Concrete as a carbon store

In order to lower atmospheric CO2 levels to the target set in 1988 - 350 ppm (parts per million) - an estimated 400 billion tons of carbon must be removed from the atmosphere. This amounts to approximately 1,500 billion tons of CO2. Empa researchers have explored a novel approach, suggesting 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," said Pietro Lura, Head of Empa's Concrete and Asphalt laboratory. The initiative aims to bind excess CO2 while transforming it into a valuable resource.

Building materials as a solution

Using surplus renewable energy, CO2 can be converted into methane or methanol, which can then be 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," explained Lura. Researchers compared the global use of materials like concrete, asphalt, and plastics with the carbon removal target, including unavoidable emissions. They concluded that the worldwide demand for building materials far exceeds the carbon surplus in the atmosphere. However, challenges remain in efficiently incorporating carbon into these materials without compromising their properties.

Compared to other CO2 reduction methods, such as underground storage, the Mining the Atmosphere approach offers several advantages. These include long-term stability, high carbon storage density, and decentralized implementation. Additionally, carbon-rich building materials can replace conventional ones. "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," added Lura.

The concept is designed to contribute not only to CO2 reduction but also to a carbon-binding economy with ecological and economic benefits. "Carbon from the atmosphere can be used, for example, to produce polymers, bitumen for asphalt, or ceramic materials like silicon carbide. Additionally, high-value materials such as carbon fibers, carbon nanotubes, and graphene could make the process economically viable - with concrete accounting for the largest share of carbon storage," said Lura.

Accelerating the process

In an ideal scenario, building materials like concrete could bind up to ten gigatons of carbon annually. However, this potential would only be fully realized after 2050, when sufficient renewable energy becomes available. Beyond the 400 gigatons of carbon surplus, at least an additional 80 gigatons would need to be removed from unavoidable emissions by 2100. According to various scenarios, the surplus CO2 could be fully absorbed into building materials within 50 to 150 years, achieving the target CO2 level of 350 ppm.

Silicon carbide is seen as a critical component in these efforts. "Silicon carbide offers enormous advantages, as it binds carbon practically forever and has excellent mechanical properties. However, its production is extremely energy-intensive and presents challenges in terms of cost-effectiveness and sustainability," noted Lura.

Removing the entire anthropogenic carbon surplus with porous aggregate alone would take more than 200 years. A combination of porous carbon and silicon carbide could significantly enhance the efficiency of carbon storage in concrete, improving its durability and stability. "Nevertheless, the aim should be to remove as much CO2 as possible from the atmosphere each year to achieve 350 ppm CO2 in a realistic timeframe, alongside other measures. It is equally important to minimize emissions to ensure the recovery process is not wasted," emphasized the Empa researcher.

Mining the Atmosphere

Empa's Mining the Atmosphere initiative focuses on actively removing excess CO2 from the atmosphere to prevent irreversible climate change. The goal is to establish a new global economic model and industrial sector that uses CO2 as a key raw material. CO2 is first converted into base chemicals like methane or methanol, which are then used to replace conventional building materials and petrochemical products. At the end of their lifecycle, these carbon-rich materials can be safely stored in specialized landfills to ensure permanent carbon sequestration. Synthetic methane also offers a way to transport energy from sunny regions to areas with winter energy deficits.

However, successful implementation will require advancements in materials research and process development, especially to optimize the use of decentralized and fluctuating renewable energy sources. Additionally, new business models, economic incentives, and regulatory frameworks will be essential to transition to a carbon-neutral society.

Research Report:Mining the atmosphere: A concrete solution to global warming