For elements are needed to make cement; Calcium, Silicon, Aluminum and Iron. These can be found in limestone, sand and clay. The components are crushed and made into a fine powder. When mixed with water, the components bind. When mixed with sand and gravel, it forms concrete. See the following link for a nice page on how cement is made: http://www.buildeazy.com/newplans/eazylist/cement.html
Ferro Cement is another type of cement. The definition of which can be found here: http://ferrocement.com/intro-Ferro/intro.en.html