Civil Engineering Reference
In-Depth Information
CHAPTER 2
Portland, Blended, and
Other Hydraulic Cements
becomes harder and stronger. Most of the hydration and
strength development take place within the first month,
but then continues, though more slowly, for a long time
with adequate moisture and temperature. Continuous
strength increases exceeding 30 years have been recorded
(
Washa and Wendt 1975
and
Wood 1992
).
THE BEGINNING OF AN INDUSTRY
Early builders used clay to bind stones together into a solid
structure for shelter and protection. The oldest concrete dis-
covered so far dates from around 7000 BC and was found in
1985 when a concrete floor was uncovered during the con-
struction of a road at Yiftah El in Galilee, Israel. It consisted
of a lime concrete, made from burning limestone to produce
quicklime, which when mixed with water and stone, hard-
ened to form concrete (
Brown 1996
and
Auburn 2000
).
A cementing material was used between the stone
blocks in the construction of the Great Pyramid at Giza
in Ancient Egypt around 2500 BC. Some reports say it was
a lime mortar while others say the cementing material was
made from burnt gypsum. By 500 BC, the art of making lime-
based mortar arrived in Ancient Greece. The Greeks used
lime-based mate-
rials as a binder
between stone and
brick and as a
rendering material
over porous lime-
stones commonly
used in the con-
struction of their
temples and pal-
aces.
Examples of
early Roman con-
crete have been
found dating back
to 300 BC. The
very word con-
Fig. 2-1. Portland cement is a fine powder that when mixed
with water becomes the glue that holds aggregates together
in concrete. (58420)
Portland cements are hydraulic cements composed prima-
rily of hydraulic calcium silicates (Fig. 2-1). Hydraulic
cements set and harden by reacting chemically with water.
During this reaction, called hydration, cement combines
with water to form a stonelike mass, called paste. When the
paste (cement and water) is added to aggregates (sand and
gravel, crushed stone, or other granular material) it acts as
an adhesive and binds the aggregates together to form
concrete, the world's most versatile and most widely used
construction material.
Hydration begins as soon as cement comes in contact
with water. Each cement particle forms a fibrous growth on
its surface that gradually spreads until it links up with the
growth from other cement particles or adheres to adjacent
substances. This fibrous build up results in progressive
stiffening, hardening, and strength development. The stiff-
ening of concrete can be recognized by a loss of workabil-
ity that usually occurs within three hours of mixing, but is
dependent upon the composition and fineness of the
cement, any admixtures used, mixture proportions, and
temperature conditions. Subsequently, the concrete sets
and becomes hard.
Hydration continues as long as favorable moisture and
temperature conditions exist (curing) and space for hydra-
tion products is available. As hydration continues, concrete
Fig. 2-2. Isle of Portland quarry stone
(after which portland cement was
named) next to a cylinder of modern
concrete. (68976)
