Sustainability and construction rarely go hand in hand.

That is precisely why civil engineering students Rebecca Rysgaard Højmose and Maria Pilegaard Petersen chose to make this combination the focus of their bachelor’s project last autumn, where they explored the potential of using clay soil as a building material.

“Construction is a major culprit because it emits so much CO₂. That’s why we wanted to use our bachelor’s project to investigate more sustainable solutions,” says Rebecca Rysgaard Højmose.

Clay soil has been used as a building material in Denmark for centuries, but during the 19th century it was gradually replaced by fired bricks, lime, cement and concrete, which were easier to mass-produce, more uniform, and both stronger and quicker to build with.

Today, however, interest in clay soil is beginning to return.

“Studies show that rammed earth has the potential to significantly reduce CO₂ emissions,” says Maria Pilegaard Petersen.

Green construction requires thorough research

Although reintroducing this traditional building material could prove both sustainable and climate-friendly, a number of important questions remain unanswered.

These are precisely the questions Rebecca Rysgaard Højmose and Maria Pilegaard Petersen—who have since begun their master’s degree—addressed in their bachelor’s project.

“Before the industrialisation of construction, when materials such as fired bricks and concrete really gained ground, buildings were neither as tall nor designed with the same focus on indoor climate and health,” says Maria Pilegaard Petersen.

For that reason, they wanted to investigate not only what clay soil can be used for, but also what it is unsuitable for.

“You could compare it to asbestos. It was used extensively before people realised how harmful it was to health. We want to do the opposite: first investigate health risks and construction potential, and only then consider using the material,” says Rebecca Rysgaard Højmose.

The aim is to define concrete parameters for when clay soil meets the requirements currently placed on building materials.

Their research suggests, among other things, that clay soil may help stabilise indoor temperature and humidity levels.

However, the results are not yet conclusive when compared with conventional building materials, which they believe highlights the need for further research.

In addition to having a lower climate impact than many commonly used building materials, clay soil has the advantage that it can be excavated in several places across Denmark and therefore does not require CO₂-intensive transportation.

Even so, clay soil is still only used to a limited extent in Denmark. One reason is the lack of knowledge about how the material performs under Danish climatic conditions.

Invisible gas may challenge green construction

Building height and indoor climate were not the only aspects examined by Rebecca Rysgaard Højmose and Maria Pilegaard Petersen in their project.

They also focused on radon—a radioactive and harmful gas that is released when the element radium in the soil breaks down.

Radon occurs naturally in the ground and can seep into buildings if they are not adequately protected. The gas is both invisible and odourless, yet poses a significant health risk.

Because clay soil is made from materials excavated directly from the ground, the two students wanted to investigate whether the material could potentially introduce radon into buildings—a question that has not previously been studied.

In their experiments, they measured the extent to which radon is released from clay soil and compared the results with fired bricks. The findings show that radon emissions from clay soil were significantly higher than those from the bricks.

On this basis, they conclude that the use of clay soil may pose a potential health risk.

Nevertheless, Rebecca Rysgaard Højmose and Maria Pilegaard Petersen are not ready to dismiss clay soil as a building material.

Although the results do not provide definitive answers in themselves, they see their work as a step in the right direction towards clarifying how clay soil can be used most effectively—and safely—in future construction, or whether alternative materials should be explored instead.