You can grind it up and use it for things other than mortar, such as
landfill, but that's probably not what you meant. From about.com:
cement: The binding element in both concrete and mortar.
concrete: A product composed of cement, sand and gravel or other coarse
aggregate. When water is mixed in with this product, it activates the
cement, which is the element responsible for binding the mix together
to form one solid object.
mortar: A product composed of cement and sand. When water is mixed in
with this product, the cement is activated. Whereas concrete can stand
alone, mortar is used to hold together bricks, stones or other such