Researchers at the Chair of Wood Materials Science at ETH Zurich, in collaboration with Empa, have developed a process to turn sawmill sawdust into a recyclable, fire‑resistant composite using the mineral struvite (crystalline ammonium magnesium phosphate). To overcome earlier difficulties in combining struvite crystals with wood particles, the team uses an enzyme extracted from watermelon seeds to control crystallization of struvite from an aqueous suspension of the precursor newberyite. The enzyme promotes growth of large crystals that fill cavities between sawdust particles and bind them. The wet mixture is pressed for two days, removed from the mould and dried at room temperature.
The resulting board is stronger under compression perpendicular to the grain than the original spruce timber, a property that makes it suitable for interior fittings. Struvite improves fire resistance because it breaks down when heated, releasing water vapor and ammonia; these gases absorb heat, are non‑combustible and displace air, which hinders fire spread and causes faster charring. Cone calorimeter tests at the Polytechnic University of Turin found untreated spruce ignites after around 15 seconds, while the struvite sawdust composite takes more than three times as long. After ignition, a protective inorganic and carbon layer forms rapidly.
Compared with cement‑bonded particleboards, which contain 60 to 70% cement by weight and are heavy, the struvite sawdust board contains about 40% binder and is significantly lighter. The composite is also easy to recycle: it can be ground, heated to just over 100°C (212°F) to release ammonia, the sawdust sifted out, and the binder dissolved and re‑precipitated as newberyite for reuse. Struvite can additionally serve as a slow‑release fertilizer. Future work will optimise and scale up production; wider use will depend on binder cost, although struvite deposits from sewage treatment plants could provide a raw material. The research appears in Chem Circularity and the source is ETH Zurich.
- Key steps: press mixture, dry, test fire resistance.
- Recycling: heat to release ammonia, recover sawdust and binder.
- Next: cost analysis, larger scale tests and optimisation.
Difficult words
- crystallization — process where dissolved material forms solid crystals
- enzyme — a protein that speeds up chemical reactions
- precursor — a substance that exists before forming another
- binder — a material that joins particles together
- charring — the process of burning that leaves carbon residue
- compression — pressure that squeezes material into less space
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- What advantages and disadvantages do you see in using sawdust and struvite for interior fittings and recycling?
- How could the availability of struvite from sewage treatment plants affect the cost and wider adoption of this material?
- What additional tests or improvements would you recommend before scaling up production?
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