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Collaboration between CNRS and MIT has produced a cement that generates heat and conducts electricity.
Concrete has been instrumental in developing civilization since its invention many millennia back. It is used in numerous construction applications, from bridges and buildings. Yet, concrete’s function remains primarily structural despite centuries of innovation.
Researchers have been working on a multiyear change in collaboration with the French National Center for Scientific Research and MIT Concrete Sustainability Hub (CSHub). They work together to make concrete more durable by adding new functionalities, such as electron conductivity. Concrete could use electron conductivity for many unique purposes, including self-heating and energy storage.
Their method involves the controlled introduction of a cement mixture of highly conductive nanocarbon material. They validate their approach in a paper published in Physical Review Materials and present the conductivity parameters.
Nancy Soliman (the paper’s lead author) is a postdoc at MIT CSHub and believes this research can add a new dimension to a material already a viral building material.
She explains that this is a first-order model for conductive cement. It will provide [the knowledge] necessary to promote the scaling-up of such multifunctional materials.”
From the nanoscale to state-of-the-art
Nanocarbon materials have gained popularity over the past few decades due to their unique combination properties, including conductivity. Engineers and scientists have previously proposed the creation of materials that could impart conductivity to concrete and cement if they are incorporated.
Soliman wanted to ensure that the nanocarbon material she selected could be scaled up for this new project. She and her colleagues settled on nanocarbon black, a low-cost carbon material with excellent conductivity. Their predictions about conductivity proved to be correct.
Soliman says concrete is an insulative material by nature. But, when you add nanocarbon black particles, it transforms from an insulator into a conductive material.”
Chanut and Soliman could warm this mortar sample made from nanocarbon-doped concrete by running a current through it (see the thermometer display to the right).
Soliman and her coworkers found that nanocarbon black could be incorporated into their mixtures at a mere 4 percent volume. This allowed them to reach the percolation threshold, the point at which the samples could carry an electric current.
This current could also generate heat, which was what they noticed. This is the Joule effect.
Nicolas Chanut is a coauthor of the paper and a postdoctoral researcher at MIT CSHub. He explains that joie heating (or resistive heat) is partly caused by interactions between moving electrons and atoms within the conductor. “The accelerated electrons of the electric field exchange kinetic energy each time they collide and a bit. This causes particles’ vibrations within the lattice, resulting in heat and a rise in temperature.
They found that even a minimal voltage, as low as 5 V, could raise the temperature of their samples by approximately 5 cm 3. This is about 41 degrees Celsius (about 100 Fahrenheit). Although a water heater may reach similar temperatures, it is essential to consider how this material would work compared to other heating methods.
Chanut says that this technology could be used for radiant indoor heating. Indoor radiant heating is usually done by heating water through pipes below the floor. This system is difficult to maintain and build. The heating system will be easier to install and support if cement is used as a heating element. The cement has a very high nanoparticle dispersion, making it more homogeneous in heat distribution.
Scratch tests were used to test the mechanical properties. You can see the results on the surfaces of the samples.
There are many outdoor applications for nanocarbon cement. Chanut and Soliman believe nanocarbon cement can be used in concrete pavements to reduce safety, durability, and sustainability. Many of these concerns are due to the use of salt as deicing.
“We see a lot of snow in North America. Soliman notes that deicing salts are required to remove snow from roads. These salts can cause substantial damage and groundwater contamination. Heavy-duty trucks that salt roads are expensive to operate and emit heavy emissions.
Nanocarbon cement can be used to heat pavements with radiant heating. This could save millions in maintenance and operation costs and address safety and environmental concerns. This technology may benefit certain situations, such as airport runways, where exceptional pavement conditions are crucial.
Tangled wires
This state-of-the-art cement provides elegant solutions to a wide range of problems. However, multifunctionality was a technical challenge. With a way to align nanoparticles within a functioning circuit, known as volumetric wiring, it would be easier to exploit their conductivity. Researchers discovered tortuosity, which is a property that ensures perfect volumetric wiring.
Franz-Josef Ulm is a coauthor and leader of the paper. He is also a professor at MIT Department of Civil and Environmental Engineering and a faculty advisor at CSHub. It has been used to describe how ions flow in the past. It is used in this study to describe electron flow through the volumetric wire.
Ulm illustrates tortuosity by using the example of a car traveling between two points within a city. Although the distance between these two points may be two miles as the crow flies, the space could be longer due to the complexity of the streets.
This is also true for electrons traveling through cement. The length of the sample they must travel within it is consistently longer than its length. This is called tortuosity.
The best tortuosity is achieved by balancing the amount and distribution of carbon. High tortuosity will result if the carbon is not evenly distributed. The same goes for the sample. With enough carbon, the tortuosity will be sufficient to form efficient, direct wiring with high conductivity.
Even large amounts of carbon can prove to be counterproductive. Conductivity will stop improving after a certain point. If implemented at a large scale, it would only increase costs. They sought to optimize the mix because of these intricacies.
Ulm says, “We discovered we can achieve a tortuosity value two by fine-tuning carbon’s volume.” This means that the electrons travel twice as long as the sample.
Ulm and his coworkers were able to quantify these properties. Their recent paper did not only prove multifunctional cement is possible but also proved that it could be mass-produced.
Ulm explains that engineers need a quantitative model to pick up suitable materials. You want to expect specific repeatable properties before you mix materials. This paper describes precisely that. It separates what is due to environmental boundary conditions (extraneous) from what is owed to fundamental mechanisms within the material.
Soliman, Chanut, and Ulm have identified and quantified these mechanisms and hope to give engineers the information they need to create multifunctional cement at a larger scale. Their path is promising and should be easy enough, given their hard work.
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