Clean dirt for a better built environment? 

The built environment is both society’s structural foundation and one of its biggest climate polluters; cement production alone accounts for around 7% of all greenhouse gas emissions worldwide. The situation is improving, however, as researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are working on ways to reduce emissions via innovative building materials and technologies.

One of the paths to clean buildings is a muddy one: Ellina Bernard from Empa’s Concrete and Asphalt laboratory in Dübendorf is investigating clay’s potential as a sustainable building material. Clay should release significantly less CO₂than concrete. It is available in almost unlimited quantities, is recyclable, and easily processed.

Bernard cautions that clay is not yet a true miracle product, despite having been used for around 10,000 years. Clay’s geological composition varies regionally, complicating standardised production and use. Conventional cement also currently needs to be added to create a stable and durable building material, increasing clay’s ecological footprint.

Clay’s fine minerals form physical bonds as they air-dry, as opposed to cement’s chemical bonds. Concrete-like stability cannot be achieved in this way, so Bernard is looking for a suitable stabilising binder. Magnesium oxide is a promising candidate. If produced in a sustainable manner, it has a far better carbon footprint than cement, the chemical reaction of which releases large amounts of CO₂. Magnesium oxide also forms nanocrystals that shorten drying time and counteract dreaded lump formation in the clay, while interfering only gently with clay’s advantageous micro- and nanostructure.

The team has already achieved a ­compressive strength of up to 15 megapascals (MPa) in the lab with magnesium oxide clay – many times that of untreated clay. By comparison, clay with added cement reaches up to 20MPa, and a wall subject to limited load must withstand up to 10MPa. Bernard stresses, however, that lifecycle analyses are also needed to record durability, deconstruction, and recycling, in order to holistically assess the material’s sustainability.