Modern concrete uses three types of aggregates and are mixed in proportions that are expressed in three numbers. For example, 1:2:3 means one part Portland cement, two parts of clean sand, and three parts gravel or pebbles.

A common misconception about concrete is that it hardens or dries through evaporation. Actually, concrete hardens because of chemical reactions that take place when water is added to the dry concrete mixture. This produces new chemical compounds that lock in water between their molecules. This process, known as hydration, forms crystals that bind the concrete mixture together. These new crystals fill in the spaces between the aggregate, cement, and sand. The speed of hydration depends on the temperature, amount of water, and the type of cement.

The process of hydration takes curing time. The concrete mixture can lose up to half of its strength if not kept wet during the first seven days after pouring. The full hydration time for common cement is about 28 days. The longer the curing time, the more hydration takes place, and the stronger the concrete becomes. Concrete thus becomes stronger as it grows older.