't see nappies becoming part of Australia's concrete mix anytime soon.
"I know nappies are a very big waste problem, but I would never use it in concrete, that's for sure," she said.
"I just cannot imagine how it would be used."
