The wood that protects itself

Today, sawdust usually ends up in the oven. Tomorrow it could protect houses from fire. Researchers at ETH Zurich and Empa have developed a composite material that combines wood waste with the mineral struvite and achieves the fire protection class of conventional cement boards. The material protects itself, can be recycled and comes partly from sewage treatment plants.

Millions of tons of sawdust are produced worldwide every year. Most of it is burned, and the CO2 bound in the wood is released back into the atmosphere. This is precisely where the research team led by doctoral student Ronny Kürsteiner, under the direction of Ingo Bungert, Professor of Wood-based Materials at ETH Zurich, comes in. The aim is to keep the sawdust in the material cycle for longer.

The trick with the watermelon seed
The binding agent is struvite, a crystalline ammonium magnesium phosphate with known fire protection properties. The problem until now was that struvite was difficult to combine with sawdust due to its crystallization behaviour. The researchers found the solution in an enzyme from watermelon seeds. It controls crystallization in such a way that large struvite crystals are formed, which fill the cavities between the sawdust particles and bind them firmly together. The compressed material is then dried at room temperature.

Three times longer than spruce
The fire tests at the Turin Polytechnic speak for themselves. In a cone calorimeter, a standardized test method for heat exposure, untreated spruce wood caught fire after around 15 seconds. The struvite-sawdust composite took more than three times as long. Once burning, the material quickly forms a protective layer of inorganic material and carbon, which slows down the further spread of the fire. In addition, the struvite releases water vapor and ammonia under heat, which has a cooling effect and deprives the fire of oxygen.

Lighter than cement, better for the climate
Conventional cement-bonded chipboard, which is widely used in interior design today, consists of 60 to 70 percent cement by weight. They are heavy and climate-damaging to produce. The new struvite sawdust boards only require 40 percent binder. They are lighter, have a significantly better carbon footprint and do not end up in hazardous waste landfill after demolition.

Recyclable and suitable for fertilizers
The composite can be broken down into its components. Mechanically in the mill, then heated to just over 100 degrees, whereupon ammonia is released and the sawdust is sieved off. The mineral source material newberyite precipitates as a solid and can be reprocessed into struvite. If the material is not reused, it can be used as a long-term fertilizer. Struvite releases phosphorus, nitrogen and magnesium slowly and in a controlled manner into the soil.

Sewage treatment plant as a source of raw materials
Another cycle is emerging. Struvite is produced in large quantities as an unwanted by-product in sewage treatment plants, where it clogs pipes and is expensive to dispose of. “We could use these deposits as a base material for our building material,” says Kürsteiner. Whether the material will establish itself on the market depends primarily on the cost of the binder. A problem that could be solved with this source.