Researchers at ETH Zurich and Empa have found a way to press sawdust with a mineral binder to create a robust, flame-retardant material suitable for internal partitions.
Initial estimates show their material could achieve the same fire protection class as conventional cement-bonded particle board. And it is recyclable.
An article in the journal聽explains how sawdust can be mixed with struvite, a crystalline mineral composed of magnesium, ammonium and phosphate. Struvite is also an active inorganic flame retardant.
Previously it had proven difficult to combine struvite with sawdust particles due to its crystallisation behaviour. Now, the Swiss wood scientists 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 is pressed for two days then removed from the mould and dried at room temperature.
鈥淭he material is stronger under compression perpendicular to the grain than the original spruce timber,鈥� said Ronny K眉rsteiner, who developed the process as part of his doctoral thesis. The new material鈥檚 mechanical properties and fire resistance make it particularly suitable for internal fittings. That鈥檚 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 cone calorimeter. The struvite sawdust composite took 45 seconds to ignite 鈥� three times longer than untreated spruce 鈥� and then, once ignited, a protective layer of inorganic material and carbon forms quickly, protecting the material from further fire spread. 鈥淭he struvite sawdust panels essentially protect themselves,鈥� said K眉rsteiner.
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 have yet to confirm this. Cement-bonded particleboards 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.聽
Also, unlike cement-bonded particleboards, the struvite sawdust board can later be broken down into its individual components by breaking it up mechanically in a grinder and heating it to just over 100掳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 is suggested. It can also be used as a natural fertiliser that releases the bound phosphorus to promote 聽plant growth.
Whether the material will catch on in the construction industry depends primarily on the cost of the binder, said K眉rsteiner. 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. 鈥淲e could use these deposits as a raw material for our building material,鈥� he said.聽