Redefining how we see and use waste

Materials World magazine
,
4 Mar 2020

Soon we may be walking on plastic bottle roads along fly ash streets, as increasing categories of waste products are finding new life as feedstocks. Eoin Redahan takes a look at some of the pioneering projects tackling the waste problem.

If life gives you lemons, make lemonade. If it leaves you a mountain of plastic bottles, use them to build plastic roads. If it deals you polluted sludge on a riverbed, dredge it, and use the hard-standing to make new land, and when industry coughs up fly ash industrial spoils, use them to make a greener concrete. All around us, organisations are coming up with novel ways to re-purpose waste and designing new products to form circular materials economies.

Following the plastic path

MacRebur is one such innovator. The company has taken the humble plastic bottle and re-worked it into roadbuilding. The idea came about several years ago when MacRebur CEO, Toby McCartney, visited India and saw local people mending potholes by putting plastic waste into them, adding diesel, and setting fire to the mixture. From this spectacle came the inspiration to build a plastic road.

Today, MacRebur also melts plastics but the company collects all types of waste plastic and reforms them into pellets that can replace some of the bitumen used to produce asphalt for roads. As well as saving expensive resources, this approach diverts plastics from landfill.    

The company’s complex arrangements of polymers have been added to roads in 15 countries, including a 270m strip in Jeffrey’s Bay, South Africa, which contains the equivalent of 55,200 plastic bottles or 145,000 single-use carrier bags.

The percentage of plastic incorporated in the mixtures depends on the surface and the purpose of the road. According to McCartney, MacRebur has used 30% plastic content for private roads and car parks, but BS/EN roads standards mean the amount is lower on public roads – about 6% of binder volume. MacRebur claimed the plastic additives make these roads slightly more flexible than conventional roads, helping them cope better with expansion and contraction, thereby reducing cracks and potholes.

Twenty local authorities in the UK have used MacRebur’s pellets in their roads, with a large proportion being in Yorkshire. And, with more than 200 councils announcing a climate emergency, the growing environmental concern could play well for MacRebur. However, there remain plastic road sceptics. The company said it has conducted independent laboratory testing to prove its products do not leach or generate toxic fumes but others have urged caution. 

In Australia in October 2019, national road and traffic organisation AustRoads published a report called Viability of using recycled plastics in asphalt and sprayed sealing applications. In it, principal author, Christina Chin, cast doubt on laboratory trials that have not been conducted in line with her own country’s bitumen standards.

‘There are concerns about hazards road workers could be exposed to while handling recycled plastics,’ she said. ‘Some plastics, when heated, release toxic emissions such as chloride, formaldehyde, toluene and ethylbenzene. Another major concern is microplastics leaching out from our pavements into waterways.’

Nevertheless, in the UK, many authorities have bought into the MacRebur approach. While McCartney conceded that it has not always been easy working with local authorities, which sometimes lack the motivation or means to change or innovate, momentum is with them. In the meantime, MacRebur, in a nod to its origin story, now produces a cold-mix pothole filler which a major retailer will be selling at several of its sites, and paving stones from waste plastics.

Other companies are also investing in recycled plastics as material resources. For instance, global petrochemical firm Shell announced that it will use plastic waste as a liquid feedstock to make high-end chemicals for various products. However, while its ambition to use one million tonnes of plastic waste a year in its chemicals plants by 2025 is laudable, it is worth a note of caution due to the energy-intensive pyrolysis technique employed.

Elsewhere, at De Montfort University (DMU) Leicester, UK, waste plastics are being turned into bricks. DMU Senior Lecturer in Mechanical Engineering, Dr Karthikeyan Kandan, said the 100% upcycled plastic waste bricks provide 10 times better insulation than standard clay bricks. Taking inspiration from the way the baya weaver bird builds its nest, Kandan 3D-printed latticed bricks in a process he said involves ‘criss-crossing strips of plastics materials to form a grid or weave’ to boost their performance.

However, market intelligence provider, S&P Global Platts, produced a research paper which stated that while the price of virgin plastics has fallen, the price of recycled plastics has remained consistent. It warned that the additional costs needed to process recycled flakes will make them less attractive to use if the price of virgin flakes keep falling.

Greening cement

According to a 2016 Chatham House report, concrete is responsible for an estimated 8% of global carbon emissions. As such, there is a great deal of interest in commercially scalable processes that could reduce this. Researchers at Western Sydney University, Australia, found an option while repurposing waste byproducts from magnesium mining and other industries.

