Electrifying cement with nanocarbon

Since its invention several millennia ago, concrete has played an important role in the advancement of civilization and has been used in countless construction applications – from bridges to buildings. And yet, despite centuries of innovation, its function has remained largely structural.

A multi-year effort by researchers from MIT Concrete Sustainability Hub (CSHub), in collaboration with the French National Center for Scientific Research (CNRS), aimed to change that. Their collaboration promises to make concrete more sustainable by adding new features – namely electron conduction. Electron conduction would allow the use of concrete for a variety of new applications, ranging from self-heating to energy storage.

Their approach is based on the controlled introduction of highly conductive nanocarbon materials into the cement mixture. In a paper in Physical Review Materials, they validate this approach while proposing the parameters that determine the conductivity of the material.

Nancy Soliman, lead author of the paper and a postdoc at the MIT CSHub, believes that this research has the potential to add a whole new dimension to what is already a popular construction material.

“This is a first-order model of the conductive cement,” she explains. “And it will bring [the knowledge] needed to scale up this kind [multifunctional] material. ”

From the nanoscale to the latest

Over the past few decades, nanocarbon materials have increased due to their unique combination of properties, including conductivity. Scientists and engineers have previously proposed the development of materials that can give conductivity to cement and concrete when incorporated into them.

For this new job, Soliman wanted to ensure that the nanocarbon materials they chose were affordable enough to be produced on a large scale. She and her colleagues decided on nanocarbon black – a cheap carbon material with excellent conductivity. They found that their conductivity predictions were confirmed.

“Concrete is an insulating material by nature,” says Soliman, “but when we add black nanocarbon particles, it moves from an insulator to a conductive material.”

By incorporating only 4 percent of their mixtures nanocarbon black, Soliman and her colleagues found that they could reach the percolation threshold, the point at which their samples could carry a stream.

They noted that this current also had an interesting result: it can generate heat. This is the result of what is known as the Joule effect.

“Joule heating (or resistance heating) is caused by interactions between the moving electrons and atoms in the conductor,” explains Nicolas Chanut, a co-author on paper and a postdoc at MIT CSHub. The accelerated electrons in the electric field exchange kinetically. each time they collide with an atom, causing vibrations of the atoms in the lattice, which manifests as heat and an increase in temperature in the material. ‘

In their experiments, they found that even a small voltage – up to 5 volts – could increase the surface temperatures of their samples (about 5 cm³ in size) to 41 degrees Celsius (about 100 degrees Fahrenheit). Although a standard water heater can reach comparable temperatures, it is important to consider how this material will be implemented compared to conventional heating strategies.

“This technology can be ideal for indoor floor heating,” explains Chanut. “Indoor radiant heating is usually done by circulating heated water in pipes running under the floor. But it can be difficult to construct and maintain. As the cement itself becomes a heating element, the heating system becomes simpler to install and more “Furthermore, the cement provides a more homogeneous heat distribution due to the very good distribution of the nanoparticles in the material.”

Nanocarbon cement can also be used outdoors. Chanut and Soliman believe that nanocarbon cement can alleviate concerns about durability, sustainability and safety when implemented in concrete walkways. Many of the concerns stem from the use of salt for the icing sugar.

“In North America, we see a lot of snow. To get this snow off our roads, we need to use desalination salts that can damage the concrete and pollute the groundwater,” says Soliman. The trucks that are heavily used to salt roads are also heavy emissions and expensive to drive.

By enabling radiant heating in sidewalks, nanocarbon cement can be used to ice sidewalks without roads, which can save millions of dollars in repair and operating costs while correcting safety and environmental issues. In certain applications where the maintenance of exceptional pavement conditions is most important – such as airport runways – this technology can be particularly advantageous.

Tangled wires

Although this modern cement offers elegant solutions to a variety of problems, achieving multifunctionality presents a variety of technical challenges. Without aligning the nanoparticles in a functioning circuit (known as the volumetric wiring) within the cement, their conductivity would be impossible to exploit. To ensure an ideal volumetric wiring, researchers investigated a property known as a turtle.

“Tortuosity is a concept we introduce to the analogy from the field of diffusion,” explains Franz-Josef Ulm, a leader and co-author of the paper, a professor in the MIT Department of Civil and Environmental Engineering, and the faculty advisor at CSHub. “In the past, it described how ions flow. In this work, we use it to describe the flow of electrons through the volumetric wire.”

Ulm explains tortoise with the example of a car driving between two points in a city. Although the distance between the two points in the crow’s flight may be two kilometers, the actual distance may be greater due to the streets’ circus.

The same goes for the electrons moving through cement. The path they must take within the sample is always longer than the length of the sample itself. The extent to which the path is longer is the tortoise.

Achieving the optimal turtle means balancing the amount and distribution of carbon. If the carbon is too heavily distributed, the volumetric wiring will become sparse, leading to a high turtle. Similarly, without carbon in the sample, the turtle will be too large to form a direct, efficient wiring with high conductivity.

Even the addition of large amounts of carbon can be counterproductive. At some point, the conductivity will stop improving and will theoretically only increase the cost if implemented on a large scale. Because of these complications, they have tried to optimize their mixes.

“We have found that refining the volume of carbon can achieve a turtle value of 2,” says Ulm. “This means that the path taken by the electrons is only twice the length of the sample.”

Ulm and his colleagues were of utmost importance in quantifying such traits. The purpose of their recent article was not only to prove that multifunctional cement is possible, but that it is also viable for mass production.

“The most important point is that a quantitative model is needed for an engineer to pick things up,” Ulm explains. “Before mixing materials, you want to be able to expect certain repeatable properties. This is exactly what this paper sets out; it separates what is due to the boundary conditions -[extraneous] environmental conditions – due to the fundamental mechanisms in the material. “

By isolating and quantifying these mechanisms, Soliman, Chanut and Ulm hope to provide engineers with exactly what they need to implement multifunctional cement on a broader scale. The path they have chosen is promising – and thanks to their work, it should not be too tortuous.

This story is published under the auspices of MIT News (web.mit.edu/newsoffice/), a popular news site about MIT research, innovation and teaching.