In order to reduce the CO₂ concentration in the atmosphere to the target level of 1988 – i.e. to 350 ppm (parts per million) – an estimated 400 billion tons of carbon must be removed from the atmosphere. This is a huge amount, equivalent to around 1,500 billion tons of CO₂. Empa researchers have now calculated that this excess carbon could be stored in building materials such as concrete by the middle of the next century. “These calculations are based on the assumption that sufficient renewable energy will be available after 2050 to remove CO₂ from the atmosphere – a very energy-intensive endeavor. This assumption allows us to use different scenarios to analyze how realistic and efficient the concept of our Mining the Atmosphere initiative is,” says Pietro Lura, Head of Empa's Concrete and Asphalt laboratory. The large-scale research initiative has set itself the goal of not only binding excess CO₂, but also using it as a valuable raw material.

Building materials are crucial

Surplus renewable energy is used to convert CO₂ into methane or methanol, which in turn are further processed into polymers, hydrogen or solid carbon. “Even if sufficient renewable energy is available, the central question remains as to how these huge quantities of carbon can be stored in the long term. Concrete seems predestined for this, as it can absorb enormous quantities,” explains Lura. The researchers therefore compared the mass of materials used worldwide, such as concrete, asphalt and plastics, with the amount of carbon that needs to be removed from the atmosphere – including emissions that are difficult to avoid. “The mass of building materials required worldwide far exceeds the excess carbon in the atmosphere. However, it remains a challenge how quickly and efficiently carbon can be introduced into these materials without deteriorating their properties,” concludes Lura.

Compared to other CO₂ reduction measures such as underground storage, the Mining the Atmosphere approach offers several advantages: It ensures long-term stability as well as a high storage density of carbon and enables decentralized implementation. At the same time, conventional CO₂-emitting building materials can be replaced. “Carbon must be incorporated into stable materials, as direct storage can be dangerous – for example, due to the risk of fire. Ideally, these carbon-enriched building materials are used over several recycling cycles before they are finally disposed of safely,” says Lura.

According to the Empa researcher, this concept should not only contribute to the reduction of CO₂, but also enable a carbon-binding economy that offers both ecological and economic benefits. “Carbon from the atmosphere can be used, for example, to produce polymers, bitumen for asphalt or ceramic materials such as silicon carbide. In addition, other high-value materials such as carbon fibers, carbon nanotubes and graphene could make the whole process economically viable – with concrete clearly accounting for the largest share of carbon storage.”

Hard carbon rocks as an accelerator

So how long would it take to remove all the excess CO₂ from the atmosphere? In an optimal scenario, building materials such as concrete could bind up to ten gigatons of carbon per year. However, this potential would only be fully exploited from 2050, when sufficient renewable energy is available after the energy transition. In addition to the surplus 400 gigatons of carbon, at least an additional 80 gigatons would have to be removed from emissions that are difficult to avoid by 2100. According to the various scenarios, the surplus CO₂ could be completely absorbed in building materials within 50 to 150 years – which would bring the CO₂ level back to the target level of 350 ppm.

The key to the most optimistic scenarios lies in the production of silicon carbide, which can be used as a filler in building materials. “Silicon carbide offers enormous advantages, as it binds carbon practically forever and has excellent mechanical properties. However, its production is extremely energy-intensive and represents one of the greatest challenges, both in terms of cost-effectiveness and sustainable implementation,” says Pietro Lura.

It would take more than 200 years to eliminate the entire anthropogenic carbon surplus with carbon in the form of porous aggregate alone. A combination of porous carbon and silicon carbide is therefore a viable solution. This would allow large quantities of carbon to be stored in concrete, which would also be more durable and stable than conventional concrete. “Nevertheless, the aim should be to remove as much CO₂ as possible from the atmosphere each year in order to achieve 350 ppm CO₂ in a realistic timeframe together with other measures. At the same time, it is crucial to continuously minimize our emissions so that the recovery process is not in vain,” says the Empa researcher.

Mining the Atmosphere
To achieve climate targets and prevent irreversible changes to the climate system, it is not enough to reduce greenhouse gas emissions. It is also necessary to actively remove excess CO₂ from the atmosphere. This is precisely where Empa's large-scale research initiative, Mining the Atmosphere, comes in. The aim is to create a completely new global economic model and an associated industrial sector that uses CO₂ as the raw material of the future. CO₂ is first converted into basic chemicals such as methane or methanol. These are then further processed to replace conventional building materials and petrochemical products. At the end of their life cycle, these carbon-rich materials will be stored in special landfills to permanently bind the carbon. Thanks to the synthetic methane, energy can also be transported from sunny locations to countries with an energy gap in winter.

However, according to the Empa researchers, implementation requires further progress in materials research and process development, particularly in order to make optimum use of decentrally generated and fluctuating renewable energies. In addition, a focus on new business models, economic incentives and suitable regulatory frameworks is necessary to make a carbon-neutral society a reality.