Every time a tree trunk is sawn, it creates sawdust. Millions of tonnes of sawdust are produced every year worldwide, with most of it is burned to generate energy. This releases the carbon dioxide stored in the wood back into the atmosphere – which is not ideal from an environmental perspective. Now, a team of researchers at the Chair of Wood Materials Science at ETH Zurich and Empa has developed a process that can convert sawdust into a recyclable and environmentally friendly composite using the mineral struvite, a crystalline, colourless ammonium magnesium phosphate. This, in turn, keeps the sawdust in the material cycle for longer.
The researchers’ new material, based on mineralised sawdust, is an excellent fire retardant. Image Credit: Dan Vivas Glaser / from Kürsteiner R et al. Chem Circularity 2026, CC BY 4.0
Struvite has long been known for its fascinating fire protection properties. Previously, however, it had proven difficult to combine the mineral with sawdust particles due to its crystallisation behaviour. Now, ETH researchers are using an enzyme extracted from watermelon seeds to control the crystallisation of struvite from an aqueous suspension of the mineral precursor Newberyite. This process creates large crystals that fill the cavities between the sawdust particles and bind them together firmly. The material, which is pressed for two days, is then removed from the mould and dried at room temperature.
Wood Elements that Protect Themselves
“The material is stronger under compression perpendicular to the grain than the original spruce timber,” explains Ronny Kürsteiner, who developed the process as part of his doctoral thesis. The new material’s mechanical properties and excellent fire resistance make it particularly suitable for internal fittings. That’s because struvite is not only non-combustible, it also helps to actively increase fire resistance. When heated, the mineral breaks down, releasing water vapour and ammonia. This process absorbs heat from the surrounding environment, producing a cooling effect. The non-combustible gases that are released also displace the air, hindering the fire from spreading further and causing the material to char more quickly.
The ETH team partnered with researchers at the Polytechnic University of Turin, who tested the material in a so-called cone calorimeter – a standardised test that simulates how a material behaves when exposed to an external heat source. While untreated spruce ignites after around 15 seconds, the struvite sawdust composite takes more than three times as long. Once ignited, a protective layer of inorganic material and carbon forms quickly, protecting the material from further fire spread. “The struvite sawdust panels essentially protect themselves,” says Kürsteiner.
Initial estimates have shown that the material could achieve the same fire protection class as conventional cement-bonded particleboards, although larger-scale flame retardancy tests are still required to confirm this. Cement-bonded particleboards are currently widely used in interior fittings for flame protection applications. They contain 60 to 70 per cent cement by weight, making them heavy and giving them a poor carbon footprint due to the high level of energy involved in cement production. The struvite sawdust board, on the other hand, contains just 40 per cent binder, making it significantly lighter.
Easy to Recycle
Another advantage of this innovative composite compared to other composite building materials is that, unlike cement-bonded particleboards, for example, it doesn’t end up as waste after demolition. Once removed, the struvite sawdust board can be broken down into its individual components by breaking it up mechanically in a grinder and heating it to just over 100 °C. This releases the ammonia and allows the sawdust to be sifted out. After dissolution of the reclaimed material, the precursor newberyite is then precipitated again as a solid.
Newberyite can then be mixed with sawdust once more to form the struvite composites. This new material could therefore be an important contributor to the circular economy in the future. It can also be used as a natural fertiliser, which has interesting implications for agriculture because it releases the bound phosphorus that plants need for growth in a slow and controlled manner.
"Next, the researchers intend to continue optimising and scaling up the production process. Whether the material will catch on in the construction industry depends primarily on the cost of the binder, says Kürsteiner. Struvite is relatively expensive compared to polymer binders or cement. This could change, however, if they can tap into another cycle: struvite accumulates in large quantities in sewage treatment plants, where it clogs the sewage pipes. “We could use these deposits as a raw material for our building material,” says Kürsteiner.