Aggregate — solid materials like sand, gravel and crushed stone — is combined with cement to make concrete, the world’s most widely used human-made material. But sand is in short supply, so there is a need for alternate materials to serve as concrete aggregate.
Scientists at Northwestern University (NU; Evanston, Ill.; www.northwestern.edu) have developed a method for making sand-like materials for concrete by combining carbon dioxide with minerals from seawater. The process, described in a recent issue of the journal Advanced Sustainable Systems, creates a path to sequester atmospheric CO2 while also generating a substitute material for sand in concrete.
The approach involves applying electric current across electrodes in seawater, while simultaneously bubbling CO2 gas through the seawater. The electricity forms hydrogen gas and hydroxide ions in solution, while the CO2 increases the concentration of bicarbonate anions (HCO3–) in the water. The hydroxide and bicarbonate ions combine with naturally occurring calcium and magnesium in seawater to form CaCO3, which acts directly as a carbon sink, and Mg(OH)2, which further interacts with CO2 to sequester carbon.
Lead investigator Alessandro Rotta-Loria, a civil and environmental engineering professor at NU, likens the method to the way coral and mollusks use to form their shells. That process harnesses metabolic energy to convert dissolved ions into calcium carbonate, while the NU researchers applied electrical energy to initiate the process, then boosted mineralization with the injection of CO2.
In a series of experiments, the researchers found they could control the composition and properties of the solid materials formed by altering the voltage and current of the applied electricity, along with the flowrate, timing and duration of CO2 injection. The seawater recirculation in the reactor also plays a role and can be controlled as well.
Depending on the conditions, the resulting solids are flakier and more porous, or denser and harder (photo) — but are always primarily composed of calcium carbonate and magnesium hydroxide. Researchers can grow the materials around an electrode or directly in solution.
This ability “gives us some flexibility to develop materials suited to different applications,” Rotta-Loria says. In addition to being used in concrete as a substitute for sand or gravel, they could be used to manufacture cements, plasters and paints.