Developing advanced carbon dioxide-absorbing building materials

The current high levels of carbon dioxide (CO₂) emissions are a significant contributor to global warming. Cement-based materials have demonstrated potential in capturing and solidifying CO₂ as minerals through carbonation, presenting an opportunity to address the challenges of climate change. As a result, extensive research has been conducted to enhance the carbonation process in cement-based materials.

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. In this reaction, the dissolved CO₂ forms carbonate ions (CO₃^2-) and further reacts with calcium ions (Ca²⁺) from C–S–H to produce calcium carbonate precipitates.

However, due to the unstable nature of cement paste compounds, the complete explanation of carbonation mechanisms remains unclear despite numerous studies involving varied parameters.

Previous research has indicated that the carbonation process is significantly influenced by relative humidity (RH), the solubility of CO₂, the calcium/silicate (Ca/Si) ratio, and the concentration and saturation level of water in C–S–H. Additionally, it is crucial to comprehend the impact of ions and the movement of water through the nanoscale pores in C–S–H layers, which are referred to as gel-pore water.

To address this, Associate Professor Takahiro Ohkubo from the Graduate School of Engineering at Chiba University, with his team of researchers, conducted a thorough investigation to understand the impact of factors such as relative humidity, CO₂ solubility, and Ca/Si ratios on the carbonation process. Their research provides valuable insights into the mechanism of carbonation reaction under different conditions, shedding light on important aspects of material science and engineering.

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

The researchers conducted a comprehensive study on the carbonation process by synthesizing C-S-H and subjecting them to accelerated carbonation using 100% CO₂, significantly higher than atmospheric levels.

“Natural carbonation in cement materials occurs over several decades by absorbing atmospheric CO₂, making it difficult to study in a lab setting. Accelerated carbonation experiments with elevated CO₂ concentrations provide a practical solution to this challenge,” explains Associate Professor Ohkubo.

They varied the RH conditions and Ca/Si ratios during the synthesis of the samples. Furthermore, they used ²⁹Si NMR to analyze the C-S-H samples and ¹H NMR relaxometry to study the water exchange processes under a deuterium dioxide (D₂O) atmosphere.

The researchers discovered that the carbonation reaction induced structural changes, leading to the collapse of the C-S-H chain structure and alterations in pore size. These changes were heavily influenced by the Ca/Si ratio of the C-S-H chain and the RH conditions. Notably, lower RH conditions and a high Ca/Si ratio resulted in smaller-sized pores, thereby hindering efficient carbonation by suppressing the leaching of Ca²⁺ ions and water from the interlayer space to gel-pores.

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

Associate Professor Ohkubo further adds, “Our findings can contribute to developing new building materials that can absorb large amounts of atmospheric CO₂. Additionally, carbonation reactions are also common in organic matter, and hence, our new approach will help to understand the carbonation of compounds in the natural environment.”

This study provides insights into the carbonation process of cement-based materials, presenting a possible approach to reducing CO₂ emissions.

Journal reference:

1. Taiki Uno, Naohiko Saeki, Ippei Maruyama, Yuya Suda, Atsushi Teramoto, Ryoma Kitagaki, and Takahiro Ohkubo. Understanding the Carbonation Phenomenon of C–S–H through Layer Structure Changes and Water Exchange. The Journal of Physical Chemistry C, 2024; DOI: 10.1021/acs.jpcc.4c01714