Recently we started production of SRC with 97.5 LSF, 2.35 SM, and 0.75 AM raw meal composition. After that free lime become normal (i.e 1.1 - 1.20). Do you think that with OPC we were operating with high alumina modulus? (Continuation question)

I don't think the high free CaO was associated with the alumina modulus itself. However, the mix of raw materials must have changed in the switch to SRC production. Which raw materials have reduced significantly? Whichever one is the most likely culprit. We can rule out the coal as presumably this remains the same blend as when OPC was produced.

I understand that the cement manufacturing industry is considering the use of copperas (ferrous sulphate) in the manufacture of cement to reduce chromium 6 allergy problems for the end user. My company produces the only trouble free copperas saturator on the market. Our saturator accepts crystal copperas generates a stream of aqueous ferrous sulphate solution for dosing mainly to sewage to remove phosphates or to control odour and septicity in sewers and sewage works. I am interested to understand the mechanisms underlieing the use of copperas for cement production.

You are right that cement companies are considering the addition of ferrous sulphate to their cement in order to meet the stricter limits on hexavalent chromium that are being introduced. The application involves adding solid ferrous sulphate to the cement either at the milling or packing stage and relying on this to reduce any hexavalent chromium present in the cement. The industry knows that this solution has its limitations, not least being that the activity of the solid ferrous sulphate will deterioratr with time and this will confer a "shelf-life" to cement treated in this way to meet the directive.