Green cement: concrete, the number one construction material, is a major source of climate-warming gases. Meet one engineer who hopes to build a better world with.(Nikolaos Vlasopoulos)

"You know, cement is everywhere," Nikolaos Vlasopoulos, an environmental engineer at Imperial College in London, says while sitting in a brightly lit college conference room in a hulking seven-story building held up by the topic of conversation. "It's all around us."

Last year, the world produced 3.6 billion tons of cement--the mineral mixture that solidifies into concrete when added to water, sand and other materials--and that amount could increase by a billion tons by 2050. Globally, the only substance people use more of than concrete, in total volume, is water.

Cement's virtues, Vlasopoulos says, have long been plain: It is inexpensive, pourable and, somewhat inexplicably, becomes hard as a rock. But one other important detail is seldom acknowledged: Cement is dirty. Not dirty as in it won't come off your clothes--although that problem has dogged construction workers for centuries. The key ingredient is limestone, mostly calcium carbonate, the remains of shelled marine creatures. The recipe for making cement calls for heating the limestone, which requires fossil fuels. And when heated, limestone sends carbon dioxide gas wafting into the atmosphere, where it traps heat, contributing to global warming. Cement production is responsible for 5 percent of the world's human-produced carbon dioxide emissions; in the United States, only fossil fuel consumption (for transportation, electricity, chemical manufacturing and other uses) and the iron and steel industry release more of the greenhouse gas. And with booming countries such as China and India using cement to construct their rise, cement's dirtiness looms as one of the foremost downsides of globalization.

If cement's enormous contribution to air pollution is largely overlooked by the general public, Vlasopoulos, 31, has been aware of it for some time. He grew up in Patras, a Greek port. His father was an engineer and his mother worked in a bank, and during Vlasopoulos' summers home from Dimokrition PanepistimionThrakis college, where he studied environmental engineering, he worked in a cement factory with his uncle. This was fortuitous. His job was to assemble the equipment that measured carbon dioxide emission levels. They were high; …