A team of researchers at the Chair of Wood Materials Science at ETH Zurich and Empa has developed a process that converts normally burned sawdust into a recyclable, environmentally friendly composite material that is fire-resistant and biodegradable.
The team behind the new sawdust-based material said it could also provide a solution for the additive material struvite that accumulates in sewage treatment, which is mixed with the sawdust to imbue the final material with its fire-resistant properties.
Struvite Imbues Sawdust with Fire-Resistant Properties
According to a statement detailing the new approach, sawmills worldwide produce millions of tons of sawdust each year. Most of this leftover material is burned to generate energy. However, that process also releases the carbon dioxide stored in the wood back into the atmosphere, a process that the research team notes is “not ideal from an environmental perspective.”
To keep the sawdust in the material cycle for an extended period, the ETH Zurich and Empa team explored adding different materials to the sawdust to expand its potential applications. Ultimately, the team landed on struvite, a crystalline, colorless ammonium magnesium phosphate known for its fire-resistant properties.
Because struvite’s crystallization behavior has made the material difficult to combine with sawdust particles, previous attempts have found dubious results. Still, the promise of adding struvite to sawdust was enticing enough that the researchers decided to search for a solution.
Watermelon Seeds Help Manage Crystallization Process
After some research, the team decided to try adding an enzyme extracted from watermelon seeds. According to a statement announcing the work, this enzyme controls the crystallisation of struvite “from an aqueous suspension of the mineral precursor Newberyite.”
Instead of the typical crystalline formation process, this additive causes struvite to form large crystals that fill the cavities between sawdust particles. This process binds the two normally resistant materials at the molecular level, forming a hybrid sawdust/struvite material.
Next, the research team ‘pressed’ the hybrid material for two days. Once this step was finished, the team removed the final sample from the mold and dried it at room temperature.
When the research team tested their hybrid sawdust/struvite sample, it displayed several advanced properties. For example, Ronny Kürsteiner, who developed the process as part of his doctoral thesis, said: “The material is stronger under compression perpendicular to the grain than the original spruce timber.”
Tests also confirmed that the new material is not only fire-resistant but also actively increases the fire resistance of surrounding materials. Specifically, when the enhanced sawdust is heated, the mineral breaks down.
This process results in the release of water vapor and ammonia, both of which help retard combustion by absorbing heat. The release of non-combustible gases also displaces the air, which also hinders the fire’s ability to spread.
“The struvite sawdust panels essentially protect themselves,” says Kürsteiner.
Similar Fire Protection of Cement-Bonded Particle Boards
To quantify the material’s fire-resistance capabilities, the ETH team partnered with researchers at the Polytechnic University of Turin. This collaboration involved testing the material in a calorimeter to assess its behavior when exposed to heat.
These tests revealed that the struvite/sawdust composite took around 45 seconds to ignite. The research team notes that this is three times as long as untreated spruce takes to catch flame. They suspect this delayed response is due to a protective inorganic layer that forms on the composite material’s surface when it is heated.
“Initial estimates have shown that the material could achieve the same fire protection class as conventional cement-bonded particleboards,” the team notes, while adding that larger-scale flame retardancy tests will be required to confirm this. This could be significant, as cement-bonded particle boards are widely used in interior fittings specifically for their flame retarding properties.
“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 team explained. “The struvite sawdust board, on the other hand, contains just 40 per cent binder, making it significantly lighter.”
Material is Reusable and Recyclable
When discussing the other potential advantages of their fire-resistant sawdust/struvite composite, the ETH Zurich team noted that the material is reusable. Most cement-bonded particle board ends up as waste, adding to a construction project’s carbon footprint.
“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,” the team explained.
Because of its organic nature, the material can also be used as a natural fertilizer. The team notes that this is an intriguing option, since the new material slowly and controlled releases bound phosphorus that plants need to grow in a slow and controlled manner.
Next, the research team intends to further optimize the process and scale it up to demonstrate its commercial potential. Although struvite is currently more costly than other potential additives, the team notes that it accumulates in sewage treatment plants, “where it clogs the sewage pipes.”
“We could use these deposits as a raw material for our building material,” Kürsteiner said.
The study “Enzyme-mediated consolidation of lignocellulosic materials with a flame-retardant and fully recyclable mineral binder” was published in Chem Circularity.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.