To make concrete, aluminum- and iron-rich rocks are the magic ingredients, but only in the correct proportions. Once it has all cooled down, the result is a powder the gray-white color of the moon. If you put your hands through it you find that it has the silky texture of ash—there is something atavistic about it—but your hands soon feel dried out as if under a subtle type of attack. This is a very special material with a very dull name: cement.
