Control of Product Quality
In addition to control of temperature (mentioned above), the main requirement is to obtain a consistent fineness of the product. From the earliest times, fineness was measured by sieving the cement. As cements have become finer, the use of sieves is less applicable, but the amount retained on a 45 μm sieve is still measured, usually by air-jet sieving or wet-sieving. The amount passing this sieve (typically 95% in modern general-purpose cements) is related to the overall strength-development potential of the cement, because the larger particles are essentially unreactive.
The main measure of fineness today is specific surface. Because cement particles react with water at their surface, the specific surface area is directly related to the cement's initial reactivity. By adjusting the fineness of grind, the manufacture can produce a range of products from a single clinker. Tight control of fineness is necessary in order to obtain cement with the desired consistent day-to-day performance, so round-the-clock measurements are made on the cement as it is produced, and mill feed-rates and separator settings are adjusted to maintain constant specific surface.
A more comprehensive picture of fineness is given by particle size analysis, yielding a measure of the amount of each size range present, from sub-micrometer upwards. This used to be mainly a research tool, but with the advent of cheap, industrialized laser-diffraction analyzers, its use for routine control is becoming more frequent. This may take the form of a desk-top analyzer fed with automatically gathered samples in a robotized laboratory, or, increasingly commonly, instruments attached directly to the output ducts of the mill. In either case, the results can be fed directly into the mill control system, allowing complete automation of fineness control.
In addition to fineness, added materials in the cement must be controlled. In the case of gypsum addition, the material used is frequently of variable quality, and it is normal practise to measure the sulfate content of the cement regularly, typically by x-ray fluorescence, using the results to adjust the gypsum feed rate. Again, this process is often completely automated. Similar measurement and control protocols are applied to other materials added, such as limestone, slag and fly-ash.
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