Portland cement clinker grinding

Grinding Aid. An additive to the charge in a ball mill or rod mill to accelerate the grinding process the additive has surface-active or lubricating properties. Grinding aids find particular use in the grinding of portland cement clinker, but in the UK their use is precluded by the conditions laid down in B.S. 12. 

Portland cement clinker and the assessment of its quality are discussed. Other sections deal with cement grinding, storage and hydration. The types and strength classes of cement, as well as cement testing procedures and associated matters, are also considered. Finally, some information on standard specifications for cement in various countries is given. 

Fig.11 Portland cement clinker under-burned (porous) micrograph obtained with reflected light free lime black pocket belite light-coloured textured areas alite dark textured areas pores (here filled with resin) grey areas with grinding scratches...

A quantity of sulphate (in the form of gypsum or a mixture of gypsum and anhydrite-II) is always added to the Portland cement clinker in the finish grinding... 

Portland cement also consumes large quantities of gypsum. About 5% of gypsum is added to the cement clinker before grinding. Addition of gypsum aids in increasing the early strength of the cement and prevents undesirable false set. 

Portland cement is an aluminosilicate powder which sets to a solid mass on treatment with water. It is usually manufactured by grinding limestone and clay to a fine powder, mixing with water to form a slurry, and burning the mixture, with a flame of gas, oil, or coal dust, in a long rotary kiln. At the hot end of the kiln, where the temperature is about 1500 C, the aluminosilicate mixture is sintered together into small round marbles, called clinker. The clinker is ground to a fine powder in a ball mill (a rotating cylindrical mill filled with steel balls), to produce the final product. Over 100,000,000 barrels of cement per year is made in the United States. 

As an economical and rapid method to control the quality of portland cement, the value of routine clinker microscopy should be an inescapable conclusion from the numerous observations and interpretations given on previous pages. Quality control of clinker without microscopy of raw feed, in the writer s opinion, is less than adequate. Profound cause-effect relationships exist between the raw feed/particle size distribution, energy required for grinding and burning, clinker quality, and cement performance. Visually appreciating the characteristics of raw feed via microscopical examination gives additional comprehension to quality control. 

As the last step in Portland cement manufacture, Portland clinker, in combination with limited amounts of gypsum and/or anhydrite, has to be ground to a fine powder. A ball mill is the standard equipment used for this purpose. It was reported recently (Goldstein, 1997) that the power consumption in the grinding process may be reduced if—in place of a single clinker—a blend of two clinkers is used, of which one has a higher and the other a lower lime saturation factor. 

Composite cements may be produced by grinding the mineral addition (or additions) together with Portland clinker and calcium sulfate, or by blending them with Portland cement produced separately. 

Subsequently, the high-almnina clinker is crashed and ground to a fine powder in a ball mill. The clinker is very hard to grind, and the process is associated with a high power consumption. Unlike Portland cement, no set controlling constituents, such as gypsum, need to be intergroimd with the burnt material. 

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