Portland cement
Portland cement is the most common type of cement in general usage, as it is a
basic ingredient of concrete, mortar and plaster. It consists of a mixture of
oxides of calcium, silicon and aluminium. Portland cement and similar materials
are made by heating limestone (as source of calcium) with clay or sand (as
source of silicon) and grinding the product (clinker), with a source of sulfate
(most commonly gypsum). The resulting powder, when mixed with water, will become
a hydrated solid over time.
Portland cement was developed from cements (or correctly hydraulic limes) made
in Britain in the early part of the 19th century, and its name is derived from
its similarity to Portland stone, a type of building stone that was quarried on
the Isle of Portland in Dorset, England. Joseph Aspdin, a British bricklayer, in
1824 was granted a patent for a process of making a cement which he called
Portland cement. However, his cement was not Portland cement but an artificial
hydraulic lime similar to the material known as "Roman Cement" which was
Patented in 1796 by James Parker. Aspdin's process was similar to that patented
in 1822 and used since 1811 by James Frost who called his cement "British
Cement". The name "Portland Cement" is recorded in a directory published in 1823
being associated with a William Lockwood and possibly others. Aspdin's son
William in 1843 made an improved version of the "Roman Cement" and he initially
called it "Patent Portland Cement" but he had no patent and his cement still was
not Portland Cement. In 1848 William Aspdin further improved his cement and in
1853 moved to Germany where he was involved in cement making. (See "The Cement
Industry 1796-1914: A History" by A J Francis 1977) The first true Portland
cement was probably made by the factory of "Portlandzementfabrik Stern" in
Germany about 1867. This works is mentioned by Henry Reid in his 1868 book on
Cement Manufacture wherein his states that the cement is superior to any other
by a large margin. The German Government issued a Standard on Portland Cement in
1878 (possibly the first standard of any kind). The Stern cement would have
complied with that standard.
There are three fundamental stages in the production of Portland cement:
Preparation of the raw mixture
Production of the clinker
Preparation of the cement
The chemistry of cement is very complex, so cement chemist notation was invented
to simplify the formula of common molecules found in cement.
The raw materials for Portland cement production are a mixture (as fine dust in
the 'Dry process' or in the form of a slurry in the 'Wet process') of calcium
oxide, silicon oxide, aluminium oxide, ferric oxide, and magnesium oxide. The
raw materials are usually quarried from local rock, which in some places is
already practically the desired composition and in other places requires the
addition of clay and limestone, as well as iron ore, bauxite or recycled
materials.
The raw mixture is heated in a kiln, a gigantic slowly rotating and sloped
cylinder, with temperatures increasing over the length of the cylinder up to
~1480°C. The temperature is regulated so that the product contains sintered but
not fused lumps. Too low a temperature causes insufficient sintering, but too
high a temperature results in a molten mass or glass. In the lower-temperature
part of the kiln, calcium carbonate (limestone) turns into calcium oxide (lime)
and carbon dioxide. In the high-temperature part, calcium oxides and silicates
react to form dicalcium and tricalcium silicates (C2S C3S). Small amounts of
tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF)are also
formed. The resulting material is clinker, and can be stored for a number of
years before use. Prolonged exposure to water decreases the reactivity of cement
produced from weathered clinker.
The energy required to produce clinker is ~1700 J/g. However, because of heat
loss during production, actual values can be much higher. The high energy
requirements and the release of significant amounts of carbon dioxide makes
cement production a concern for global warming.
In order to achieve the desired setting qualities in the finished product, about
2% gypsum is added to the clinker and the mixture is pulverized very finely. The
powder is now ready for use, and will react with the addition of water.
The most common use for portland cement is the production of concrete. Concrete
is a composite material consisting of aggregate (gravel), cement, and water. As
a construction material, concrete can be cast in almost any shape desired, and
once hardened, can become a structural (load bearing) element.
When water is mixed with Portland cement, the product sets in a few hours and
hardens over a period of weeks. The initial setting is caused by a reaction
between the water, gypsum, and tricalcium aluminate (C3A), forming the
crystalline hydration products calcium-alumino-hydrate (CAH), ettringite (Aft),
and monosulfate (Afm). The later hardening and the development of cohesive
strength is due to the reaction of water and tricalcium silicate (C3S), forming
an amorphous hydrated product called calcium-silicate-hydrate(CSH gel). In each
case the hydration products surround and cement together the individual grains.
The hydration of dicalcium silicate (C2S) proceeds more slowly than that of the
above compounds slowly increasing later-age strength. The ultimate cementing
agent is probably gelatinous silica (SiO2). All three reactions mentioned above
release heat.
Plastic cement is a type of Portland cement with the addition of a plasticizing
material (limestone or hydrated lime), as well as other materials to reduce
setting time and facilitate workability (see superplasticizer). Plastic cement
is used primarily for spreading onto walls to make exterior stucco, as Portland
cement (used primarily for concrete) would have poor spreadability. In this
usage, the term "plastic" does not refer to the addition of an organic polymer.
Rather, it refers to the addition of a substance to increase the workability of
the mixture.
In 2002 the world production of hydraulic cement was 1,800 million metric tons.
The top three producers were China with 704, India with 100, and the United
States with 91 million metric tons for a combined total of about half the world
total by the world's three most populous states.
"For the past 18 years, China consistently has produced more cement than any
other country in the world.China's cement export peaked in 1994 with 11 million
tons shipped out and has been in steady decline ever since. Only 5.18 million
tons were exported out of China in 2002. Offered at $34 a ton, Chinese cement is
pricing itself out of the market as Thailand is asking as little as $20 for the
same quality." Jan 7, 2004
"Demand for cement in China is expected to advance 5.4% annually and exceed 1
billion metric tons in 2008, driven by slowing but healthy growth in
construction expenditures. Cement consumed in China will amount to 44% of global
demand, and China will remain the world's largest national consumer of cement by
a large margin.
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