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.

I am working out the percentage by-pass required for a large precalciner-preheater kiln for the production of low alkali cement. As the presence of chloride in the raw meal is noted, the volatility of the K₂O and Na₂O is supposed to increase. Can you provide me some relationship between the increase in the volatility of the alkalis with the increase in Cl content.

There is no absolute relationship between chloride content and alkali volatility. This depends on the other anoins present and also the burning zone temperature in your kiln. However, it is possible to say that the presence of chloride will increase the volatility of alkalis.