Northwestern University scientists have developed a new carbon-negative building material using seawater, electricity, and carbon dioxide (carbon dioxide). As Earth’s climate continues to warm, researchers around the globe are exploring ways to capture carbon dioxide from the air and store it deep underground. While this approach has multiple climate benefits, it does not maximize the value of the enormous amounts of atmospheric carbon dioxide.
Northwestern’s new strategy addresses this challenge by permanently locking away carbon dioxide and turning it into valuable materials that could be used to manufacture concrete, cement, plaster, and paint. This carbon-negative material generation process also releases hydrogen gas, a clean fuel with various applications, including transportation.
Northwestern’s Rotta Loria (who led the study) is the Louis Berger Associate Professor of Civil and Environmental Engineering at Northwestern’s McCormick School of Engineering. Jeffrey Lopez, an assistant professor of chemical and biological engineering at McCormick, served as a key coauthor on the study.
Rotta Loria and Lopez co-advised the study, which included contributions from Nishu Devi, a postdoctoral fellow and lead author; Xiaohui Gong and Daiki Shoji, Ph.D. students; and Amy Wagner, a former graduate student. The study also benefited from the contributions of key representatives from the Global R&D department of Cemex, a global building materials company dedicated to sustainable construction. This work is part of a broader collaboration between Northwestern and Cemex.
This new study builds on previous work from Rotta Loria’s lab to store carbon dioxide long-term in concrete and electrify seawater to cement marine soils. Now, he utilizes insights from those two projects by injecting carbon dioxide and applying electricity to the lab’s seawater.
To generate the carbon-negative material, the researchers started by inserting electrodes into seawater and applying an electric current. The low electrical current split water molecules into hydrogen gas and hydroxide ions. While leaving the electric current on, the researchers bubbled carbon dioxide gas through seawater. This process changed the chemical composition of the water, increasing the concentration of bicarbonate ions.
Finally, the hydroxide ions and bicarbonate ions reacted with other dissolved ions like calcium and magnesium, that occur naturally in seawater. And the reaction produced solid minerals, including calcium carbonate and magnesium hydroxide. Calcium carbonate directly acts as a carbon sink, while magnesium hydroxide sequesters carbon through further interactions with carbon dioxide.
Rotta Loria compares the process to the technique coral and mollusks use to form their shells, which harnesses metabolic energy to convert dissolved ions into calcium carbonate. Instead of using metabolic energy, these researchers applied electrical energy to initiate the process and boost mineralization with the injection of carbon dioxide.
With experimentation, the researchers made two significant discoveries. Not only could they grow these minerals into sand, but they also were able to change the composition of these materials by controlling experimental factors, such as the voltage and current of electricity, the flow rate, timing and duration of carbon dioxide injection, and the flow rate, timing and duration of seawater recirculation in the reactor.
Depending on the conditions, the resulting substances are flakier and more porous or denser and harder— but always mainly composed of calcium carbonate and/or magnesium hydroxide. And researchers can grow the materials around an electrode or directly in solution.
These materials can be used in concrete as a substitute for sand and/or gravel — a crucial ingredient that accounts for 60-70% of this ubiquitous building material. Or they could be used to manufacture cement, plaster and paint — all essential finishes in the built environment.
Depending on the ratio of minerals, this material can hold over half its weight in carbon dioxide. With a composition of half calcium carbonate and half magnesium hydroxide, for example, 1 metric ton of the material has the capacity to store over one-half a metric ton of carbon dioxide. Rotta Loria also says the material if used to replace sand or powder would not weaken the strength of concrete or cement.
Rotta Loria said the industry could apply the technique in highly scalable, modular reactors — not directly into the ocean — to avoid disturbing ecosystems and sea life. This approach would enable the full control of the chemistry of the water sources and water effluent, which would be reinjected into open seawater after adequate treatment and environmental verifications.
Rotta Loria also foresees reintroducing some carbon dioxide into concrete and cement to make more sustainable materials for construction and manufacturing.
Cemex and Northwestern University’s McCormick School of Engineering supported the study, which was published in the journal Advanced Sustainable Systems.
KEY QUOTES:
“We have developed a new approach that allows us to use seawater to create carbon-negative construction materials. Cement, concrete, paint and plasters are customarily composed of or derived from calcium- and magnesium-based minerals, which are often sourced from aggregates — what we call sand. Currently, sand is sourced through mining from mountains, riverbeds, coasts and the ocean floor. In collaboration with Cemex, we have devised an alternative approach to source sand — not by digging into the Earth but by harnessing electricity and carbon dioxide to grow sand-like materials in seawater.”
“Our research group tries to harness electricity to innovate construction and industrial processes. We also like to use seawater because it’s a naturally abundant resource. It’s not scarce like fresh water.”
“We showed that when we generate these materials, we can fully control their properties, such as the chemical composition, size, shape and porosity. That gives us some flexibility to develop materials suited to different applications.”
“We could create a circularity where we sequester carbon dioxide right at the source. And, if the concrete and cement plants are located on shorelines, we could use the ocean right next to them to feed dedicated reactors where carbon dioxide is transformed through clean electricity into materials that can be used for myriad applications in the construction industry. Then, those materials would truly become carbon sinks.”
- Northwestern’s Alessandro Rotta Loria, who led the study
“The appeal of such an approach is the attention that is being given to the ecosystem and using science to harness the elements in the contemporary environment to develop valuable products for several industries and preserve resources.”
- Davide Zampini, vice president of global R&D at Cemex
