Clean dirt

The built environment is both the structural foundation of a society and one of its biggest climate polluters. Cement production alone accounts for around seven percent of all greenhouse gas emissions worldwide. Empa researchers are therefore working on various ways to reduce these emissions with innovative building materials and technologies. One of these paths to clean construction is a muddy one: Ellina Bernard of Empa's "Concrete & Asphalt" laboratory in Dübendorf and the Chair of Sustainable Construction at ETH Zurich is currently investigating the potential of clay as a sustainable building material. Because compared to concrete, clay should release much less CO2. In addition, it is available in almost unlimited quantities, is recyclable and can be easily processed - certainly also together with other "hipsters" of modern building culture such as organic waste materials from hemp processing.

Earthen paste for housing

The potential of this concrete alternative would be enormous. It is true that concrete could not be replaced by clay for all construction purposes. But possible - in addition to a variety of non-load-bearing structures - load-bearing walls of residential buildings. And after all, more than half of all building permits in Switzerland, for example, are issued precisely for residential buildings. This can be used so-called poured earth in a formwork or pressed clay in the form of prefabricated building blocks. And these air-dried clay bricks have a correspondingly more favorable energy balance than their fired counterparts, the bricks.

A true miracle cure? "Not yet," says Empa researcher Bernard. For although clay has been used for around 10,000 years and is thus one of the more primitive building materials in human history, the earthy paste has still not really come to grips with the problem. On the one hand, the geological composition of the natural material varies all over the world, which makes standardized production and use difficult. On the other hand, conventional cement is currently added to the clay to create a stable and durable building material. However, this addition causes the ecological footprint of the clay to slip back into the red zone. Ellina Bernard and her team therefore want to explore the earthy material, define standards for its composition and mechanical strength, and thus simultaneously develop a clean alternative building material for industrial application. For this ambitious project, the Empa researcher is being funded with one of the SNSF's coveted "Ambizione" grants.

The gentle power of magnesium

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, Ellina Bernard delves into the very heart of the matter. Unlike cement, which is held together by chemical bonds, the fine clay minerals in loam form physical bonds during air drying. 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 in this by geologist Raphael Kuhn, who is currently writing his dissertation on clay additives. One promising candidate is magnesium oxide. If extracted in a suitably sustainable manner, it has an excellent carbon footprint compared with calcium-containing cement, whose chemical reaction releases large quantities of CO2. In addition, magnesium oxide shortens the drying time, counteracts the dreaded formation of lumps in the clay by forming nanocrystals, and yet only gently interferes with the advantageous micro- and nanostructure of the clay elementary particles.

In initial laboratory experiments, the team has already achieved a compressive strength of up to 15 megapascals with various clay formulations - many times that of untreated clay. For comparison, clay with added cement reaches up to 20 megapascals. "But this is 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 capture the durability, deconstruction and recycling of the materials.

Ambizione grant from the SNSF

Ellina Bernard's project "Deciphering the role of magnesium in earth materials for sustainable construction" is funded by the Swiss National Science Foundation (SNSF) with an "Ambizione" grant. The grant is awarded to outstanding young researchers with an independent project who have already completed their doctorate.