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 ?

The standard consistency test certainly has significance regarding the quality of the cement. You need to think in terms of the "concrete making potential" of the cement. In the test the amount of required to produce a paste of standard consistency is determined. This relates directly to the water demand and workability of concrete made from the cement. If more water must be added to achieve the desired consistency or workability then the strength development of the concrete will be impaired and more cement will need to be added to the concrete to compensate for the additional water. The reason for most standards adopting mortar testing of cement is to promote consistency and comparability between tests and laboratories over time. By using a tightly specified grading of testing sand the influences of the aggregates on the strength testing are eliminated or minimised. This means that the strength development potential of the cement is being measured by the standard mortar test.

We are analysing particle size distribution of raw meal, coal mill product and cement mill product through CELAS PSA having facility to determine 0.7 to 400 micron. Q1. What is the best particle distribution (on various micron sizes) of raw mill in terms of best mill performance, burnability and kiln performance and which plant in the world is producing? Q2. What is the best particle distribution(on various micron sizes) of coal mill in terms of best mill and kiln performance? Our coal ash is 28. Q3. What is the best particle distribution( on various micron sizes) of cement mill product to have optimum power consumption, best strength and good performance in concrete.

There is no straightforward answer to your questions. For raw mix the optimum particle size distribution depends on the mineral composition of your raw materials. If you have a high content of quartz then you must have a very tight fine particle size distribution with low content of oversize particles where the quartz will concentrate. On the other hand with a homogenous raw mix with no quartz it is not beneficial to have too small a particle size distribution as this will increase the dust losses from the top stage of the preheater. Similarly with coal there is no hard and fast rule. It is said that the 90 micron residue should not be more than 50 per cent of the volatile content of the coal. Increasing the fineness beyond that is counter-productive. For cement the optimum particle size distribution depends on the cement performance characteristics that your customers have come to expect. Ring formation is caused by the formation of liquids in the kiln at a particular position. These penetrate the refractory lining and cause a localised thickening of the coating. Fe₂O₃ can be a cause of these problems and I am not surprised that raising the alumina modulus has solved the problem. Many cement factories operate with higher alumina modulus than 1.7 and I would recommend keeping the kiln feed mix design with the higher alumina modulus. A snowman is the formation of a large build-up on the first grate of the cooler where the clinker falls from the kiln rather than a build-up in the kiln.