Page 52 - Handbook of Thermal Analysis of Construction Materials

36 Chapter 2 - Introduction to Portland Cement Concrete

Although added in small amounts, admixtures may influence many
of the properties of concrete, from the time water comes into contact with
the dry ingredients of concrete, to its long term behavior. Generally,
concrete contains one or more admixtures and their role has been studied
extensively by thermal techniques and, hence, a separate chapter is devoted
to describe their application.

1.0 PRODUCTION OF PORTLAND CEMENT

According to ASTM C-150 portland cement is a hydraulic cement
produced by pulverizing clinker consisting essentially of hydraulic calcium
silicates, usually containing one or more types of calcium sulfate as an
interground addition.
The raw materials for the manufacture of portland cement contain,
in suitable proportions, silica, aluminum oxide, calcium oxide, and ferric
oxide. Since the compounds in cement science are complex, a simplified
representation is often used in cement nomenclature: C = CaO; S = SiO ;
2
–
A = Al O ; F = Fe O ; H = H O; S = SO ; K = K O; and N = Na O. In
2
2
3
2
3
2
3
2
addition, “W” or “w” represents water, “C” or “c,” cement, and “S” or “s,”
solid. Thus, “W/C” or “w/c” is water:cement ratio and “W/S” or “w/s” is
water:solids ratio. A source of lime is provided by calcareous ingredients
such as limestone or chalk and the source of silica and aluminum oxide
being shales, clays, or slates. The iron bearing materials are iron and pyrites.
Ferric oxide not only serves as a flux but also forms compounds with lime
and alumina. The raw materials also contain small amounts of other
compounds such as magnesia, alkalis, phosphates, fluorine compounds,
zinc oxide, and sulfides. The cement clinker is produced by feeding the
crushed, ground, and screened raw mix into a rotary kiln and heated at a
temperature of about 1300–1450°C. Approximately 1100–1400 kcal/g of
energy is consumed in the formation of clinker. The sequence of reactions
is as follows.
• At a temperature of about 100°C (drying zone), free water
is expelled.
• In the pre-heating zone (100–750°C), firmly bound water
from the clay is lost.

• In the calcining zone (750–1000°C), calcium carbonate is
dissociated.