Civil Engineering Reference
In-Depth Information
crete is derived from the Latin word “concretus” meaning
grown together or compounded. The Romans perfected the
use of pozzolan as a cementing material. Sometime during
the second century BC the Romans quarried a volcanic ash
near Pozzuoli; thinking it was sand, they mixed it with lime
and found the mixture to be much stronger than they had
produced previously. This discovery was to have a signifi-
cant effect on construction. The material was not sand, but
a fine volcanic ash containing silica and alumina, which
when combined chemically with lime, produced what
became known as pozzolanic cement. This material was
used by builders of the famous Roman walls, aqueducts
and other historic structures including the Theatre at
Pompeii (seating 20,000 spectators), and the Colosseum
and Pantheon in Rome. Pozzolan seems to have been
ignored during the Middle Ages when building practices
were much less refined than earlier and the quality of
cementing materials deteriorated. The practice of burning
lime and the use of pozzolan was not introduced again
until the 1300s.
Efforts to determine why some limes possess hy-
draulic properties while others (those made from essen-
tially pure limestones) do not were not made until the 18th
century. John Smeaton, often referred to as the “father of
civil engineering in England,” concentrated his work in
this field. He found that an impure, soft limestone, contain-
ing clay minerals made the best hydraulic cement. This
combined with a pozzolan, imported from Italy, was used
in his project to rebuild the Eddystone Lighthouse in the
English Channel, southwest of Plymouth England. The
project took three years to complete and began operation in
1759; it was recognized as a significant accomplishment in
the development of the cement industry. A number of
discoveries followed as efforts within a growing natural
cement industry were now directed to the production of a
consistent quality material.
The difference between a hydraulic lime and natural
cement is a function of the temperature attained during
calcination. Furthermore, a hydraulic lime can hydrate in a
“lump” form, whereas natural cements must be crushed
and finely ground before hydration can take place. Natural
cement is stronger than hydraulic lime but weaker than
portland cement. Natural cement was manufactured in
Rosendale, New York in the early 1800s (
White 1820
) and
was first used to build the Erie Canal in 1818 (
Snell and
Snell 2000
).
The development of portland cement was the result of
persistent investigation by science and industry to produce
a superior quality natural cement. The invention of port-
land cement is generally credited to Joseph Aspdin, an
English mason. In 1824, he obtained a patent for his prod-
uct, which he named portland cement because when set, it
resembled the color of the natural limestone quarried on
the Isle of Portland in the English Channel (Fig. 2-2)
(
Aspdin 1824
). The name has endured and is used through-
out the world, with many manufacturers adding their own
trade or brand names.
Aspdin was the first to prescribe a formula for port-
land cement and the first to have his product patented.
However, in 1845, I. C. Johnson, of White and Sons, Swans-
combe, England, claimed to have “burned the cement raw
materials with unusually strong heat until the mass was
nearly vitrified,” producing a portland cement as we now
know it. This cement became the popular choice during the
middle of the 19th century and was exported from England
to various parts of the world. Production also began in
Belgium, France, and Germany about the same time and
export of these products from Europe to North America
began about 1865. The first recorded shipment of portland
cement to the United States was in 1868. The first portland
cement manufactured in the United States was produced at
a plant in Coplay, Pennsylvania, in 1871.
MANUFACTURE OF PORTLAND CEMENT
Portland cement is produced by pulverizing clinker which
consists primarily of hydraulic calcium silicates. Clinker
also contains some calcium aluminates and calcium alu-
minoferrites and one or more forms of calcium sulfate
(gypsum) is interground with the clinker to make the
finished product.
Materials used in the manufacture of portland cement
must contain appropriate amounts of calcium, silica,
alumina, and iron components. During manufacture,
chemical analyses of all materials are made frequently to
ensure a uniformly high quality cement.
Steps in the manufacture of cement are illustrated in
the flow charts in Figs. 2-3 and 2-4. While the operations of
all cement plants are basically the same, no flow diagram
can adequately illustrate all plants. There is no typical port-
land cement manufacturing facility; every plant has signif-
icant differences in layout, equipment, or general ap-
pearance (Fig. 2-5).
Selected raw materials (Table 2-1) are transported from
the quarry (Fig. 2-6), crushed (Fig. 2-7), milled, and propor-
tioned so that the resulting mixture has the desired chemi-
cal composition. The raw materials are generally a mixture
Fig. 2-5. Aerial view of a cement plant. (70000)
