Almost everything about the Bank of America tower, a soaring skyscraper under construction near Times Square in New York City, has been designed to minimize the use of energy. Take the concrete. Making the stuff from scratch is very energy intensive, so the builders are using a mix of 55% concrete and 45% slag, a waste product from blast furnaces. Mixing slag with concrete saves energy and makes the concrete stronger. The tower will save so-called gray water from washrooms and use it to flush the toilets. The building will also generate much of its own electricity from natural gas, a less potent carbon emitter than coal. These features will account for $3.5 million of a total building cost of $1.2 billion, but the owners expect to recoup that in a few years with all the energy they'll save. When it's finished next year, the tower will be the second highest in the city, but it stands alone as the greenest building in New York.