They claimed to have found a way to re-use magnesite – a waste product from mining produced by mixing magensium oxide powder and a concentrated magnesium chloride solution – in order to manufacture a magnesium chloride cement (MOC) that leads to less carbon-intensive concrete. Up to now, MOC had been confined to indoor applications such as floor tiles, decorative panels, and insulation boards due to its poor water-resistance.   

By adding the industrial byproducts fly ash and silica fume along with chemical additives, the researchers said it has improved the water-resistance of the MOC. Furthermore, it stated that this novel material has better compressive and flexural strength than conventional concrete, and may even be capable of absorbing CO2.

Despite the positives, the material has drawbacks as it is four times the price of conventional cements and is not sufficiently corrosion-resistant to be used alongside steel rebar in large-scale structures.

‘That’s the big hurdle to jump,’ said Lead Researcher Professor Yixia Zhang. ‘If you just use this as an internal structure, we have already achieved the water-resistance that enables the wide application of this material, from indoor to outdoor. If you want to further promote this material, and use it to make concrete for large-scale structures, it will take time.’

The three-year Australian Research Hub-funded project is coming to an end but Zhang said there has been significant interest from industry for the MOC, and she is hoping to secure funding for further research of tailored MOC products. ‘Research and funding are crucial for the potential large-scale application of this green cement to replace, or partly replace, conventional cement for concrete,’ she said.    

Other initiatives re-using mining waste range from the audacious to the workaday. On the small scale, Australian company Karst is crushing calcium carbonate stone waste from mining and construction to create paper. Elsewhere, copper slag is being sold for sand blasting, mining waste is used as aggregate in fly ash-based geopolymers, and engineering innovation platform Ennomotive is running a competition to redevelop mine tailings into glass.

Arguably, the biggest gains are to be made in construction, where spoil heaps could be employed for backfill. But before that could happen, the toxic threat must be stripped from these mounds. It will be interesting to see how effective bioremediation becomes as a means of decontaminating mining spoil. There is also the option of stabilisation and solidification (S/S) techniques to re-purpose contaminated waste.

Strong and stable?

A further project based in the port of Gothenburg, Sweden, involves processing contaminated sediments that have been dredged, mixed with binding elements – such as cement and granular blast furnace slag – and encapsulated in concrete for hard-standing. Then, this hard-standing will be covered in asphalt and used to extend the port.

According to COWI, which carried out some of the work, the environmental toxins, including the highly toxic organotin compound TBT, are safely enclosed and once the formula cures the binding action should remove the threat of leaching.

Also focusing on S/S, Italy-based researchers from Politecnico di Bari have investigated the ability of treating polluted clay sediments off the Gulf of Taranto using an interesting S/S method with biochar. The team looked into treating dredged sediments polluted by mercury, lead, copper, zinc and polycyclic aromatic hydrocarbons with cement and a lime that uses carbon and biochar additives to remediate the material and make it suitable for construction. The biochar is also a byproduct of pyrolysis.

There is a need to find a purpose for dredged sediments. Citing a 2011 SedNet study, lead researcher Francesco Todaro noted that 200 million cubic metres of sediments are dredged in Europe each year. Of this, more than half is contaminated and expensive to dispose of, with less than 5% effectively re-used. 

‘Until the early 1990s, in most cases the dredged sediments were disposed of in deeper seas or deposited on land,’ said Todaro. ‘Fortunately, this tendency has been changing in recent years. Conventions for the protection of the marine environment and some new European regulations concerning waste have been introduced to set guidelines for a proper disposal of dredged material into the sea and to avoid the traditional perception of these contaminated materials as waste, considering them as a commercially exploitable resource.’

Tightening of regulations should encourage material re-use, however, this is not possible while it is contaminated. Todaro and his colleagues looked into applying biochar and other forms of remediation such as nano zero-valent iron to degrade organic contaminants and treatments with autochthonous microorganisms. Todaro even posited the idea of using other additives in the process, such as shells, to reduce treatment costs.

The EU’s Using Sediment as a Resource project also focused on dredging river sediments that clog waterways, increase the threat of flooding, and harbour pollutants. Several pilot projects were launched to re-use the matter including geotechnical treatment of polluted sand for waterway embankments, using marine-dredged sediment for coastal defences, and in place of aggregates in roads and natural clay in bricks.

Whether waterbed sludge, abandoned plastic bags or industrial detritus, the spoils of our excesses are slowly being put to use as the spoils of the war on waste.