Oliver Batchelor, of Ilam School asks :-

Why does concrete go hard after it is poured?

Des Bull, a civil engineer at the University of Canterbury, responded.

Concrete is made up of four things: cement, water, sand and stones.

Cement is a special mixture of rocks that have been heated up (much hotter than needed to boil water). These rocks are broken up into a very fine powder which we call cement.

The cement and water mix together to make a 'cement paste'. The cement powder changes when it is mixed with water. The paste slowly goes hard and forms a new type of rock.

When the 'cement paste' is added to sand and stones it dries to form a new type of rock in which the 'cement paste' glues the sand and stones together. This is what we call concrete.