Scientists from the northwest university have used seawater, electricity and carbon dioxide to develop new carbon-negative building materials.
Carbon negative materials do not maximize the value of vast amounts of atmospheric CO2, despite providing multiple climate benefits.
The new process addresses this challenge by permanently locking CO2 and turning it into a valuable material that can be used to manufacture concrete, cement, gypsum and paints.
The process of generating carbon-negative materials also releases hydrogen gas, a clean fuel with a variety of uses, including transportation.
Cultivation of carbon-negative materials in seawater
This new study is based on previous studies to store CO2 in concrete for a long period of time and electrify seawater into marine soils.
“We have developed a new approach that allows us to use seawater to create carbon-negative building materials,” said Alessandrolottaloria of Northwestern, who led the study.
“Cement, concrete, paints and plaster are made up of or derived from calcium and magnesium-based minerals, which are often sourced from aggregates.
“Today, sand is sourced from mining on the mountains, riverbed, coastal and seabeds. In collaboration with CEMEX, we devised a source by cultivating sand-like materials in the seawater by digging into sand.”
To generate carbon-negative materials, researchers began by inserting the electrodes into the seawater and applying current. Low current divides water molecules into hydrogen gas and hydroxide ions. While the current was turned on, researchers bubbled CO2 gas through seawater.
This process changed the chemical composition of water and increased the concentration of bicarbonate ions.
Finally, hydroxide and bicarbonate ions reacted with other dissolved ions, such as calcium and magnesium, which are naturally occurring in seawater. This reaction produced solid minerals containing calcium carbonate and magnesium hydroxide.
Calcium carbonate acts as a direct carbon sink, while magnesium hydroxide sequesters carbon through further interaction with CO2.
Double discovery
Through the experiment, the researchers made two important discoveries: They were able to not only grow these minerals into sand, but also alter the composition of the carbon-negative materials.
They did this by controlling experimental factors such as the voltage and current of electricity, flow rate, timing and duration of CO2 injection, flow rate, timing and duration of seawater recirculation in the reactor.
Depending on the conditions, the resulting material is flexier, more porous or more dense, but always consists primarily of calcium carbonate and/or magnesium hydroxide.
These carbon-negative materials can be used in concrete as a sand and/or gravel alternative. This is an important ingredient that accounts for 60-70% of this ubiquitous material.
Additionally, it can be used in the manufacture of cement, plaster and paint. This is the essential finish for all in a built environment.
Store carbon in structures
Depending on the mineral ratio, the material can hold more than half the weight of CO2.
For example, one metric ton of carbon negative material has a capacity of more than one metric ton of half the calcium carbonate and half the magnesium hydroxide composition.
When used to replace sand or powder, it does not weaken the strength of concrete or cement.
“This approach gives us complete control over the chemistry of the water source and water drainage, which is reinjected into the open sea only after proper treatment and environmental verification,” explained Lottaloria.
“You can create circularity that isolates CO2 in the source.
“If concrete and cement plants are located on the coastline, the adjacent ocean can be used to supply dedicated reactors, where CO2 is converted via clean electricity into carbon-negative materials that can be used for countless applications in the construction industry.”
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