Clean dirt

The potential of this concrete alternative would be enormous. Admittedly, clay could not replace concrete for all construction purposes. However, in addition to a large number of non-load-bearing constructions, load-bearing walls of residential buildings are possible. And after all, more than half of all building permits in Switzerland, for example, are issued for residential buildings. So-called poured earth can be used in a casing, pressed clay in the form of prefabricated building blocks. And these air-dried clay bricks have a more favorable energy balance than their baked counterparts, bricks.

A true miracle product? “Not yet,” says Empa researcher Bernard. Although clay has been used for around 10,000 years, making it one of the more primitive building materials in human history, the earthy paste still hasn’t really taken off – yet. On the one hand, the geological composition of the natural material varies all over the world, which makes standardized production and use difficult. Secondly, conventional cement is currently added to the clay to create a stable and durable building material. This addition, however, pushes the ecological footprint of clay back into the red zone. Ellina Bernard and her team thus want to explore the earthy material, define standards for its composition and mechanical strength, and at the same time develop a clean alternative building material for industrial use. For this ambitious project, the Empa researcher has been awarded one of the SNSF’s Ambizione grants.

There is something mysterious about the transformation of a muddy paste of water and earth into a rock-solid product. To unravel and ultimately control this process, Ellina Bernard delves into the very heart of the matter. Unlike cement, which is held together by chemical bonds, the fine minerals in clay form physical bonds as they air-dry. Stability like that of concrete cannot be achieved in this way. That is why the researcher is looking for a suitable stabilizing binder.She is supported by geologist Raphael Kuhn, who is currently writing his dissertation on clay additives. One promising candidate is magnesium oxide.

If it is produced in a sustainable manner, it has an excellent carbon footprint compared to calcium-containing cement, whose chemical reaction releases large amounts of CO₂. In addition, magnesium oxide shortens the drying time, counteracts the dreaded formation of lumps in clay by forming nanocrystals, and yet only gently interferes with clay’s advantageous micro- and nanostructure.

In initial laboratory experiments, the team has already achieved a ­compressive strength of up to 15 megapascals with the magnesium oxide clay – many times that of untreated clay. By comparison, clay with added cement reaches up to 20 megapascals, and a wall subject to rather limited load, such as inside an apartment, must be able to withstand up to 10 megapascals.

“But that’s just the beginning,” says Ellina Bernard. Since she wants to assess the sustainability of building materials holistically, the laboratory experiments must also be accompanied by life cycle analyses that record the durability, deconstruction and recycling of the materials.