When we talk about mineralisation, we mean the implementation of mineral water-insoluble crystals into the wood. As I cannot simply press the crystals into the wood, they have to be made to grow in the wood. This is usually done by dissolving salts in water. These salt solutions are then impregnated into the wood one after the other via pressure. The salts can react via a so-called precipitation reaction with each other to form minerals, which in turn are insoluble in water. The purpose of doing this is to specifically change the properties of the wood. In particular, the focus of the mineralization is on improving the fire resistance of wood.
As you can see everywhere, wood as a construction material becomes more and more attractive. Not only small buildings such as cabins are made of wood but also high-rise buildings. 30 – 40 years ago, it was unthinkable to build high-rise buildings out of wood. Today they are no longer a rarity. The Mjosa tower in Norway (85.5m), the HoHo-Wien in Austria (84m) or the Ascent MKE Buidling Milwaukee, USA (86.6m) are just a few examples. Since wood is a combustible material, it is obvious that a special concept must be applied to protect material and people, especially for such buildings from fire incidents. However, most commercially available fire retardants are e.g. corrosive or are suspected of being harmful to organisms, humans and the environment. But the biggest problem is that most of the fire retardants that are used for wood are very soluble in water. They are no longer effective if they are washed out of the wood by rain, for example. This is where minerals are used that are insoluble in water and can therefore withstand weather-related leaching.
At different Universities mineralization treatments were evidenced to enforce the fire-retardant properties of wood. In our study, the investigated minerals are: Calcium carbonate (calcite), magnesium phosphate (struvite) and calcium oxalate (whewellite). These minerals showed promising results according to their ability to increase the fire retardancy of wood. However, little is known about intended or unintended side effects due to the mineral treatments. In particular when using under outdoor conditions we need to know the impact of the minerals on the wood. Therefore, following things are important to know:
· Do the minerals remain fixed in the wood and retain their fire-retardant properties?
· Does the wood maintain its color over time?
· Do the minerals interact with surface coatings?
To answer these questions, we treated fir wood with calcium oxalate (whewellite), calcium phosphate (found in apatite), and magnesium phosphate (struvite). We then applied various coatings to some of the mineralized boards to observe their interaction with the minerals during outdoor weathering. Finally, we exposed the boards to the outdoor climate on the roof of the Bern University of Applied Sciences in Biel, Switzerland. Biel has a humid temperate climate (Cfb).
After 16 months of outdoor exposure, we observed that mineralized wood generally showed improved color stability, mainly due to suppressed mold growth on the surfaces. We found good evidence with a scanning electron microscope that the minerals were still present on the weathered wood surfaces. However, there were signs of unwanted interactions between the minerals and the coatings. After 15 months, the surface treatments on the mineralized specimens began to detach. This raises the question of how to improve this behavior.
Keywords: mineralization, weathering, wood modification
Authors
Tom Franke
Bern University of Applied Sciences, Biel, Switzerland
Thomas Volkmer
Bern University of Applied Sciences, Biel, Switzerland
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