Concrete is an important material for making different buildings and structures. It is a composite made of Portland cement, sand, gravel or aggregate, and water in varying proportions depending on the task.
Concrete is used more than any other man-made material in the world. As of 2006, about 7.5 billion cubic meters of concrete are made each year—more than one cubic meter for every person on Earth.
The ingredients are mixed together into a paste, a bit like making dough for bread. The concrete is then poured into a frame. After a few hours it sets hard. Concrete solidifies due to a chemical reaction known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a strong stone-like material.
link source : https://simple.wikipedia.org/wiki/Concrete