Concrete with stone steps
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.