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Cement

 

From Wikipedia, the free encyclopedia

In the most general sense of the word, cement is a

binder, a substance which sets and hardens

independently, and can bind other materials together.

The name "cement" goes back to the Romans who used

the term "opus caementitium" to describe masonry

which resembled concrete and was made from crushed

rock with burnt lime as binder. The volcanic ash and

pulverized brick additives which were added to the burnt

lime to obtain a hydraulic binder were later referred to

as cementum, cimentum, cäment and cement. Cements

used in construction are characterized as hydraulic or

non-hydraulic.

 

The most important use of cement is the production of

mortar and concrete - the bonding of natural or artificial

 

aggregates to form a strong building material which is

durable in the face of normal environmental effects.

 

History

Early uses

The earliest construction cements are as old as

construction[1], and were non-hydraulic. Wherever

primitive mud bricks were used, they were bedded

together with a thin layer of clay slurry. Mud-based

materials were also used for rendering on the walls of

timber or wattle and daub structures. Lime was probably

used for the first time as an additive in these renders,

and for stabilizing mud floors. A "daub" consisting of

mud, cow dung and lime produces a tough and water-

 

proof coating, due to coagulation, by the lime, of

proteins in the cow dung. This simple system was

common in Europe until quite recent times. With the

advent of fired bricks, and their use in larger structures,

various cultures started to experiment with higher-

strength mortars based on bitumen (in Mesopotamia),

gypsum (in Egypt) and lime (in many parts of the

world).

It is uncertain where it was first discovered that a

combination of hydrated non-hydraulic lime and a

pozzolan produces a hydraulic mixture, but concrete

made from such mixtures was first used on a large scale

by the Romans. They used both natural pozzolans (trass

or pumice) and artificial pozzolans (ground brick or

pottery) in these concretes. Many excellent examples of

structures made from these concretes are still standing

 

, notably the huge monolithic dome of the Pantheon in

Rome. The use of structural concrete disappeared in

medieval Europe, although weak pozzolanic concretes

continued to be used as a core fill in stone walls and

columns.

 

Hydraulic cements

Hydraulic cements are materials which set and harden

after combining with water, as a result of chemical

creactions with the mixing water and, after hardening,

retain strength and stability even under water. The key

requirement for this is that the hydrates formed on

immediate reaction with water are essentially insoluble

in water. Most construction cements today are

hydraulic, and most of these are based upon portland  cement, which is made primarily from limestone, certain

 

clay minerals, and gypsum, in a high temperature

process that drives off carbon dioxide and chemically

combines the primary ingredients into new compounds.

Non-hydraulic cements include such materials as (non-

hydraulic) lime and gypsum plasters, which must be kept

dry in order to gain strength, and oxychloride cements

which have liquid components. Lime mortars, for

example, "set" only by drying out, and gain strength only

very slowly by absorption of carbon dioxide from the

atmosphere to re-form calcium carbonate.

Setting and hardening of hydraulic cements is caused by

the formation of water-containing compounds, forming

as a result of reactions between cement components and

water. The reaction and the reaction products are

referred to as hydration and hydrates or hydrate phases

, respectively. As a result of the immediately starting

reactions, a stiffening can be observed which is very

 

small in the beginning, but which increases with time. After reaching a certain level, this point in time is

referred to as the start of setting. The consecutive further

consolidation is called setting, after which the phase of

hardening begins. The compressive strength of the

material then grows steadily, over a period which ranges

from a few days in the case of "ultra-rapid-hardening"

cements, to several years in the case of ordinary

cements.

 

Modern cement

Modern hydraulic cements began to be developed from

the start of the Industrial Revolution (around 1700),

driven by three main needs:

·   Hydraulic renders for finishing brick buildings in wet

·  

·   climates

 

 

·   Hydraulic mortars for masonry construction of harbor

·  

·   works etc, in contact with sea water.

·  

·   Development of strong concretes.

·  

In Britain particularly, good quality building stone

became ever more expensive during a period of rapid

growth, and it became a common practice to construct

prestige buildings from the new industrial bricks, and to

finish them with a stucco to imitate stone. Hydraulic

limes were favored for this, but the need for a fast set

time encouraged the development of new cements. Most

famous among these was Parker's "Roman cement" This

was developed by James Parker in the 1780s, and

finally patented in 1796. It was, in fact, nothing like any

material used by the Romans, but was a "Natural

 

cement" made by burning septaria - nodules that are

found in certain clay deposits, and that contain both clay

minerals and calcium carbonate. The burnt nodules

were ground to a fine powder. This product, made into a

mortar with sand, set in 5-15 minutes. The success of

"Roman Cement" led other manufacturers to develop

rival products by burning artificial mixtures of clay and

chalk.

