| A
concrete sidewalk is a human-made layer of rock. It is composed
of rock fragments and binding material shaped by confoming
borders. Simple tools mix sand, water, and portland cement
together to form a rock-like sidewalk. A wooden form holds the
mixture in place until it hardens. Like a fossil in a rock, a
leaf or footprint in wet cement remains after the cement
hardens. Cement forms when limestone and clay are burned
together and then ground into dust. Iowa is the leading producer
of portland cement. |
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