What is cement?

Something I was always curious about.

• In general, cement is any substance that can bind two things together, and can harden or set independently of those things.
• Cement is made from heating limestone with small amounts of other materials (such as clay).
• The most common form of cement is Portland cement.
• The most common uses of cement is to make concrete, which is cement + aggregate (such as gravel/sand) and water.
• China, India and the US are the top producers of cement (in that order), with China producing far more than the other two.  China alone almost produces half of the world's cement.  China also consumes the most in the world.
• The production of concrete is extremely bad for the environment.  Not including the quarrying and transport, it generates about 900 kg of carbon dioxide per 1000 kg of cement and releases heavy metals in both gaseous and solid form (in the cement itself).

http://en.wikipedia.org/wiki/Cement