Revolutionary Discoveries in Carbon Dioxide Capture with Cement Innovations

Cement-based materials might play a significant role in combating climate change by capturing and storing carbon dioxide from the air as minerals through a process called carbonation. However, despite numerous studies, the precise mechanisms of this process remain unclear. Recently, researchers have undertaken a detailed examination of the carbonation reaction with a new approach, highlighting the importance of structural changes and water movement, thus paving the way for innovative building materials that can absorb carbon dioxide more effectively.
Cement-based materials might play a significant role in combating climate change by capturing and storing carbon dioxide from the air as minerals through a process called carbonation. However, despite numerous studies, the precise mechanisms of this process remain unclear. Recently, researchers have undertaken a detailed examination of the carbonation reaction with a new approach, highlighting the importance of structural changes and water movement, thus paving the way for innovative building materials that can absorb carbon dioxide more effectively.

Carbon dioxide (CO₂) emissions are one of the main contributors to global warming. Cement-based materials have shown potential in capturing and transforming CO₂ into solid minerals through carbonation, presenting a possible way to address climate change challenges. As a result, significant research efforts have focused on enhancing carbonation efficiency in cement-based materials.

In simple terms, carbonation in cement paste starts when CO₂ dissolves in water and interacts with calcium silicate hydrates (C-S-H), which are formed during the hydration of raw materials. This interaction leads to the creation of carbonate ions (CO₃^2-), which then react with calcium ions (Ca²⁺) from C-S-H to generate calcium carbonate precipitation. However, despite extensive research under various conditions, the full understanding of carbonation mechanisms remains elusive due to the instability of cement paste compounds.

Prior research has indicated several factors that significantly affect carbonation, including relative humidity (RH), CO₂ solubility, calcium/silicate (Ca/Si) ratio, and the concentrations and saturation of water within C-S-H. Additionally, the movement of ions and water through the tiny pores in C-S-H layers, referred to as gel-pore water, plays a crucial role that needs further study.

To explore these aspects, Associate Professor Takahiro Ohkubo and his research team from Chiba University, alongside experts from The University of Tokyo, University of Ryukyus, Hiroshima University, and Hokkaido University, investigated the carbonation reaction mechanisms under various Ca/Si ratios and RH levels. Their work was published in The Journal of Physical Chemistry C on July 08, 2024. “The influence of water movement and carbonation-related structural changes is still an unresolved issue. In our study, we employed a new method that utilizes ²⁹Si nuclear magnetic resonance (NMR) and ¹H NMR relaxometry, which are effective tools for examining water transport in C-S-H,” explains Associate Professor Ohkubo.

To investigate the carbonation process, the researchers created C-S-H samples and accelerated the carbonation reaction by applying 100% CO₂, which is significantly higher than natural atmospheric levels. “Natural carbonation in cement materials can take decades as they absorb CO₂ from the air, making laboratory studies challenging. Accelerated carbonation experiments with increased CO₂ concentrations offer a practical alternative,” says Associate Professor Ohkubo. The samples were prepared under different RH levels and Ca/Si ratios, followed by analysis using ²⁹Si NMR and evaluation of water exchange processes using ¹H NMR relaxometry in a deuterium dioxide (D₂O) atmosphere.

The researchers discovered that the structural adjustments caused by carbonation, such as the breakdown of the C-S-H chain structure and alterations in pore size, were significantly influenced by both the Ca/Si ratio and RH levels. Furthermore, conditions with low RH and a high Ca/Si ratio led to smaller pore sizes, which inhibited the leaching of Ca²⁺ ions and water from the interlayer spaces to gel-pores, thereby reducing carbonation efficiency. “Our research indicates that carbonation results from a combination of structural changes and mass transfer, highlighting the need to focus on their interactions rather than solely on structural changes,” states Associate Professor Ohkubo.

Moreover, Associate Professor Ohkubo emphasizes the broader implications of this research: “Our findings could aid in the development of new building materials capable of absorbing significant amounts of atmospheric CO₂. Furthermore, carbonation reactions are also prevalent in organic materials, so our novel approach may enhance our understanding of the carbonation processes occurring in the natural environment.”

In conclusion, this study provides valuable insights into the carbonation reactions of cement-based materials, offering a potential strategy for reducing CO₂ emissions.