Sawdust and a mineral from sewage pipes could replace cement in fireproof walls

Buildings kill people twice. First in fires — roughly 120,000 fire-related deaths occur worldwide each year, many in structures whose interior fittings ignited too quickly. Then in landfills, where construction and demolition waste accounts for more than a third of all solid waste in industrialized nations. Cement-bonded particleboard, the go-to material for fire-rated interior walls and ceilings, contributes to both problems. It is heavy, energy-intensive to produce, and essentially impossible to recycle. Once a building fitted with it comes down, the boards go to rubble piles.

Meanwhile, the timber industry generates its own mountain of waste. Every saw cut on every log produces sawdust — millions of tonnes per year globally — and most of it gets burned for energy, releasing stored carbon back into the atmosphere in a single combustion event. Two waste streams, two environmental burdens, and until recently, no obvious intersection between them.

A team at ETH Zurich and the Swiss Federal Laboratories for Materials Science (Empa) has now found that intersection. By binding sawdust particles with struvite — a crystalline ammonium magnesium phosphate mineral — they have produced a composite panel that is lighter than cement board, delays ignition by more than three times compared with raw spruce, actively fights fire as it heats up, and can be broken back down into its original components for reuse. The work, led by doctoral researcher Ronny Kürsteiner, was published in Chemical Circularity.

A mineral that fights back when heated

Struvite's fire-resistance properties have been recognized for years. The mineral is non-combustible on its own, but it does something more interesting than simply refusing to burn: when heated, struvite decomposes, releasing water vapor and ammonia gas. That decomposition absorbs heat from the surrounding material — a built-in cooling mechanism. The released gases, both non-combustible, displace oxygen around the flame front, starving the fire of fuel and slowing its spread.

The problem has always been getting struvite to cooperate with wood. The mineral's crystallization behavior made it difficult to combine with sawdust particles in any structurally meaningful way. Crystals formed too small, too irregularly, or in the wrong places, leaving gaps and producing weak composites.

Kürsteiner's solution came from an unexpected source: watermelon seeds. An enzyme extracted from the seeds controls how struvite crystallizes out of an aqueous suspension of its mineral precursor, newberyite. With the enzyme directing the process, large crystals grow into the cavities between sawdust particles, filling voids and locking everything together. The mixture is pressed in a mold for two days, then removed and dried at room temperature — no kiln, no high-energy curing step.

Stronger than spruce, slower to ignite

The resulting material surprised even the researchers who made it. Under compression perpendicular to the grain, the struvite-sawdust composite is stronger than the original spruce timber the sawdust came from. That mechanical performance, combined with its fire behavior, makes it a candidate for interior fittings — partition walls, ceiling panels, protective linings — where fire resistance codes are strict.

To quantify that fire behavior, the ETH team partnered with researchers at the Polytechnic University of Turin, who ran the composite through a cone calorimeter test. This standardized procedure exposes a material sample to a controlled external heat source and measures how quickly it ignites, how much heat it releases, and how fast the flame spreads. Untreated spruce ignited after approximately 15 seconds. The struvite-sawdust panel held off ignition for more than three times that duration.

Once ignition did occur, the panel's behavior diverged sharply from bare wood. A protective layer of inorganic residue and carbon formed rapidly on the surface, acting as a shield against further heat penetration. The material charred rather than sustaining open flame. Kürsteiner describes the panels as essentially "protecting themselves" — a phrase that sounds like marketing until you look at the cone calorimeter curves.

Initial estimates suggest the composite could achieve the same fire protection classification as conventional cement-bonded particleboard. Larger-scale flame retardancy testing is still needed to confirm that rating officially, but the early numbers are encouraging. And the weight difference is substantial: cement-bonded boards contain 60 to 70 percent cement by mass, making them dense and cumbersome. The struvite-sawdust board uses just 40 percent binder.

Demolition without the landfill

Fire performance alone would make the material interesting. The recyclability story is what makes it genuinely different.

When a building is renovated or demolished, cement-bonded particleboard becomes construction waste. The cement and wood fibers are so thoroughly intermingled that separation is impractical, so the boards get crushed and dumped. The struvite-sawdust composite, by contrast, can be disassembled. Mechanically breaking up the panel in a grinder and heating the fragments to just above 100 degrees Celsius releases the ammonia, allowing the sawdust to be sifted out. The remaining inorganic material is dissolved, and newberyite — the mineral precursor — precipitates out as a solid, ready to be mixed with fresh sawdust and formed into new panels.

That closed loop extends beyond construction. Struvite contains phosphorus in a form that plants can absorb, and recovered material can serve as a slow-release fertilizer. The phosphorus releases gradually rather than in a single flush, which is better for soil health and reduces runoff. A building panel that ends its life as crop nutrition is, at minimum, a more creative fate than a landfill.

The cost question and a sewage solution

Whether the material moves from laboratory to job site depends largely on economics. Struvite is currently more expensive than Portland cement or the polymer binders used in conventional particleboard. For a construction industry that optimizes relentlessly on cost per square meter, that premium is a hard sell regardless of environmental benefits.

But Kürsteiner sees a potential workaround hiding in municipal infrastructure. Struvite accumulates in large quantities inside sewage treatment plants, where it crystallizes on pipe walls and clogs flow lines. Wastewater operators spend considerable effort and money removing these deposits. Using recovered sewage struvite as raw material for building panels would turn a waste-management headache into a feedstock supply — reducing costs for the composite and maintenance bills for water utilities simultaneously.

That supply chain does not exist yet. Scaling the production process, establishing consistent mineral quality from wastewater sources, and running the full suite of building-code fire tests all remain ahead. The enzyme-mediated crystallization process itself needs to be optimized for throughput; a two-day pressing cycle is acceptable in a research lab but would need to come down for industrial production.

Where sawdust stops being waste

The deeper appeal of the work is conceptual as much as practical. Construction materials have historically been designed for performance and cost, with end-of-life as an afterthought. Cement board does its job admirably for decades, then becomes rubble. The struvite-sawdust composite was designed from the start to come apart — to re-enter the material cycle rather than exit it.

There is something satisfying about the raw ingredients, too. Sawdust that would have been burned. A mineral scraped from sewage pipes. An enzyme from watermelon seeds. None of these are high-tech or rare. The innovation is in how they fit together — and in the recognition that fire safety and sustainability do not have to be competing priorities in a building's walls.

The study is early-stage, and plenty could go wrong between a cone calorimeter test and a fire-rated product on a construction site. Larger panels may behave differently than small samples. Cost parity with cement board is not guaranteed even with wastewater-sourced struvite. But the combination of mechanical strength, active fire suppression, full recyclability, and low-energy manufacturing is unusual enough to warrant attention from an industry that badly needs alternatives.