US scientists turn CO2 into solid material that could make concrete and cement more sustainable

The researchers created solid materials that could be used in concrete as a substitute for sand and/or gravel. Or they could be used to manufacture cement, plaster and paint — all essential finishes in the built environment.

Researchers at Northwestern University have created a new carbon-negative construction material using seawater, electricity, and carbon dioxide (CO₂).

As global temperatures rise, scientists worldwide are working on methods to extract CO₂ from the atmosphere and store it underground. While this technique helps combat climate change, it doesn’t fully capitalize on the potential of captured CO₂.

Northwestern's innovative approach tackles this issue by permanently storing CO₂ and transforming it into useful materials that can be utilized in the production of concrete, cement, plaster, and paint. Additionally, the process generates hydrogen gas — a clean energy source with various uses, including in transportation.

We’ve developed a novel method that uses seawater to produce carbon-negative construction materials,” said Alessandro Rotta Loria, who led the research at Northwestern University. “Traditional materials like cement, concrete, paint, and plaster typically rely on calcium- and magnesium-based minerals, which are usually obtained from natural sand.

This sand is commonly extracted through mining activities in mountains, riverbeds, coastal areas, and even the ocean floor. In partnership with Cemex, we’ve created an alternative — instead of extracting sand from the Earth, we use electricity and carbon dioxide to generate sand-like materials directly in seawater.”

Rotta Loria holds the position of Louis Berger Associate Professor of Civil and Environmental Engineering at Northwestern’s McCormick School of Engineering. The study also involved Jeffrey Lopez, an assistant professor of chemical and biological engineering at McCormick, who played a significant role as coauthor.

The research team, co-advised by Rotta Loria and Lopez, included postdoctoral fellow and lead author Nishu Devi; Ph.D. students Xiaohui Gong and Daiki Shoji; and former graduate student Amy Wagner. The project also received valuable input from members of Cemex’s Global R&D division. This study forms part of an ongoing collaboration between Northwestern University and Cemex to promote sustainable construction practices.

Tapping seawater, a naturally abundant resource

This new study expands on earlier research from Rotta Loria’s lab, which focused on long-term CO₂ storage in concrete and using electrified seawater to stabilize marine soils. Now, combining knowledge from both areas, his team injects CO₂ into seawater while applying electricity in a lab setting.

“Our team explores how electricity can be used to transform construction and industrial methods,” Rotta Loria explained. “We favor seawater in our experiments because it’s abundant and doesn’t compete with the limited supply of freshwater.”

To create the carbon-negative material, the team inserted electrodes into seawater and applied a mild electric current. This split the water molecules, releasing hydrogen gas and generating hydroxide ions. While maintaining the electrical flow, they introduced CO₂ gas into the seawater, which altered its chemical balance and raised the levels of bicarbonate ions.

These bicarbonate and hydroxide ions then reacted with naturally occurring calcium and magnesium ions in the seawater. The result was the formation of solid minerals like calcium carbonate and magnesium hydroxide. Calcium carbonate captures and stores CO₂ directly, while magnesium hydroxide removes additional CO₂ through subsequent reactions.

Rotta Loria compares the method to the natural process used by corals and mollusks to build their shells, where they use metabolic energy to transform dissolved ions into calcium carbonate. In contrast, the researchers used electrical energy to trigger the same process and enhanced it by adding CO₂ to accelerate mineral formation.

“This approach is especially compelling because it considers environmental sustainability — using scientific innovation to work with natural elements in today’s world to create valuable products for various industries while conserving resources,” said Davide Zampini, vice president of global R&D at Cemex.

Developing materials for different uses

Through their experiments, the researchers made two key findings. First, they discovered they could grow the minerals into sand-like materials. Second, they found they could alter the composition and characteristics of these materials by adjusting various experimental parameters — such as the voltage and electrical current, the rate, timing, and duration of CO₂ injection, and the recirculation of seawater in the reactor.

Depending on how the process is controlled, the resulting materials can range from light and porous to dense and solid. However, they consistently consist mainly of calcium carbonate and/or magnesium hydroxide. These materials can be formed either around an electrode or freely within the solution.

“We demonstrated that we can precisely tailor the properties of the materials we produce — including their chemical makeup, size, shape, and porosity,” Rotta Loria explained. “This flexibility allows us to customize the materials for a variety of applications.”

The produced materials can potentially replace sand and gravel in concrete — components that typically make up 60-70% of its volume — or be used in the production of cement, plaster, and paint, all of which are essential in construction and finishing work.

Storing carbon in structures

The material's CO₂ storage capacity depends on its mineral composition. When made with equal parts calcium carbonate and magnesium hydroxide, one metric ton of the material can absorb more than half a metric ton of CO₂. According to Rotta Loria, substituting this material for sand or powder in concrete or cement would not compromise structural strength.

He also suggests that the method could be implemented using scalable, modular reactors located on land, rather than in the ocean, to protect marine ecosystems.

“This strategy allows for complete control over the water chemistry and ensures that any water returned to the ocean is properly treated and environmentally verified,” he explained.

The cement industry is a major contributor to global emissions, accounting for around 8% of worldwide CO₂ output, according to the World Economic Forum — and this figure increases when concrete production is included. Rotta Loria envisions this new process as a way to recycle some of that CO₂ back into construction materials, creating more sustainable options for the building and manufacturing sectors.

“We have the potential to create a circular system where CO₂ is captured directly at its source,” said Rotta Loria. “If cement and concrete plants are situated along coastlines, nearby ocean water could supply specialized reactors. In these reactors, CO₂ would be converted—using clean electricity—into useful construction materials. In this way, the materials themselves would serve as effective carbon sinks.”

The research, featured in the journal Advanced Sustainable Systems, was backed by Cemex and Northwestern University’s McCormick School of Engineering.