Building Materials, Third Edition

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The operations in the wet process of cement manufacture
are mixing, burning and grinding. The crushed raw
materials are fed into ball mill (Fig. 5.6) and a little water
is added. On operating the ball mill, the steel balls in it
pulverize the raw materials which form a slurry with
water. This slurry is passed to silos (storage tanks), where
the proportioning of the compounds is adjusted to ensure
desired chemical composition. The corrected slurry
having about 40 per cent moisture content, is then fed
into rotary kiln (Fig. 5.4) where it loses moisture and
forms into lumps or nodules. These are finally burned at
1500-1600°C. The nodules change to clinker at this
temperature. Clinker is cooled and then ground in tube
mills. While grinding the clinker, about 3 per cent gypsum
is added. The cement is then stored in silos from where
it is supplied. A flow diagram of manufacturing cement
by wet process is shown in Fig. 5.7.

Comparison of Wet and Dry Process:The chief advantages of the wet process are the low cost
of excavating and grinding raw materials, the accurate control of composition and homogeneity
of the slurry, and the economical utilization of fuel through the elimination of separated drying
operations. On the other hand the longer kilns, essential in the wet process, cost more and are
less responsive to a variable clinker demand than the short kilns which can be used in the dry
process.

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Experience has shown that it is practically impossible to make large quantities of cement
without any variation in quality. To be sure, some mills working with raw materials which run
very uniformly and using the best of equipment and methods of operation will have very few
unsuccessful 'burns' in a year, whereas others will be less fortunate. Nevertheless the consumer
has little chance of ascertaining how his particular consignment of cement was made; therefore,
if he has under way a construction of any importance, he ought to satisfy himself regarding the
quality of his purchase. He should test his cement not only to see that he gets what he has paid
for but also to forestall the possibility of a failure through the use of defective material.
In engineering construction the main qualifications demanded of a cement are permanency of
structure, strength, and a rate of setting suitable to the demands of the work. To determine
these qualifications, both physical and chemical tests are made, the former, on account of
importance, more often than the latter.
As a result of long experience the physical tests which have come into general use in
determining the acceptability of cement are: (1) soundness or constancy of volume, (2) strength,
(3) time of set or activity, and (4) fineness. In order that the results of such tests made by
different parties may accord as nearly as possible, it is necessary that a standard method be
rigidly adhered to and that only experienced operators, who fully appreciate the necessity of
eliminating personal equation from all manipulations, be employed.

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