The time taken for concrete to harden depends on its purpose. It can harden within a few minutes, a couple of hours, in the air, or under water. In cities like Singapore, massive amounts of concrete are needed to build, e.g. skyscrapers, expressways, subways and airport tarmacs.
Concrete connects the past to the future, and is everywhere you look. Without concrete, there would not be safe roads, safe traveling in general, comfortable homes or sturdy buildings to work in.