How Seawater and Carbon Dioxide Become Building Material

Researchers have discovered that seawater and carbon dioxide can be converted into a solid construction material that locks up more carbon than is emitted during its manufacture.

The result reframes a fundamental building component as a potential climate asset, particularly in places where energy-intensive industries operate along the coast.

Seawater and carbon dioxide become stone

In a bench-scale reactor, seawater produced light-coloured, sand-like particles that could replace quarried aggregate in concrete.

Alessandro Rotta Loria, working with fellow researchers at Northwestern University, showed that this material can be formed directly within a carefully managed chemical environment.

The solids can develop in more than one format: either as loose powder suspended in the liquid or as grains that accumulate on an electrode.

This adaptability relies on a precise chemical equilibrium, raising the more detailed issue of how changing conditions affects the resulting material.

How the reaction works

When an electric current passes through seawater, it splits water molecules and creates hydroxide ions, charged particles that reduce acidity in the surrounding water.

Meanwhile, carbon dioxide bubbled through the seawater forms bicarbonate ions, dissolved carbon-containing compounds that can produce minerals.

These components react with the calcium and magnesium already present in seawater, hardening into calcium carbonate - the mineral in limestone and shells - alongside a magnesium-rich solid.

The reaction also releases hydrogen gas, creating a second output that could help make the process economically viable.

Shaping the mineral mix

Adjusting the voltage and current, carbon dioxide supply, or water movement by small amounts created particles ranging from light flakes to compact grains.

In certain conditions, the solids adhered to the electrode; in others, bubbles of hydrogen dislodged them and released them into the solution.

“We showed that when we generate these materials, we can fully control their properties, such as the chemical composition, size, shape, and porosity,” said Rotta Loria.

Such control is important because concrete, plaster and fillers each require particular particle sizes, densities and levels of porosity.

Why sand matters

Aggregates - the sand and gravel that provide bulk - account for approximately 60 to 75 percent of the volume of a standard concrete mixture.

Substituting some of that material with particles grown in a reactor could reduce reliance on sand extracted from rivers, shorelines and the seabed.

Since reactor-grown particles can be made as fine powders or larger grains, this chemistry could be used for a range of construction products.

It transforms captured carbon into a material manufacturers require on a vast scale, making it more valuable than simply storing it underground.

Some mixes become carbon negative

The most effective mixtures are carbon-negative, meaning that they retain more carbon dioxide than the process generates.

A mixture containing equal amounts of calcium carbonate and the magnesium-rich solid can store more than half of its own mass in carbon dioxide.

Further carbonation - a process that draws carbon dioxide into solid materials - enables the magnesium-rich component to retain even more.

This additional treatment also alters the material, moving its potential beyond carbon storage and towards structural performance.

Strength after curing

Over 30-day runs, deposits continued to build up around the electrode, eventually producing chunks measuring about 2.5 cm rather than loose powder.

Their internal porosity - the open space within a solid - allowed ions to continue travelling through the chunks, sustaining further growth.

With additional carbonation, compressive strength increased from around 14 kilograms per square centimetre (200 pounds per square inch) to more than 61 (870).

Very alkaline environments still caused some aggregates to break down, indicating that their durability will depend on their final construction application.

Keeping oceans separate

Rather than discharging carbon dioxide into the open ocean, the team envisages carrying out the chemistry in modular coastal reactors.

Operators of these nearshore units could manage the incoming seawater, collect by-products and treat the remaining liquid before returning it.

“We could create a circularity where we sequester CO₂ right at the source,” Rotta Loria said.

This distinction is important because an otherwise promising climate technology is harder to justify if it harms neighbouring ecosystems.

Future research directions

A 2018 Chatham House analysis estimated that cement manufacture produces around 8 percent of worldwide carbon dioxide emissions.

For this material to reduce cement’s climate impact, the electricity powering the system must remain low-carbon and reasonably affordable.

Greater voltages also initiated chlorine-related reactions at the positive electrode, underlining the need for industrial systems to control side reactions carefully.

More demanding abrasion tests are also required, as construction materials can fail through wear and impacts just as readily as through compression.

Building ingredients that store emissions

As well as concrete, the mineral products could be used in cement, plaster, paint and restoration work requiring calcium- and magnesium-rich solids.

Because particles can either form on electrodes or separate into the solution, factories could tailor production to distinct supply chains.

This manufacturing versatility may account for industrial interest, since carbon capture has greater longevity when it becomes a marketable ingredient.

A gas considered waste in one process can therefore become a raw material in another.

Seawater, electricity and captured carbon can now be used to create building ingredients that retain emissions, replace mined resources and produce useful hydrogen.

Whether this approach becomes routine in construction will rest on reactor design, clean electricity, cost and convincing results from larger-scale trials.