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Spot the difference

28 February 2011 No Comment

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In an earlier blog I looked at some of the drivers for changing from one particle size measurement technique to another, as well as some of the challenges that it brings. A more detailed look at switching from sieving to laser diffraction followed, and today I want to focus on a topic of particular interest to the cement industry – the move from Blaine measurement towards laser diffraction analysis.

One of the most important characteristics of an analytical technique is its ability to differentiate between samples that perform differently within an application. Blaine measurement is a traditional technique used in the cement industry to characterize fineness, an important performance indicator. However two cement samples with identical Blaine figures can show different particle size distributions when measured by laser diffraction. Should we classify these two cements as different or the same?

Understanding Blaine

Blaine analysis measures the specific surface area of a sample, the surface area per unit mass. Finer particle sizes have higher specific surface area: more area per unit mass, so Blaine is an indirect measure of particle size.

But, Blaine delivers just a single averaged figure. In the example above, Sample 2 contains more fines than sample 1 but also more coarse material. These two differences cancel each other out so average specific surface area remains the same, illustrating why two quite different samples can have the same Blaine. The more important question is “do we need to know about the difference indicated by the particle size distribution?”

Size Distribution Measurements

For answers, we need to look at how the data are used. Cement manufacturers measure Blaine to quantify product quality. The hydration speed of cement particles is a function of particle size and determines the strength of the set cement. Generally, finer particles hydrate more quickly, giving greater strength, and within certain limits finer cement is better cement.

Returning to our samples, these materials will behave differently when used. Very fine cement particles, in the 2 – 3 micron range can cause exothermic cracking. Conversely particles over 50 microns may not hydrate, compromising product strength. Sample 1 contains less of both types of particles and consequently is a superior cement, even though Blaine ranks it the same as sample 2. So, the size distribution data are relevant!

Switching to laser diffraction

The practicalities of switching to laser diffraction are equally compelling. Blaine measurement is a largely manual technique and real-time measurement is not really feasible. Laser diffraction, however, is fast and automated. In the lab this means greater productivity and better reproducibility. For processing it means on-line instrumentation and real-time measurement.