What are the main ingredients contained by Portland cement?

A cement is a binder, a substance that sets and hardens and can bind other materials together.  When mixed with water to it can bind sand & gravel into hard, solid mass called concrete. A Mixture of Limestone and Clay is ground and burnt at a very high temperature to form Clinker. The Clinker is ground to a fine powder with addition of Gypsum (up to 5 %) to form Cement. The essential components of Cement are Lime, Silica, Alumina and Iron Oxide.

Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world, It is hydraulic cement  that not only hardens by reacting with water but also forms a water-resistant product  It is a fine powder produced by grinding Portland cement clinker (more than 90%) with Gypsum.

For ordinary Portland cement :

limestone (TC  75-90%) or (Cao  44 - 55 )

Clay include three element or two element sometimes ( sio2 - Al2O3 - Fe2O3 )

Sometimes increase AL2O3 (raw martial )or Iron  

Lime

63.1

64.5

60

Silica

20.6

20.7

22.5

Alumina

6.3

5.2

5.2

Iron Oxide

3.6

2.9

4.6

The ingredients of cement include limestone, shells and chalk combined with shale, slate, clay, silica sand, blast furnace slag and iron ore. These ingredients are heated at high temperatures to form a rock-like substance, which is then ground into cement powder.

Portland cement is the material used to form concrete. When mixed with water, it forms a paste that binds with sand and rock that hardens into concrete. Portland cement was invented in the early 19th century by a bricklayer in Leeds, England, who created the cement by burning powdered limestone and clay in his kitchen oven.