Specifying a set-point for the grinding circuit on the basis of particle size

A key failing of Blaine is that it provides a single averaged result for each cement sample rather than a size distribution. This means that two cement samples with a similar, but not identical, distribution cannot be differentiated using the technique. Laser diffraction rapidly yields a full particle size distribution raising the question of how to use this additional information in the best way possible. How will the differences between these two samples, detectable by particle size analysis but not by Blaine measurement, impact strength, i.e. product performance?

Variation between Blaine and Particle Size Distribution

Fines at the very low end of a cement sample size distribution, material in the sub 2 micron range, react with water very quickly because of their high specific surface area. Such rapid hydration can cause the cement to cure exothermically and crack. An upper limit on the fraction of material in this size range is sensible. At the other end of the distribution, particles larger than 50 microns may fail to hydrate in a reasonable time-frame, ultimately reducing compressive strength.

In general aspecification of around 60 – 70% in the 3 to 32 micron range has benefits in terms of strength. Particles at the lower end of this range ensure good early strength while larger particles confer high long term strength.

Optimum strength performance

Applying this understanding to analyzing the results shown for two samples with the same Blaine number, it becomes clear that sample 1 is likely to have much better strength characteristics than sample 2 which contains more fines and more excessively coarse material.

The specification of the set-point for the grinding circuit will of course vary from grade to grade but with laser diffraction analysis in place precise definition on the basis of strength targets becomes possible. The <2 micron, 3 to 32 micron, and > 50 micron fraction sizes are all commonly reported and controlled, to give market-leading product quality.