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Structural Concrete Frames 345
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White Portland cement
Low-heat Portland cement
Portland blastfurnace cement
Water-repellent cement
High-alumina (aluminous) cement
Ordinary Portland cement
Ordinary Portland cement is the cheapest and most commonly used cement, accounting
for about 90% of all cement production. It is made by heating limestone and clay to a
temperature of about 1300°C to form a clinker, rich in calcium silicates. The clinker is
ground to a fine powder with a small proportion of gypsum, which regulates the rate of
setting when the cement is mixed with water. This type of cement is affected by sulphates
such as those present in groundwater in some clay soils. The sulphates have a disintegrating
effect on ordinary Portland cement. For this reason sulphate-resisting cements are produced
for use in concrete in sulphate bearing soils, marine works, sewage installations and
manufacturing processes where soluble salts are present.
Rapid hardening Portland cement
Rapid hardening Portland cement is similar to ordinary Portland except that the cement
powder is more finely ground. The effect of the finer grinding is that the constituents of
the cement powder react more quickly with water, and the cement develops strength more
rapidly. Rapid hardening cement develops in three days, a strength which is similar to that
developed by ordinary Portland in seven days. With the advantage of the cement’s early
strength development, it is possible to speed up construction. With rapid hardening cement,
the initial set is much shorter and formwork systems can be removed earlier. Although
rapid hardening is more expensive than ordinary Portland cement, it is often used because
of its early strength advantage. Rapid hardening Portland cement is not a quick setting
cement. Several months after mixing there is little difference in the characteristics of ordinary
and rapid hardening cements.
Sulphate-resisting Portland cement
The effect of sulphates on ordinary cement is to combine with the constituents of the
cement. As the sulphates react there is an increase in volume on crystallisation, which
causes the concrete to disintegrate. Disintegration is severe where the concrete is alternately
wet and dry, as in marine works. To counteract this, the aluminates within the cement,
which are affected by sulphates, are reduced to provide increased resistance to the effect of
sulphates. Because it is necessary to carefully control the composition of the raw materials
of this cement, it is more expensive than ordinary cement. High-alumina cement described
later is also a sulphate-resisting cement.
White Portland cement
White Portland cement is manufactured from china clay and pure chalk or limestone and
is used to produce white concrete finishes. Both the raw materials and the manufacturing
process are comparatively expensive; therefore, the cement is mainly used for the surface
of exposed concrete and for cement renderings. Pigments may be added to the cement to
produce pastel colours.