A rock is formed by natural processes and the structure of concrete is not. Rock is naturally occurring mineral material that forms part of the earth's surface or has become detached from the earth's surface but is still a solid piece or is made of pieces that have been bound together through a natural process. Rock is formed by one of three general types of processes. This results in three classifications: igneous, metamorphic or sedimentary.
Concrete is a man-made material containing natural rock products. It is a mixture of water, Portland cement and aggregate (sand and/or gravel). The water and cement react chemically to form a strong, hard material that binds the aggregate together.
Portland cement comes from limestone (calcium carbonate) and is produced by heating limestone in a kiln to produce calcium oxide, a fine powder. It may also contain silica, iron oxide and/or gypsum.