I need to know how and why companies use air cooled blast furnace slag in the manufacture of cement.

Air-cooled blast furnace slag can be added to the inlet of the cement kiln in the patented Cemstar process developed by TXI cement in the USA. I know a number of US cement companies are doing this. This is in the manufacture of cement powder. Another use is as an aggregate in the manufacture of concrete.

Could you please to explain the possibility of steel slag in cement industry as raw material if it was added in the kiln inlet ?

Steel slag can certainly be added to the kiln inlet to boost the output from the kiln. This process has been patented by TXI in the USA and is known as Cemstar. The limiting factor is the high iron content of the steel slag, which effectively limits the possible addition rate. This subject is discussed in more detail in the February edition of International Cement Review.