John Smeaton made an important contribution to the

development of cements when he was planning the

construction of the third Eddystone Lighthouse (1755-9)

in the English Channel. He needed a hydraulic mortar

that would set and develop some strength in the twelve

hour period between successive high tides. He

performed an exhaustive market research on the

available hydraulic limes, visiting their production sites,

 

and noted that the "hydraulicity" of the lime was directly

related to the clay content of the limestone from which

it was made. Smeaton was a civil engineer by

profession, and took the idea no further. Apparently

unaware of Smeaton's work, the same principle was

identified by Louis Vicat in the first decade of the

nineteenth century. Vicat went on to devise a method of

combining chalk and clay into an intimate mixture, and,

burning this, produced an "artificial cement" in 1817.

James Frost[3], working in Britain, produced what he

called "British cement" in a similar manner around the

same time, but did not obtain a patent until 1822. In

1824, Joseph Aspdin patented a similar material, which

he called Portland cement, because the render made

from it was in color similar to the prestigious Portland stone.

 

 

All the above products could not compete with

lime/pozzolan concretes because of fast-setting (giving

insufficient time for placement) and low early strengths

(requiring a delay of many weeks before formwork

could be removed). Hydraulic limes, "natural" cements

and "artificial" cements all rely upon their belite content

for strength development. Belite develops strength

slowly. Because they were burned at temperatures below

1250 °C, they contained no alite, which is responsible

for early strength in modern cements. The first cement

to consistently contain alite was that made by Joseph

Aspdin's son William in the early 1840s. This was what

we call today "modern" Portland cement. Because of the

air of mystery with which William Aspdin surrounded

his product, others (e.g. Vicat and I C Johnson) have

claimed precedence in this invention, but recent

 

analysis[4] of both his concrete and raw cement have

shown that William Aspdin's product made at

Northfleet, Kent was a true alite-based cement.

However, Aspdin's methods were "rule-of-thumb": Vicat

is responsible for establishing the chemical basis of

these cements, and Johnson established the importance

of sintering the mix in the kiln.

William Aspdin's innovation was counter-intuitive for

manufacturers of "artificial cements", because they

required more lime in the mix (a problem for his father),

because they required a much higher kiln temperature

(and therefore more fuel) and because the resulting

clinker was very hard and rapidly wore down the

millstones which were the only available grinding

technology of the time. Manufacturing costs were

therefore considerably higher, but the product set

 

reasonably slowly and developed strength quickly, thus

opening up a market for use in concrete. The use of

concrete in construction grew rapidly from 1850

onwards, and was soon the dominant use for cements.

Thus Portland cement began its predominant role.

 

Types of modern cement

 

Blue Circle Southern Cement works near Berrima, New South Wales, Australia.

Portland cement

Portland cement is the most common type of cement in

general usage, as it is a basic ingredient of concrete,

mortar and most non-speciality grout. The most

common use for Portland cement is in the production of

concrete. Concrete is a composite material consisting of

 

 

aggregate (gravel and sand), 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. Portland cement may

be gray or white.

For details of the manufacture of Portland cement, see

the main article.

Portland cement blends

These are often available as inter-ground mixtures from

cement manufacturers, but similar formulations are

often also mixed from the ground components at the

concrete mixing plant.

Portland Blastfurnace Cement contains up to 70%

ground granulated blast furnace slag, with the rest

Portland clinker and a little gypsum. All compositions

produce high ultimate strength, but as slag content is

 

 

increased, early strength is reduced, while sulfate

resistance increases and heat evolution diminishes. Used

as an economic alternative to Portland sulfate-resisting

and low-heat cements.

Portland Flyash Cement contains up to 30% fly ash.

The flyash is pozzolanic, so that ultimate strength is

maintained. Because flyash addition allows a lower

concrete water content, early strength can also be

maintained. Where good quality cheap flyash is

available, this can be an economic alternative to ordinary

Portland cement.

Portland Pozzolan Cement includes fly ash cement,

since fly ash is a pozzolan, but also includes cements

made from other natural or artificial pozzolans. In

countries where volcanic ashes are available (e.g. Italy,

Chile, Mexico, the Philippines) these cements are often

 

the most common form in use.

Portland Silica Fume cement. Addition of silica fume

can yield exceptionally high strengths, and cements

containing 5-20% silica fume are occasionally produced.

However, silica fume is more usually added to Portland

cement at the concrete mixer

Masonry Cements are used for preparing bricklaying

mortars and stuccos, and must not be used in concrete.

They are usually complex proprietary formulations

containing Portland clinker and a number of other

ingredients that may include limestone, hydrated lime,

air entrainers, retarders, waterproofers and coloring

agents. They are formulated to yield workable mortars

that allow rapid and consistent masonry work. Subtle

variations of Masonry cement in the US are Plastic

Cements and Stucco Cements. These are designed to

 

produce controlled bond with masonry blocks.

