Cement is a large contributor to carbon emissions, accounting for 8 percent of total global emissions, the fourth-largest source of CO2 pollution worldwide. Efforts to reduce these emissions have involved adding fly ash to concrete. It provides almost a one-to-one substitute for cement; plus, it is a waste biproduct from coal-fired power plants. Another reduction strategy is to lock in carbon from the air to cure certain concrete mixes. Adding rice hulls, high in silicon, as a substitute for cement has also been used successfully.
Recent research has unveiled a promising method to create sustainable, potentially carbon-negative cement: seawater electrolysis. Passing an electric current through seawater splits its molecules into hydrogen, chlorine, and oxygen gases. The process precipitates out ions of calcium carbonate and magnesium hydroxide, both considered wastes in the production of green hydrogen gas. Fortuitously, calcium carbonate can be a key ingredient in cement.
Ancient Romans produced concrete that has endured 2,000 years compared to our concrete that has a life expectancy of 80 to 100 years. Some of the Romans’ structures, aqueducts, and ceiling domes are still standing and in use today. Scientists are discovering that calcium is the binding agent in Roman concrete, making it remarkably strong. It also self-heals: As cracks appear, moisture intrusion recrystallizes the lime chunks into calcium carbonate, resealing any fissures.
The main objection to adoption today is the slowness of mineral formation through electrolysis. A research team at MIT has been experimenting with different voltages, pH levels, and rates at which they inject carbon dioxide into the water. They are finding that adjustments affect the volume, density, and crystal structure of the resulting minerals. Some were flaky and some denser, creating ideal options for different construction uses like concrete, plaster, and even paint.
If powered by renewable electricity, this technique not only reduces emissions, but can pull CO2 from the atmosphere. As the minerals form, they effectively lock away CO2 for hundreds and even thousands of years. The resulting cementitious materials can sequester up to half their weight in trapped CO2, thus becoming a permanent carbon sink. When the process is powered by clean electricity, the materials can be carbon negative.
Instead of mining limestone, a practice that damages ecosystems and releases carbon, we can generate needed materials from abundant seawater, while helping clean the air. Adoption by the slow-to-change construction industry remains the biggest challenge. If it can change, however, from being a polluter to an environmental protector, this could be a blueprint for other industries. The big lesson is rethinking waste as a resource and not a throwaway. This opens the door to circular resilient solutions.
