Roman Cement = Lime + Pozzolana
Pozzolana = Volcanic Ash
Roman Cement hardens under water

(Portland) Cement = Limestone + Clay
The Limestone and Clay are cooked to molten form, it becoms solid upon cooling (the result is called a clinker).  This material is then ground with gypsum to form cement.
Gypsum is a sedimentary rock mineral.

Cement + Sand + Gravel = Concrete

Cement + Sand = Mortar

Gypsum heated to drive out water = Plaster (of Paris)
Plaster is a fine white powder.

Cement + Sand + Lime = Stucco

Water must be added to any one of the above before its application.