In addition to control of temperature of cement mill (mentioned above), the main requirement to cement mill is to obtain a consistent fineness of the product. From the earliest times, fineness was measured by sieving the cement out of cement mill. As cements have become finer, the use of sieves is less applicable, but the amount out of cement mill retained on a 45 μm sieve is still measured, usually by air-jet sieving or wet-sieving.

    The amount out of cement mill 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 out of cement mill are essentially unreactive. The main measure of fineness of cement out of cement mill today is specific surface. Because cement particles react with water at their surface in cement mill, the specific surface area is directly related to the cement's initial reactivity inside cement mill.

    By adjusting the fineness of grind inside cement mill, the manufacture can produce a range of products from a single clinker. Tight control of fineness of cement mill is necessary in order to obtain cement with the desired consistent day-to-day performance, so round-the-clock measurements out of cement mill are made on the cement as it is produced, and cement mill feed-rates and separator settings are adjusted to maintain constant specific surface of cement out of cement mill.