The significance of cement can't be understated. A powdery substance made with calcined lime and clay, it is a binder that enables the creation of concrete (along with water and gravel or sand). Concrete, according to Smil (whom Gates calls his favorite author), is the most important man-made material in history. It has been used in everything from the Pantheon to the Hoover Dam to China's incredibly big Three Gorges Dam.
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