Carbonation involves the dissolution of CO2 in water, followed by its interaction with calcium silicate hydrates (C-S-H) produced during the hydration process of cement. The dissolved CO2 forms carbonate ions (CO32-), which react with calcium ions (Ca2+) from the C-S-H, resulting in the formation of calcium carbonate. Despite considerable research, the full mechanism of carbonation is not entirely understood due to the complexity and instability of cement paste compounds.
Previous studies have demonstrated that carbonation is influenced by factors such as relative humidity (RH), CO2 solubility, the calcium/silicate (Ca/Si) ratio, and the water content in C-S-H. Understanding how ions and water move through the nanometer-sized pores within C-S-H layers, known as gel-pore water, is also critical.
To delve deeper into these aspects, Associate Professor Takahiro Ohkubo from Chiba University's Graduate School of Engineering, along with researchers including Taiki Uno from Chiba University, Professor Ippei Maruyama and Naohiko Saeki from The University of Tokyo, Associate Professor Yuya Suda from University of Ryukyus, Atsushi Teramoto from Hiroshima University, and Professor Ryoma Kitagaki from Hokkaido University, investigated carbonation mechanisms under different Ca/Si ratios and RH conditions. Their findings were published in 'The Journal of Physical Chemistry C' on July 08, 2024.
"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 research team synthesized C-S-H samples and exposed them to accelerated carbonation using 100% CO2, far beyond natural atmospheric concentrations.
"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. The samples were tested under different RH conditions and Ca/Si ratios, and the team used 29Si NMR to examine the structural changes, while water movement was studied using 1H NMR relaxometry under deuterium dioxide (D2O).
Their findings reveal that the structural changes resulting from carbonation, such as the collapse of the C-S-H chain structure and variations in pore size, were heavily dependent on the Ca/Si ratio and RH conditions.
Lower RH conditions and a higher Ca/Si ratio led to smaller pores, hindering the leaching of Ca2+ ions and water from the interlayer to the gel-pores, which resulted in less effective carbonation. "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," adds Associate Professor Ohkubo.
Regarding the broader implications of the study, Associate Professor Ohkubo notes, "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 also help to understand the carbonation of compounds in the natural environment."
In summary, this research provides valuable insights into the carbonation process in cement-based materials, offering potential solutions to reduce CO2 levels.
Related Links
Engineering at Chiba University
Carbon Worlds - where graphite, diamond, amorphous, fullerenes meet
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
