Scientists from Monash University in Australia developed the method to make cement absorb carbon dioxide more actively. For this purpose, they used glycine – the simplest amino acid. Such additive (in per cent points) changed the chemical reactions in the concrete facilitating four times the process of CO₂ banding and simultaneously increasing the strength of the received material.
This research belongs to the area of the so-called carbonized solidification, when the concrete is not just hardening, but actively absorbing carbon dioxide from the environment. In the course of this process, CO₂ interacts with cement components turning into stable calcium carbonate (CaCO₃) and fixing forever inside the material. This technology is believed to be one of the most efficient ways of decreasing the carbon footprint in construction. The problem is that this reaction is too slow, needs high temperature or pressure and often makes the cement structure weaker.
The Australian researchers proposed to interfere into this process at molecular level. They added minimal amount of glycine (less than 1% of the cement mass), which plays the key role in natural mechanisms of solid minerals (shells and corals) formation. After that, the samples went through a standard carbonization cycle to compare with standard cement.
At the preliminary solidification stage, glycine changed the chemical behavior of the cement. It facilitated the dissolving of mineral particles and the release of calcium ions performing as a buffer: partially neutralized the solution alkalinity, bonded calcium and facilitated its transfer into the solution. Eventually, the calcium concentration in the pores turned out almost four times less than in the reference samples, and the cement structure became more permeable for carbon dioxide.
After the CO₂ feeding stage started, the reaction went much faster. After one hour already, the modified sample had the same amount of calcium carbonate as the standard cement after four or five hours. At the same time, the process took place in standard conditions – without heating and without pressure increase.
After the material with glycine solidified, it turned out to be stronger and had higher density. The overall volume of pores decreased approximately by 22%, and the tensile strength grew by 17%. The scientists explain this by formation of organic and non-organic hybrid structures – the compounds, in which glycine molecules are partially built into calcium carbonate crystals and make them stronger.
The researchers emphasize that glycine was selected only as a model molecule – the simplest one of amino acids. Further on, they plan to test other compounds, such as arginine, serine or aspartic acid to identify the ones capable of most efficient control over the carbonization process. In future, such research may lead to creating a new generation of bio-active cements capable of faster capturing carbon dioxide and forming stronger and more durable structure.
