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Portland cement

 

Sampling fast set concrete made from Portland cement

Portland cement is the most common type of cement in general usage in many parts of the world, as it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a maximum of about 5% gypsum which controls the set time, and up to 5% minor constituents (as allowed by various standards). As defined by the European Standard EN197.1, "Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO₂ and 2CaO.SiO₂), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO₂ shall not be less than 2.0. The magnesium content (MgO) shall not exceed 5.0% by mass." (The last two requirements were already set out in the German Standard, issued in 1909). Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 °C for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as Low Heat (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3CaO.Al₂O₃) formed. The major raw material for the clinker-making is usually limestone (CaCO₃). Normally, an impure limestone which contains SiO₂ is used - the CaCO₃ content can be as low as 80%. Secondary raw materials (materials in the rawmix other than limestone) depend on the purity of the limestone. Some of the secondary raw materials used are: clay, shale, sand, iron ore, bauxite, fly ash and slag. When a cement kiln is fired by coal, the ash of the coal acts as a secondary raw material.

 

History

Portland cement was developed from cements (or correctly hydraulic limes) made in Britain in the early part of the nineteenth 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. His cement was an artificial hydraulic lime similar in properties to the material known as "Roman Cement" (patented in 1796 by James Parker) and his 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 also 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 this cement and he initially called it "Patent Portland cement" although he had no patent. In 1848 William Aspdin further improved his cement and in 1853 moved to Germany where he was involved in cement making.^[1] Many people have claimed to have made the first Portland cement in the modern sense, but it is generally accepted that it was first manufactured by William Aspdin at Northfleet, England in about 1842^[2]. The German Government issued a standard on Portland cement in 1878.

Types of Portland cement

Genera

lThere are different standards for classification of Portland cement. The two major standards are the ASTM C150 used primarily in the U.S. and European EN-197. EN 197 cement types CEM I, II, III, IV, and V do not correspond to the similarly-named cement types in ASTM C 150.

 

Production

Schematic explanation of Portland cement production

There are three fundamental stages in the production of Portland cement:

1.                Preparation of the raw mixture

2.                Production of the clinker

3.                Preparation of the cement

The chemistry of cement is very complex, so cement chemist notation was invented to simplify the formula of common oxides found in cement. This reflects the fact that most of the elements are present in their highest oxidation state, and chemical analyses of cement are expressed as mass percent of these notional oxides.

 

Rawmix preparation

 

A limestone prehomogenization pile being built by a boom stacker

 

A completed limestone prehomogenization pile

The raw materials for Portland cement production are a mixture (as fine powder in the 'Dry process' or in the form of a slurry in the 'Wet process') of minerals containing 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 individual raw materials are first crushed, typically to below 50 mm. In many plants, some or all of the raw materials are then roughly blended in a "prehomogenization pile". The raw materials are next ground together in a rawmill. Silos of individual raw materials are arranged over the feed conveyor belt. Accurately controlled proportions of each material are delivered onto the belt by weigh-feeders. Passing into the rawmill, the mixture is ground to rawmix. The fineness of rawmix is specified in terms of the size of the largest particles, and is usually controlled so that there are less than 5-15% by mass of particles exceeding 90 μm in diameter. It is important that the rawmix contains no large particles in order to complete the chemical reactions in the kiln, and to ensure the mix is chemically homogenous. In the case of a dry process, the rawmill also dries the raw materials, usually by passing hot exhaust gases from the kiln through the mill, so that the rawmix emerges as a fine powder. This is conveyed to the blending system by conveyor belt or by a powder pump. In the case of wet process, water is added to the rawmill feed, and the mill product is a slurry with moisture content usually in the range 25-45% by mass. This slurry is conveyed to the blending system by conventional liquid pumps.

Formation of clinker

The raw mixture is heated in a cement kiln, a slowly rotating and sloped cylinder, with temperatures increasing over the length of the cylinder up to a peak temperature of 1400-1450 °C. A complex succession of chemical reactions take place (see cement kiln) as the temperature rises. The peak temperature is regulated so that the product contains sintered but not fused lumps. Sintering consists of the melting of 25-30% of the mass of the material. The resulting liquid draws the remaining solid particles together by surface tension, and acts as a solvent for the final chemical reaction in which alite is formed. Too low a temperature causes insufficient sintering and incomplete reaction, but too high a temperature results in a molten mass or glass, destruction of the kiln lining, and waste of fuel. The resulting material is clinker. On cooling, it is conveyed to storage. Some effort is usually made to blend the clinker, because although the chemistry of the rawmix may have been tightly controlled, the kiln process potentially introduces new sources of chemical variability. The clinker can be stored for a number of years before use. Prolonged exposure to water decreases the reactivity of cement produced from weathered clinker.

The enthalpy of formation of clinker from calcium carbonate and clay minerals is ~1700 kJ/kg. 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. See "Environmental effects" below.

