NMR is a key-technique for understanding the structure of carbonated cement paste, capturing carbon dioxide emissions

Cement is the most widely used building materials and is one of the main contributors to CO₂ emissions in our world today. Professor Jørgen Skibsted and his group at Aarhus University (AU) is trying to better understand the dynamics of cement. They use solid-state Nuclear Magnetic Resonance (NMR) spectroscopy, a method ideal for looking at amorphous materials, since it can understand the dynamics of cement in a way other methods cannot. Jørgen’s goal is to create a new type of cement, which captures and binds the CO₂, so that emissions can be lowered to the benefit to our environment

Cement is one of the most important materials in the present world and has been for decades. As a main component in concrete, which is used in most buildings, but also other things as pipes, medical implants, and even decorative structures, it would not be an understatement to call cement the foundation of our society.

Unfortunately, the cement production process is also the main reason why the concrete industry makes up 8% of all CO₂ emissions globally each year, so until the overall emissions of CO₂ from cement are cut worldwide, the environment will continue to be polluted with over four billion tons of CO₂ annually. This is the problem that Professor Jørgen Skibsted and his research team have set out to solve.

It did not take long for Jørgen to find his interest in cement, already in 1988 during his studies, he was a summer student at the Cement and Concrete Laboratory at Aalborg Portland, a company he is still working closely together with today.

Jørgen is now Professor and deputy head of education at Aarhus University (AU), working with solid-state Nuclear Magnetic Resonance (SSNMR) spectroscopy, which he uses to study inorganic materials, where particularly one material takes up most of his time, cement.
One of his main fields of research is Portland cement based systems and developing methods of how to lower the emission of CO₂ in connection with cement production. A challenging task with long-time perspectives.

Offshore Projects

His students are working on many new exciting project, one of them being new promising materials, which possibly could lower the emission and enable recycling in deconstruction.
“One of my PhD students is working together with Aalborg Portland to find alternatives to cement in concrete mixtures looking at the material called calcified clay and the rest of the team are working with carbon capture trying to bind CO₂ to the calcium in the cement creating calcium carbonate binding the CO₂ in a solid state, so that it can only be released at higher temperatures.”

Two of his post-docs are working on projects at the Technical University of Denmark (DTU) funded by Danish Offshore Technologies and supported by the French oil company Total Energies.
One is investigating a special type of cement that can repair itself when in contact with water. A very useful property for offshore use.

“The self-repairing cement is made possible by the addition of bacteria that can react with CO₂. When in contact with water, this CO₂ then fills potential cracks and holes in the form of calcium carbonate.”

His other post-doc is working with closing old underwater oil wells. A lot of empty oil wells were left open due to an increasing demand for oil in the 70’s, and now these need to be closed. As it is important to close them probably so that no gasses nor oil seeps out into the ocean, cement is an optimal material.
How exactly cement reacts under these extreme conditions is still relatively unknown and this is exactly what Jørgen’s student is investigating.

“To mimic the condition of a deep sea well condition, we perform experiments at 60-85 degrees Celsius under a pressure of 300 Bar; these experiments are being done together with a group at DTU.”

The life of Portland Cement

Portland cement is by far the most dominant type of cement in the world constituting around 95% of the market and therefore the focus of most cement related research. The cement type is not named after any company, but rather a small island of the British coast, as the inventors thought some of its cliffs resembled the color of cement.

Portland cement consist of different calcium silicates called alite and belite and smaller occurrences of calcium aluminate phases.
The main component of cement is calcium carbonate or chalk, which releases CO₂ during heating, and it is this component, which is responsible for high CO₂ emission of cement.

“One ton of cement emits 800 kg CO₂, which is not that large of a percentage compared to other materials such as aluminum and steel. The problem is how much we use here in Denmark, yet alone in the world. In Denmark, it corresponds to each person emitting an extra 300 kg CO₂ each year.”
If we compare these numbers to the CO₂ emission of cars, it will correspond to every single family in Denmark embarking on a road trip to Dubai each year.

These numbers beg the question of how CO₂ emission from cement can be reduced.
Jørgen’s group has researched the life cycle of Portland cement and search for processes where there is potential for reducing the CO₂ emission.

“When creating cement there is an initial CO₂ emission before it is used in concrete. When concrete degrades after a hundred years or so, the crushed and milled concrete can be carbonated and reused, this product can then be used in production once again.”

After this process the CO₂ emission is reduced in two ways, first there is less initial emission as old material is reused and new material is not needed. Second, the carbonation process can capture some of the CO₂ in the cement.

“The use of this carbonated product reduces the CO₂ emission in total of close to 40%. So, if we do this, we are almost halfway to be CO₂ neutral.”

NMR spectroscopy for cement analysis

In Jørgen’s research he uses other tools as X-Ray diffraction (XRD), which is the most popular technique for studying inorganic materials, as it gives you a picture of the composition and structure quickly, but this method only works for crystalline structures.

“Less ordered systems and amorph materials does not fulfill these crystalline conditions and NMR spectroscopy can therefore be a useful method to examine such materials, as it is able to elucidate local order.”

When cement reacts with water, the hydrating products are indeed amorph, so NMR spectroscopy is ideal for studying these materials properly. The same is true for carbonation, where most of the phases created are amorphous alumina silica gels.

NMR spectroscopy is a vital tool in Jørgen’s research as it can give a simple picture of a material’s composition, which can be very valuable information especially combined with other techniques as XRD.

NMR magic

The main kind of NMR used in Jørgen’s group is Solid-State NMR (SSNMR). An important property of SSNMR is that opposed to liquid-state NMR, it does not need the material to be dissolved to analyze it. This is necessary in Jørgen’s research as cement cannot be dissolved properly without losing the dynamics and interactions. NMR therefore secures that the cement under investigation is kept as close to its natural state as possible.

“Another advantage of SSNMR is that it is nondestructive, which means the sample can be reused, unlike in other methods. This might seem insignificant but can be valuable if you want to use your samples for other experiments.”

A special kind of SSNMR spectroscopy used in Jørgen’s group is using something called Magic Angle Spinning.

In liquid state NMR you can achieve a high resolution as the molecules are moving in the solvent, giving them all sorts of different orientations relative to the strong magnetic field. Interactions in solid material will depend on how they are orientated in relation to the magnetic field and in powder these interactions would be very different.

“In magic angle spinning we use a geometric factor to remove these interactions trying to mimic the movement of liquids, which works like liquid state NMR and gives a higher resolution. This is done by rotating the sample relative the magnet at a very specific angle of 54,73 degrees.”

Strength of Danish NMR

Jørgen’s group collaborates with other universities including DTU and Aalborg university, but his group mostly work with the industry including companies as FL Schmidt, Aalborg Portland, and Heidelberg Materials in Germany.

“The different NMR groups within Denmark compliment each other and although they are doing very different research, they all have a common factor in the analysis technique, which means they can use each other in research and to solve problems. Denmark has a great NMR-infrastructure. The number of spectrometers compared to the country size is high in relation to other countries, which could be due to a great expertise in Danish NMR originating 55 years back.”

A key factor in the success of Danish NMR is continued collaboration and efforts to upgrade, develop and explore the infrastructures with continued support from public and private foundations. Being at the forefront of science requires using forefront analytical equipment. For Denmark, advancing the reduction of CO₂ emission by conversion of the processes of making cement is a unique contribution to solving the global climate crisis, and for this, NMR spectroscopy is a major important research tool.

Written by: Jonatan Emil Svendsen