Why switch from Blaine to PSD?

Particle size distribution - a more sensitive measure of cement performance

The fineness of cement is controlled because of its influence on the rate of hydration of the product in the field. This in turn impacts early and long term strength, both key performance indicators. The specific surface area of the sample, which is a function of particle size, provides an indication of how quickly and easily water will be able to penetrate, and react with, the cement particles present. This explains the industry’s traditional reliance on Blaine data.

The Blaine surface area measurement has been used since the 1940s to determine cement quality and many plants still retain this air permeability technique as a key QC tool. However, a key drawback of Blaine analysis is that it produces just a single averaged figure for each sample: two cements with a different proportion of fines can give the same Blaine number.

Two different samples with the same surface area have different size distributions

These two samples have the same averaged specific surface area and would therefore give identical Blaine numbers, even though they have very different particle size distributions. In the sample on the left, hydration will proceed at the same rate, and to the same degree, for all particles. For the sample on the right, the fine particles will hydrate quickly and completely, while the largest ones will hydrate much more slowly, and, perhaps, incompletely. The developed strength characteristics of these two cements will therefore be different. Here then the averaged figure fails to adequately describe the sample in terms of behavior in the field and, most importantly, strength.

Particle size analysis - a better tool for production

So, simply in terms of the results generated, rather than any practical considerations relating to analytical technique, particle size measurement is potentially advantageous. For the manufacturing environment the well-recognized practical limitations of Blaine are, however, extremely relevant. Blaine measurement:

• Is slow
• Uses only a small sample size
• Is ill-suited to in-process use
• Suffers from poor reproducibility

Laser diffraction particle sizing technology offers better reproducibility and, perhaps most importantly, options for real-time continuous monitoring. This technology therefore opens up a route to full automation of both analysis and control.

At this cement plant, cement fineness was measured off-line using Blaine and was correlated to the compressive strength of the cement at 1, 3, 7 and 28 days of the setting.

Taking a year's worth of QC data, the variability in certain cement product parameters was compared for off-line and on-line techniques. These parameters included:

• % fines (%<30µm)
• Blaine fineness
• compressive strengths at 1, 3, 7 and 28 days after setting

In the graph below we can see that the variability in the cement fineness and strength has decreased significantly since the installation of Malvern Insitec and the commissioning of automatic separator control.

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