Turning CO₂ into stone: A new climate technology emerges from Aarhus University

The whiteboard is covered in numbers.

Payments, calculations and achievements—it is difficult for an outsider to make sense of them, but the many figures clearly suggest that business is moving fast.

David Egholm and Jacob Bundsgaard have every reason to add a few more numbers to the board today. The two co-founders of the Aarhus University spinout Sunstones have just been awarded DKK 25 million to establish the world's first facility for CO₂ mineralisation using volcanic ash.

"We now have the opportunity to test the technology at full scale," says David Egholm as he sits down with a cup of coffee in the company's office on the fifth floor of Navitas.

Egholm is Professor of Geoscience at Aarhus University and has been affiliated with the university for more than twenty years. After the summer, however, he will dedicate himself full-time to his role as CEO of Sunstones.

Jacob Bundsgaard knows exactly what such a transition entails. Aarhus' former mayor now works as an independent adviser and has served as Chairman of the Board and co-owner of the company since early 2025.

"I'm driven by the opportunity to create something meaningful for society. That's why I believe Sunstones is worth investing both time and energy in—even though building a company around an entirely unproven technology for a completely new market is, of course, a significant risk," says Jacob Bundsgaard.

From carbon dioxide to limestone

This is no exaggeration. Sunstones is genuinely breaking new ground—or, more accurately, drilling new boreholes.

The company specialises in converting carbon dioxide into limestone, helping to remove one of the greenhouse gases most responsible for climate change.

The process begins by capturing CO₂ from biogas plants and combined heat and power stations. The gas is dissolved in water before the carbonated liquid is injected 200–300 metres underground in areas where layers of volcanic ash are found beneath the surface.

There, naturally occurring minerals in the ash—primarily calcium, but also iron and magnesium—bind with the dissolved CO₂ and convert it into stable carbonate minerals. It is a natural geological process that has helped regulate Earth's atmosphere for millions of years.

"Today, however, humanity emits around one hundred times more carbon dioxide than all the world's volcanoes combined. The minerals underground can no longer keep pace—not without our help," explains David Egholm.

Volcanic ash beneath the Island of Fur

In 2020, David Egholm was introduced to the concept of CO₂ mineralisation by colleagues at Aarhus University. Together, they decided to explore whether the technology could work outside the laboratory.

They began by collecting volcanic ash samples to test the mineralisation process under laboratory conditions.

The next step was to establish a spinout company with support from the business developers at Kitchen, Aarhus University, who also helped the university secure a patent covering key aspects of the technology.

Attention then turned to the volcanic ash deposits hidden beneath the Danish landscape. This required permits, partnerships, political dialogue, updated regulations and exploratory drilling. Among other things, the research team brought Jacob Bundsgaard on board.

"Regulatory processes can be slow in situations like this," says the former mayor. "And that's actually a good thing. As a society, we need to be certain that new technologies do more good than harm."

Behind him hangs a photograph of a clay pit on the island of Fur, where dark layers of volcanic ash cut through the cliffs and are clearly visible to visitors.

It was here that the researchers were able to observe first-hand how deep they would need to drill to reach the ancient ash deposits. Naturally, Fur became the starting point for Sunstones' field studies.

Working together with Klimafonden Skive, Aarhus University and Rybjerg Biogas, the team completed an exploratory drilling project in the area last summer. The results were so promising that Sunstones is now establishing a full-scale pilot facility at the same location, while preparing a second test well in Thy, further north-west in Denmark.

Aiming to remove one megaton of CO₂ every year

The new pilot facility, funded through the Danish Energy Agency's Energy Technology Development and Demonstration Programme (EUDP), will enable Sunstones to test the mineralisation process under real operating conditions while simultaneously maturing its commercial business model.

"Our priority is to move beyond the laboratory as quickly as possible and demonstrate what the technology can achieve in practice," says David Egholm. "Otherwise, we risk remaining stuck in the research phase. Our ambition is to have two facilities operating on commercial terms by 2029."

That ambition naturally depends on customers being willing to pay for permanent carbon removal. As a result, the company's success is closely linked to the rapidly emerging market for carbon removal credits, where organisations offset historical emissions by purchasing verified removals of greenhouse gases from the atmosphere.

Part of this development is expected to be driven by public policy and increasing environmental regulation. Denmark, for example, aims to achieve net-negative emissions by 2050, requiring the removal of an estimated seven to eight million tonnes of CO₂ every year. Similar ambitions are emerging across many other countries.

According to Jacob Bundsgaard, Sunstones' carbon removal credits could become among the most attractive on the market because they are based on a permanent, natural geological process developed in close collaboration with Aarhus University.

"Our ambition is to remove one million tonnes of CO₂ every year once we've scaled the business," he says. "Denmark's total annual emissions are around 40 million tonnes."

David Egholm adds a note of perspective.

"No single technology can solve the climate challenge on its own. The most important task remains reducing future emissions through the green transition."

Although Aarhus University and Sunstones have developed a patented drilling and well design that optimises the mineralisation process, they have not patented CO₂ mineralisation itself. The underlying geological process has existed for millions of years. Instead, the patent covers the way the wells are designed and operated to distribute the carbonated water efficiently throughout the volcanic ash layers.

Technology alone, however, is not enough to realise the company's ambitions.

Both David Egholm and Jacob Bundsgaard emphasise that future facilities should be distributed across multiple locations rather than concentrated in one area, minimising local impact while making use of suitable geological conditions wherever they are found.

"There are vast quantities of naturally occurring limestone underground—far more than we will ever create through mineralisation," says Jacob Bundsgaard.

David Egholm nods in agreement.

"We're not looking to create one major change in a single location. Instead, we envision many small-scale facilities distributed across the country."

About Sunstones

Sunstones is a spinout from the Department of Geoscience at Aarhus University. The company was founded by David Egholm, Jakob Lundbek, Jacob Bundsgaard, Hamed Sanei, Stéphane Bodin, Katrine Andresen, Rasmus Andreasen and Per Pedersen.

Among its investors is Michael Holm of Systematic Holding.

The company currently employs five people and is expanding its team through new recruitment.

On 1 August 2026, David Egholm will step down from his professorship at Aarhus University to devote himself full-time to his role as CEO and co-owner of Sunstones.