Concrete Proposal to Cut Carbon Dioxide Emissions

The making of cement accounts for up to 10 percent of the world's total emissions of carbon dioxide, a key gas involved in global warming. Now scientists and engineers are developing a cleaner way to manufacture cement.

The announcement comes during a week in which scientists around the planet are awaiting a global warming report due to be announced Friday by the Intergovernmental Panel on Climate Change.

Cement is the oldest artificial construction material, dating back to the Roman Empire. It is the primary component of the world's most widely used building material—concrete. Cement is manufactured at the rate of 2.35 billion tons per year, enough for more than 1 cubic yard of concrete for every person on Earth.

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Cement starts out as limestone and clay that pulverized and heated in a kiln to 2,730 degrees Fahrenheit, nearly hot enough to melt iron. At this heat, the powder stores energy. When the powder is mixed with water, the energy locked within it is released into chemical bonds to create the elementary ingredient of cement, known as C-S-H, or calcium-silicate-hydrate. This compound acts as a glue to bind sand and gravel together to make concrete.

Most of the carbon dioxide emissions in cement manufacturing result from heating the kiln to a temperature high enough to transfer energy into the powder. Civil engineer Franz-Josef Ulm at MIT and his colleagues are now developing materials that are hopefully as strong and cheap as cement while requiring lower temperatures during production.

In the end, the researchers suggest this cool solution can slash carbon dioxide emissions during cement manufacture by up to 10 percent. This would accomplish a fifth of the goal of the Kyoto Protocol, the United Nations agreement where industrialized nations cut their greenhouse gas emissions by 5.2 percent. [The United States has never signed the Kyoto agreement.]

Ulm and his colleagues investigated what makes cement strong by poking and prodding cement pastes from around the world with needles just nanometers or billionths of a meter in size. They found the strength of cement paste was based on granules of C-S-H stacking up into dense structures resembling the pyramid-shaped piles of oranges in grocery stores.

The researchers suggest they could swap out C-S-H with a material that stacks up just as well but requires less heat to produce, thus cutting down on carbon dioxide emissions. For instance, Ulm said candidates include compounds resembling C-S-H that replace calcium with magnesium.

"Magnesium is an earth metal, like calcium, but it is a waste material that people must pay to dispose of," he explained.

Ulm and his colleagues report their findings in the January issue of the Journal of the Mechanics and Physics of Solids.

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