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Concrete
Cement is a material that's made by heating grains
of rock and clay. When people mix cement with water,
sand, and stones, it becomes a liquid called concrete.
After a few hours, crystals grow inside the concrete
and it becomes a solid material.
About 2,000 years ago, the Ancient Romans used
concrete to make beautiful buildings, like the
Pantheon - the dome is made of concrete.
Later, people forgot how to make
concrete for more than 1,000 years!
Modern concrete was invented in
1756. Today, it's the world's most
important building material.
Concrete becomes hard very
quickly, so machines only make
the concrete when people need it.
To make concrete stronger, people put long pieces
of steel in it. This is called reinforced concrete.
Many of the world's tallest skyscrapers are
made of reinforced concrete. Huge machines
called pumps move liquid concrete
hundreds of meters above the ground.
Today, there are many different types
of concrete. Waterproof concrete is
great for building bridges. Another
type of concrete has materials in
it that take dangerous chemicals
from the air. In the future, this
will help to make
cities cleaner.
becomes transparent.