Expansive Cements contain, in addition to Portland

clinker, expansive clinkers (usually sulfoaluminate

clinkers), and are designed to offset the effects of drying

shrinkage that is normally encountered with hydraulic

cements. This allows large floor slabs (up to 60 m

square) to be prepared without contraction joints.

White blended cements may be made using white

clinker and white supplementary materials such as high-

purity metakaolin.

Colored cements are used for decorative purposes. In

some standards, the addition of pigments to produce

"colored Portland cement" is allowed. In other standards

(e.g. ASTM), pigments are not allowed constituents of

Portland cement, and colored cements are sold as

"blended hydraulic cements".

 

Non-Portland hydraulic cements

Pozzolan-lime cements. Mixtures of ground pozzolan

and lime are the cements used by the Romans, and are to

be found in Roman structures still standing (e.g. the

Pantheon in Rome). They develop strength slowly, but

their ultimate strength can be very high. The hydration

products that produce strength are essentially the same

as those produced by Portland cement.

Slag-lime cements. Ground granulated blast furnace slag is not hydraulic on its own, but is “activated” by

addition of alkalis, most economically using lime. They

are similar to pozzolan lime cements in their properties

. Only granulated slag (i.e. water-quenched, glassy slag)

is effective as a cement component.

Supersulfated cements. These contain about 80%

ground granulated blast furnace slag, 15% gypsum or

 

 

anhydrite and a little Portland clinker or lime as an

activator. They produce strength by formation of

ettringite, with strength growth similar to a slow

Portland cement. They exhibit good resistance to

aggressive agents, including sulfate.

Calcium aluminate cements are hydraulic cements

made primarily from limestone and bauxite. The active

ingredients are monocalcium aluminate CaAl2O4 (CA in

Cement chemist notation) and Mayenite Ca12Al14O33

(C12A7 in CCN). Strength forms by hydration to calcium

aluminate hydrates. They are well-adapted for use in

refractory (high-temperature resistant) concretes, e.g. for

furnace linings.

Calcium sulfoaluminate cements are made from

clinkers that include ye’elimite (Ca4(AlO2)6SO4 or

C4A3 in Cement chemist’s notation) as a primary phase

.

They are used in expansive cements, in ultra-high early

strength cements, and in "low-energy" cements.

Hydration produces ettringite, and specialized physical

properties (such as expansion or rapid reaction) are

obtained by adjustment of the availability of calcium

and sulfate ions. Their use as a low-energy alternative to

Portland cement has been pioneered in China, where

several million tonnes per year are produced.

Energy requirements are lower because of the lower kiln

temperatures required for reaction, and the lower

amount of limestone (which must be endothermically

decarbonated) in the mix. In addition, the lower

limestone content and lower fuel consumption leads to a

CO2 emission around half that associated with Portland

clinker. However, SO2 emissions are usually

significantly higher.

 

“Natural” Cements correspond to certain cements of

the pre-Portland era, produced by burning argillaceous

limestones at moderate temperatures. The level of clay

components in the limestone (around 30-35%) is such

that large amounts of belite (the low-early strength,

high-late strength mineral in Portland cement) are

formed without the formation of excessive amounts free

lime. As with any natural material, such cements have

very variable properties.

Geopolymer cements are made from mixtures of water-

soluble alkali metal silicates and aluminosilicate mineral

powders such as fly ash and metakaolin.

Environment

Cement manufacture causes environmental impacts at all

stages of the process. These include emissions of

airborne pollution in the form of dust, gases, noise and

 

vibration when operating machinery and during blasting

in quarries, and damage to countryside from quarrying

. Equipment to reduce dust emissions during quarrying

and manufacture of cement is widely used, and

equipment to trap and separate exhaust gases are

coming into increased use. Environmental protection

also includes the re-integration of quarries into the

countryside after they have been closed down by

returning them to nature or re-cultivating them.

Due to the large quantities of fuel used during

manufacture and the release of carbon dioxide from the

raw materials, cement production also generates more

carbon emissions than any other industrial process,

accounting for around 4% of the world's

anthropomorphic carbon emissions.

Cement manufacture can provide environmental benefits

 

by using wastes from certain other industries, including

slag from steel manufacture, fly ash from coal burning,

silica fume from silicon and ferrosilicon manufacturing,

and sometimes recycled concrete from demolition of

older structures.

Fuels

Fuels used in cement manufacture depend on the type of

cement. For information on fuels used in the

manufacture of cement clinker, see Portland cement,

cement kiln.

Cement business

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."

"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."

In 2006 is was estimated that China manufactured 1.235

billion metric tons of cement, which is 44% of the world

total cement production

 

 * نويسنده: محمد منصوری  * تاريخ: 2009/9/1  * موضوع: مقالات عمران   * لينک ثابت   *