A collaboration between the Massachusetts Institute of Technology (MIT) and Le Centre National de la Recherche Scientifique (CNRS) (the French National Center for Scientific Research) has yielded a cement that conducts electricity and generates heat.
Researchers tested the mechanical properties of samples by using scratch tests.
Results can be seen on the surfaces of the samples. Click to enlarge. All Photos by Andrew Logan-MIT News
Since the invention of concrete several millennia ago, it has become instrumental to the advancement of civilization, finding use in countless construction applications — from buildings [to pavements] to bridges.Yet, despite centuries of innovation, its function has remained primarily structural. A multiyear collaboration effort by MIT Concrete Sustainability Hub (CSHub) researchers and CNRS, has aimed to change that. The collaboration promises to make concrete more sustainable by adding novel functionalities—namely, electron conductivity. Electron conductivity would permit the use of concrete for a variety of new applications, ranging from self-heating to energy storage.
Nancy Soliman, Lead Author-Paper in Physical Review Materials, and Postdoc-MIT CSHub, believes that this research has the potential to add an entirely new dimension to what is already a popular construction material. The collaborative teams’ approach relies on the controlled introduction of highly conductive nanocarbon materials into the cement mixture. The paper validates this approach while presenting the parameters that dictate the conductivity of the material. Soliman explained, “This is a first-order model of the conductive cement. And it will bring [the knowledge] needed to encourage the scale-up of these kinds of [multifunctional] materials.”
From the nanoscale to the state-of-the-art Over the past several decades, nanocarbon materials have proliferated due to their unique combination of properties—chiefly conductivity. Scientists and engineers have previously proposed the development of materials that can impart conductivity to cement and concrete if incorporated within. But, for this new work, Soliman wanted to ensure the nanocarbon material they selected was affordable enough to be produced at scale. She and her colleagues settled on nanocarbon black—a cheap carbon material with excellent conductivity. They found that their predictions of conductivity were borne out.
“Concrete is naturally an insulative material. But when we add nanocarbon black particles, it moves from being an insulator to a conductive material,” Soliman said.
By incorporating nanocarbon black at just a 4% volume of their mixtures, Soliman and her colleagues found that they could reach the percolation threshold, the point at which their samples could carry a current—with an interesting upshot of ability to generate heat—known as the “Joule effect” (or resistive heating).
Nicolas Chanut, Co-Author-paper and Postdoc-MIT CSHub explained, “Joule heating is caused by interactions between the moving electrons and atoms in the conductor. The accelerated electrons in the electric field exchange kinetic energy each time they collide with an atom, inducing vibration of the atoms in the lattice, which manifests as heat and a rise of temperature in the material.”
In their experiments, they found that even a small voltage—as low as 5 volts—could increase the surface temperatures of their samples (approximately 5 cm3 in size) up to 41° Celsius (around 100° Fahrenheit). While a standard water heater might reach comparable temperatures, it’s important to consider how this material would be implemented when compared to conventional heating strategies.
INDOORS:
Chanut explained, “This technology could be ideal for radiant indoor floor heating that is done by circulating heated water in pipes that run below the floor, but that can be challenging to construct and maintain. When the cement itself becomes a heating element however, the heating system becomes simpler to install and more reliable, and the cement offers more homogenous heat distribution due to the very good dispersion of the nanoparticles in the material.”
OUTDOORS:
Nanocarbon cement could have various applications outdoors, as well. Chanut and Soliman believe that if implemented in concrete pavements, nanocarbon cement could mitigate durability, sustainability, and safety concerns—much that stem from the use of salt for de-icing.
“In North America, we see lots of snow. To remove this snow from our roads requires the use of de-icing salts, which can damage the concrete, and contaminate groundwater,” noted Soliman. The heavy-duty trucks used to salt roads are also both heavy emitters and expensive to run. By enabling radiant heating in pavements:
• Nanocarbon cement could be used to de-ice pavements
• Without road salt
• Saving potentially millions of dollars in repair and operations costs
• Remedying safety concerns
• Remedying environmental concerns
In certain applications where maintaining exceptional pavement conditions is paramount—such as airport runways—this technology could prove particularly advantageous.
TANGLED WIRES While this state-of-the-art cement offers elegant solutions to an array of problems, achieving multifunctionality posed a variety of technical challenges. For instance, without a way to align the nanoparticles into 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 tortuosity.
Franz-Josef Ulm, Leader & Co-author-paper, Professor-MIT Department of Civil & Environmental Engineering, and Faculty Advisor-CSHub said, “Tortuosity is a concept we introduced by analogy from the field of diffusion. In the past, it has described how ions flow. In this work, we use it to describe the flow of electrons through the volumetric wire.” He further explained:
like a car traveling between two points in a city:
—distance between those two points as the crow flies might be 2 miles
—actual distance driven could be greater due to the circuity of the streets
=the same for the electrons traveling through cement:
—path they must take within the sample is always longer than the length of the sample itself
—tortuosity=degree to which that path is longer
achieving the optimal tortuosity means:
—balancing the quantity
—balancing the dispersion of carbon
—if carbon is too heavily dispersed, the volumetric wiring will become sparse, leading to high tortuosity
—if not enough carbon is in the sample, tortuosity will be too great to form a direct, efficient wiring with high conductivity
—even adding large amounts of carbon could prove counterproductive
—at a certain point, conductivity will cease to improve and, in theory, would only increase costs if implemented at scale.
AS A RESULT OF THESE INTRICACIES, THEY SOUGHT TO OPTIMIZE THEIR MIXES Quantifying properties was vital to Ulm and his colleagues. Even adding large amounts of carbon could prove counterproductive. At a certain point conductivity will cease to improve and, in theory, would only increase costs if implemented at scale. As a result of these intricacies, they sought to optimize their mixes. The goal of their recent paper was not just to prove that multifunctional cement was possible, but that it was also viable for mass production.
Ulm said, “We found that by fine-tuning the volume of carbon we can reach a tortuosity value of 2. This means the path the electrons take is only twice the length of the sample. The key point is that in order for an engineer to pick up things, they need a quantitative model,” explains Ulm. “Before you mix materials together, you want to be able to expect certain repeatable properties. That’s exactly what this paper outlines; it separates what is due to boundary conditions—[extraneous] environmental conditions—from really what is due to the fundamental mechanisms within 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’ve charted is a promising one—and, thanks to their work, shouldn’t prove too tortuous. The research was supported through the Concrete Sustainability Hub by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation. For the MIT article and several others, please go to:
MIT: https://news.mit.edu/2021/electrifying-cement-nanocarbon-black-0420
For ConstructionPros.com: https://www.forconstructionpros.com/concrete/news/21391809/electrified-cement-created-through-mitcnrs-partnership
Phys.org News: https://phys.org/news/2021-04-electrifying-cement-nanocarbon-black.html
TechSpot: https://www.techspot.com/news/89381-electrically-conductive-cement-could-enable-new-applications-thanks.html