Even though there are many different types of concrete, the word "concrete" normally refers to Portland cement, which is the most widely used construction material in the world. In actual usage, concrete is normally mixed with reinforcing materials, such as sand, fibers, etc.
Mixing is a very important step. Thorough mixing is essential for the production of strong and uniform concrete. Once mixed, concrete becomes a paste that can be poured. After this, it will gradually harden. This process is called "curing". To achieve the best strength and hardness, concrete should be cured in a humid environment at an appropriate temperature. This is especially important during the first three days. In practice, this is achieved by spraying water on the concrete or placing plastic covers over it. It can take a few weeks for the concrete to reach more than 90% of its final strength.
* The Concrete Calculator uses the premixed cement density of 2,130kg/cubic meters, or 133 pounds per cubic foot (60 pounds/0.45 cubic feet, or 80 pounds/0.60 cubic feet) to obtain the weight needed. Different types of concrete can have very different levels of density.