We are using synthesized calcium sulphate anhydrite from a nearby aluminium plant. This calcium sulphate (96 per cent purity) is fine in form and free from moisture. In addition to CaSO₄2. Can Calcium sulphate be used as an alternative retarder for the Natural Gypsum? Your expert opinion on the solubility of this synthetic calcium sulphate anhydrite is appreciated.

The solubility of anhydrite varies dependent on the temperature history to which it has been subjected. However, I am sure this will be a good material to use as a set-controlling additive in your cement. You may need to use it in combination with your natural gypsum or you may be able to replace your natural gypsum with 100 per cent of this material. The only way to find out will be to conduct trials with the synthetic CaSO₄.

What are the current international standards of MTBF for kilns and mills?

There is a difference between mean time between failures and mean time between stops. Best practice for kiln MTBF in the cement industry is in excess of 750 hours, while MTBS is somewhat shorter. The problem with MTBF is that it is easier to massage the data and cheat because some stops are planned and therefore do not count. In that situation it is easy to bring forward a planned stop when you have an unexpected unplanned stop.