Study sheds light on CO₂ absorption mechanism of cement-based materials

Carbon dioxide (CO₂) are currently one of the main causes of global warming. Cement-based materials have shown promising applications in capturing and solidifying CO₂ Minerals are transformed by a process called carbonation, offering a potential solution to mitigate the challenges associated with climate change. Therefore, many studies have been conducted on carbonation of cement-based materials to improve carbonation efficiency.

In simple terms, carbonation in cement paste involves the dissolution of CO₂ in water, followed by interaction with calcium silicate hydrates (C–S–H), formed during the hydration of raw materials. During this reaction, dissolved CO₂ forms carbonate ions (CO₃^2-), and then reacts with calcium ions (Ca²⁺) of C–S–H to create calcium carbonate precipitates. However, despite extensive studies with varying parameters, the complete explanation of the carbonation mechanisms is not clearly understood due to the unstable nature of cement paste compounds.

Previous studies have shown that carbonation is strongly influenced by relative humidity (RH), CO₂ Solubility, calcium/silicate (Ca/Si) ratio, and the concentration and saturation level of water in C–S–H. In addition, it is also important to understand the influence of ions and water transport through the nano-sized pores of the C–S–H layers, known as gelatinous pore water.

To answer these questions, Associate Professor Takahiro Ohkubo of Chiba University’s Graduate School of Engineering and his team of researchers, including Taiki Uno of Chiba University, Professor Ippei Maruyama and Naohiko Saeki of the University of Tokyo, Associate Professor Yuya Suda of the University of the Ryukyus, Atsushi Teramoto of Hiroshima University, and Professor Ryoma Kitagaki of Hokkaido University, investigated the carbonation reaction mechanism under different Ca/Si ratios and relative humidity conditions.

Their study was published in Journal of Physical Chemistry C July 8, 2024.

“The role of water transport and structural changes related to carbonation remains an open question. In this study, we used a new method to investigate these factors, using ²⁹Nuclear magnetic resonance (NMR) of Si and ¹“H NMR relaxometry, which has been established as an ideal tool for studying water transport in C–S–H,” says Associate Professor Ohkubo.

To study the carbonation process, the researchers synthesized CSH and subjected it to accelerated carbonation using 100% CO₂much higher than atmospheric levels.

“Natural carbonation of cement-based materials occurs over several decades through absorption of atmospheric CO.₂which makes it difficult to study in the laboratory. Accelerated carbonation experiments with high CO concentrations₂ “Concentrations offer a practical solution to this challenge,” says Associate Professor Ohkubo.

The samples were synthesized under varying relative humidity conditions and Ca/Si ratios. In addition, they studied the C–S–H samples using ²⁹If NMR and water exchange processes using ¹H NMR relaxometry under deuterium dioxide (D₂O) atmosphere.

The researchers found that the structural changes induced by the carbonation reaction, including the collapse of the C–S–H chain structure and the changes in pore size, were strongly influenced by the Ca/Si ratio of the C–S–H chain and the relative humidity conditions. In addition, lower relative humidity conditions and a high Ca/Si ratio resulted in smaller pore sizes, suppressing Ca leaching.²⁺ ions and water from the interlayer space into the pores of the gel, resulting in inefficient carbonation.

“Our study shows that the carbonation process occurs due to a combination of structural changes and mass transfer, which signifies the importance of studying their interaction, rather than just the structural changes,” says Associate Professor Ohkubo.

Further highlighting the implications of the present study, Associate Professor Ohkubo adds: “Our findings may contribute to the development of new building materials capable of absorbing large amounts of atmospheric CO.₂. Moreover, carbonation reactions are also common in organic matter and therefore our new approach will also help to understand the carbonation of compounds in the natural environment.

In conclusion, this study sheds light on the carbonation reaction of cement-based materials, providing a potential solution to CO₂ reduction.