Extracting Cement Material From Seawater While Capturing Carbon

Concrete is an essential material in the modern world, with sand and cement actually among the world’s most massive material production by volume and weight.

Source: Visual Capitalist

The production of cement is also a very energy-intensive activity. It is also almost exclusively powered by fossil fuels, resulting in cement production being responsible for 8% of the world’s CO2 emissions.

This can be compared to the CO2 emissions of cars and vans, which are responsible for 10% of the world’s total emissions. So, making concrete more sustainable would be as impactful as turning all the world’s cars into EVs and powering them only with green energy.

A lot of the carbon emissions from cement manufacturing come from the mining, breaking, processing, and refining of the raw materials used to produce it. Like limestone, calcium carbonate-rich rocks (CaCO3) are mined and mixed with clay to obtain the raw material that becomes concrete.

There is potentially another source of calcium carbonate on Earth, which is seawater. The oceans contain a lot of dissolved minerals, with, of course, table salt (sodium and chlorine ions), but also magnesium, calcium, potassium, and even metals, with notably uranium, potentially one day sourced from the world’s oceans instead of uranium mines. Dissolved CO2 in the form of carbonate ions is also abundant in the oceans, making them one of Earth’s most powerful carbon sinks.

Source: Advanced Sustainable Systems

Scientists from Northwestern University and CEMEX Innovation Holding AG (Switzerland) are now exploring if they could exploit this sea-born abundance to produce concrete’s raw material while capturing CO2 instead of emitting it. They published their experimental results in Advanced Sustainable Systems¹, under the title “Electrodeposition of Carbon-Trapping Minerals in Seawater for Variable Electrochemical Potentials and Carbon Dioxide Injections”.

Water Electrolysis

Water (H2O) can be broken down into its constituents of hydrogen and oxygen by applying a powerful electrical current, usually with some sort of catalyst to improve the speed and efficiency of the electrochemical reaction. This is the basis of green hydrogen production, where the electricity is sourced from renewables.

However, when performing this procedure with non-pure water, and even more with sea water, the electrolysis reaction also reacts with the minerals dissolved in the water.

This is generally an unwanted reaction, as it can create deposits on the electrodes and divert energy from the intended goal of the production of hydrogen.

However, tweaking the electrolysis conditions could turn this unwanted by-product reaction into a valuable new way to produce calcium carbonate.

Producing Cement From Seawater

Unlimited Supplies

This is not necessarily a new idea, as CaCO3 as well as magnesium from seawater has a myriad of applications in the construction, manufacturing, and environmental remediation industries, including the production of concrete, cement, plasters, paintings, and fillers.

As the vast oceans covering Earth would provide a practically unlimited supply of this material, this has been considered the most sustainable potential source of these materials.

So far, just exploring the electroreduction of these minerals has not yielded a viable way to make their production from seawater economical. This is where the Northwestern University researchers brought a crucial extra step: adding CO2 to the process.

Injecting CO2 In Seawater

Because seawater is a complex mix of many minerals, when applying electrolysis, a mesh of electrochemical occurs at once, from precipitations of calcium and magnesium ions to the formation of gypsum from sulfates, the generation of chlorine & hydrogen gas, as well as a change in acidity around each electrode.

Source: Advanced Sustainable Systems

The injection of CO2 adds some more complexity as it decreases the pH of the seawater. The decrease in pH from CO2 is partially compensated by the production of OH− ions from the electrical energy.

The dissolution or precipitation of calcium carbonate is itself dependent on water acidity. For that matter, this is a phenomenon that is a concern for scientists, as the atmosphere gets richer in CO2, the oceans get more acidic.

If the electric current is powerful enough, and therefor the OH− ions production, it can be high enough to keep pH above 8.5.

At these acidity levels, the chemical reactions capture the CO2 and turn it into dissolved bicarbonate ions (HCO3-).

Source: Advanced Sustainable Systems

These bicarbonate ions then move on to react with calcium and precipitate into calcium carbonate, the base material for concrete production.

Optimizing Carbon Sequestration

In this type of reaction, the production of calcium carbonate usable for the cement industry would capture the injected CO2 instead of emitting CO2 into the atmosphere.

For any given level of power used, there exists an optimum flow rate of injected CO2 that minimizes the consumed energy while maximizing the mineral production yield. A concentration of 0.30 sccm of CO2 appeared to be such a sweet spot, where a lower power level still results in a high mass of mineral precipitating.

Source: Advanced Sustainable Systems

Creating A Usable Deposit

A problem with turning this concept into industrial application is the same problem that occurs in calcium carbonate precipitation during hydrogen production via electrolysis.

More often than not, the calcium deposit will clog the surface of the electrode, damaging the overall system and making it less efficient over time.

However, the higher power levels used in this experiment, when combined with CO2 injection, caused additional reactions, causing the precipitate calcium carbonate to detach from the electrode.

So overall, this method would be able to produce the carbonate in a way that makes it easy to collect as a mineral deposit at the bottom of the tank, without clogging the electrode.

Growing Mineral Crystals

Depending on the conditions, different mineral aggregate forms with different crystal conditions, especially calcium carbonate crystals (calcite and aragonite) and magnesium crystal (brucite).

Source: Advanced Sustainable Systems

Overall, the resulting material can be made of crystal several centimeters long (1-2 inches), and is also very porous.

The composition, porosity, and size of the aggregates synthesized using the proposed approach meet current standards for their use in materials like concrete.

Conclusion

Overall, this publication proves that the production of carbon-negative cement material is not only a theoretical possibility but a viable option when using carbon injection during the electrolysis of seawater.

Other critical parameters, such as hardness and abrasion resistance, remain to be investigated for complete confirmation that the resulting material is usable for construction projects.

This process is inherently scalable, with no obvious limitations from rare material availability, excessive energy consumption, or low yields.

By envisioning a network of interconnected, scalable reactors, this approach has the potential to be deployed at industrial scales and integrated with existing infrastructure, such as coastal industrial facilities.

Further progress in the reactor design should be able to boost the overall economical and energy efficiency, for example by optimizing electrode geometry, materials, and flow dynamics.

Ultimately, the water from which the calcium carbonate has been extracted could maybe also be an interesting material for a secondary step of hydrogen generation from seawater, as the lower ion concentration should help reduce the problems linked to mineral deposits on the electrode.

Investing in Sustainable Cement

CRH Plc

As one of the world’s leaders in cement production, CRH will be instrumental in turning cement construction into a more sustainable industry. It is the #1 in total volume of construction material provided in both the US and European markets.

The company is active in 28 countries and 3,390 locations, employing 78,500 people, with CRH Americas making 65% of its global sales.

Source: CRH

The company is expecting that robust spending by Western governments on infrastructure will help grow its business. The trends of re-industrialization and on-shoring high-tech manufacturing should also help.

Sustainability