We are producing slag cement with VRM technology (500tpd). Our cement setting time is coming down at the time of dam ring height reduced. Setting time is increased when the dam ring height was increased. Both above cases percentage of gypsum is same. Please explain why it is happening.

We have received some suggestions regarding the possible effects of dam ring height from a major cement plant in North America as follows: "I would suggest that the particle size distribution (PSD) be looked at for both instances. We have found that by changing internal settings in a VRM there are physical changes in the PSD which will impact the performance of the slag. By changing the PSD as little as 2 microns there is a significant change in the performance while not showing up in a simple test of Blaine." "If the dam ring height is increased the clinker and gypsum will stay on the grinding table longer and possibly be better ground when it exits the table and therefore the circulating load in the mill should be lower. So the two things to look at are the amount of dehydration of the gypsum with lower dam ring height compared to the higher dam ring height and also the particle size distribution of the finished cement. With a lower dam ring there is the chance that there is more overgrinding and a higher concentration of superfines." Please let us know if you find there is a relationship between dam ring height and particle size distribution or gypsum dehydration as this would make an interesting case study for publication.

I wish to know if the test of consistency of cement pastes has any significance as regards the quality of cement, eg a cement of lower consistency (<25 per cent) is better than a cement of higher consistency (> 25 per cent water). Secondly: it is observed that most of international cement standards set limits only to mortar compressive strengths although in practice the main usage of cement is in concrete applications. May I know why is that?

To calculate the velocity of the gases at the mill fan inlet you need the static pressure in addition to the dynamic pressure. If the mill fan has a capacity of 133m³/s then you need to divide this by the cross-sectional area of the inlet ducting to arrive at the design gas velocity of the fan