Page 216 - Chemical and process design handbook

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CEMENT

The terms cement and concrete are not synonymous. Concrete is artificial
stone made from a carefully controlled mixture of cement, water, and fine
and coarse aggregate (usually sand and coarse rock).
Portland cement is the product obtained by pulverizing clinker consist-
ing essentially of calcium silicate, usually containing one or more forms of
calcium sulfate, and there are five types of portland cement:

Type I. Regular portland cements are the usual products for general con-
struction.
Type II. Moderate-heat-of-hardening and sulfate-resisting portland
cements are for use where moderate heat of hydration is required or for
general concrete construction exposed to moderate sulfate action.
Type III. High-early-strength (HES) cements are made from raw mate-
rials with a lime-to-silica ratio higher than that of Type I cement and are
ground finer than Type I cements. They contain a higher proportion of
calcium silicate than regular portland cements.
Type IV. Low-heat portland cements contain a lower percentage of cal-
cium silicate and calcium aluminate, thus lowering the heat evolution.
Type V. Sulfate-resisting portland cements are those that, by their com-
position or processing, resist sulfates better than the other four types.
Type V is used when high sulfate resistance is required.

Two types of materials are necessary for the production of portland
cement: one rich in calcium (calcareous), such as limestone or chalk, and
one rich in silica (argillaceous) such as clay. These raw materials are finely
ground, mixed, and heated (burned) in a rotary kiln to form cement clinker.
Cement clinker is manufactured by both the dry process and by the wet
process (Fig. 1). In both processes closed-circuit grinding is preferred to
open-circuit grinding in preparing the raw materials because, in the for-
mer, the fines are passed on and the coarse material returned, whereas in
the latter, the raw material is ground continuously until its mean fineness
has reached the desired value. The wet process, though the original one, is

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