About 5 percent of global carbon dioxide emissions come from manufacturing cement. Kurt Zenz House, a research fellow at MIT and cofounder of a startup called C12 Energy, hopes to turn the problem into a solution. He thinks that the carbon dioxide from industrial smokestacks can be captured for use in cement production--keeping it out of the atmosphere for good.

The key to his approach is that alkaline solutions react with carbon dioxide and trap it in various compounds. For example, lye reacts with carbon dioxide to form baking soda. Combining the baking soda with seawater creates a type of cement, the glue that holds concrete together.

House says that regulations designed to limit greenhouse-gas emissions, such as a carbon tax, could eventually make this process profitable as well as environmentally sound. Meanwhile, he's researching other ways to store carbon dioxide, including sequestration under the ocean and in geologic reservoirs on land. And at C12, he's developing technology to reduce the cost of storing carbon dioxide. --Kevin Bullis

Turning carbon into cement: Kurt House has a simple recipe: Start with seawater. Extract the sodium chloride from the other minerals to make salt water. Electrolyze that, splitting the water and salt to form sodium hydroxide (lye) and hydrochloric acid. Neutralize the acid in a reaction with silicate rocks, producing sand and magnesium chloride, which can be used together or separately to melt ice on roads. Combine the highly alkaline sodium hydroxide solution with carbon dioxide streaming from a smokestack, trapping the carbon dioxide in the form of baking soda (sodium bicarbonate). Add the baking soda to seawater, which contains magnesium and calcium. The soda triggers a series of reactions, precipitating a magnesium and calcium carbonate that can be used as cement.
Credit: Arthur Mount