 

Cement grinding

A 10 MW cement mill, producing 270 tph

In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum or anhydrite) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the "specific surface", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface. Typical values are 320-380 m².kg^-1 for general purpose cements, and 450-650 m².kg^-1 for "rapid hardening" cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for 1-20 weeks production, depending upon local demand cycles. The cement is delivered to end-users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In developed countries, 80% or more of cement is delivered in bulk, and many cement plants have no bag-packing facility. In developing countries, bags are the normal mode of delivery.

 

Use

 

Decorative use of Portland cement panels on London’s Grosvenor estate^[3]

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. Users may be involved in the factory production of pre-cast units, such as panels, beams, road furniture, or may make cast-in-situ concrete such as building superstructures, roads, dams. These may be supplied with concrete mixed on site, or may be provided with "ready-mixed" concrete made at permanent mixing sites. Portland cement is also used in mortars (with sand and water only) for plasters and screeds, and in grouts (cement/water mixes squeezed into gaps to consolidate foundations, road-beds, etc).

Setting and hardening

When water is mixed with Portland cement, the product sets in a few hours and hardens over a period of weeks. These processes can vary widely depending upon the mix used and the conditions of curing of the product, but a typical concrete sets (i.e. becomes rigid) in about 6 hours, and develops a compressive strength of 8 MPa in 24 hours. The strength rises to 15 MPa at 3 days, 23 MPa at one week, 35 MPa at 4 weeks, and 41 MPa at three months. In principle, the strength continues to rise slowly as long as water is available for continued hydration, but concrete is usually allowed to dry out after a few weeks, and this causes strength growth to stop.

Setting and hardening of Portland cement is caused by the formation of water-containing compounds, forming as a result of reactions between cement components and water. Usually, cement reacts in a plastic mixture only at water/cement ratios between 0.25 and 0.75. The reaction and the reaction products are referred to as hydration and hydrates or hydrate phases, respectively. As a result of the reactions (which start immediately), a stiffening can be observed which is very small in the beginning, but which increases with time. The point in time at which it reaches a certain level is called the start of setting. The consecutive further consolidation is called setting, after which the phase of hardening begins.

Stiffening, setting and hardening are caused by the formation of a microstructure of hydration products of varying rigidity which fills the water-filled interstitial spaces between the solid particles of the cement paste, mortar or concrete. The behaviour with time of the stiffening, setting and hardening therefore depends to a very great extent on the size of the interstitial spaces, i. e. on the water/cement ratio. The hydration products primarily affecting the strength are calcium silicate hydrates ("C-S-H phases"). Further hydration products are calcium hydroxide, sulfatic hydrates (AFm and AFt phases), and related compounds, hydrogarnet, and gehlenite hydrate. Calcium silicates or silicate constituents make up over 70 % by mass of silicate-based cements. The hydration of these compounds and the properties of the calcium silicate hydrates produced are therefore particularly important. Calcium silicate hydrates contain less CaO than the calcium silicates in cement clinker, so calcium hydroxide is formed during the hydration of Portland cement. This is available for reaction with supplementary cementitious materials such as ground granulated blast furnace slag and pozzolans. The simplified reaction of alite with water may be expressed as:

2Ca₃OSiO₄ + 6H₂O → 3CaO.2SiO₂.3H₂O + 3Ca(OH)₂

This is a relatively fast reaction, causing setting and strength development in the first few weeks. The reaction of belite is:

2Ca₂SiO₄ + 4H₂O → 3CaO.2SiO₂.3H₂O + Ca(OH)₂

This reaction is relatively slow, and is mainly responsible for strength growth after one week. Tricalcium aluminate hydration is controlled by the added calcium sulfate, which immediately goes into solution when water is added. Firstly, ettringite is rapidly formed, causing a slowing of the hydration (see tricalcium aluminate):

Ca₃(AlO₃)₂ + 3CaSO₄ + 32H₂O → Ca₆(AlO₃)₂(SO₄)₃.32H₂O

The ettringite subsequently reacts slowly with further tricalcium aluminate to form "monosulfate" - an "AFm phase":

Ca₆(AlO₃)₂(SO₄)₃.32H₂O + Ca₃(AlO₃)₂ + 4H₂O → 3Ca₄(AlO₃)₂(SO₄).12H₂O

This reaction is complete after 1-2 days. The calcium aluminoferrite reacts slowly due to precipitation of hydrated iron oxide:

2Ca₂AlFeO₅ + CaSO₄ + 16H₂O → Ca₄(AlO₃)₂(SO₄).12H₂O + Ca(OH)₂ + 2Fe(OH)₃

The pH-value of the pore solution reaches comparably high values and is of importance for most of the hydration reactions.

Soon after Portland cement is mixed with water, a brief and intense hydration starts (pre-induction period). Calcium sulfates dissolve completely and alkali sulfates almost completely. Short, hexagonal needle-like ettringite crystals form at the surface of the clinker particles as a result of the reactions between calcium- and sulphate ions with tricalcium aluminate. Further, originating from tricalcium silicate, first calcium silicate hydrates (C-S-H) in colloidal shape can be observed. Caused by the formation of a thin layer of hydration products on the clinker surface, this first hydration period ceases and the induction period starts during which almost no reaction takes place. The first hydration products are too small to bridge the gap between the clinker particles and do not form a consolidated microstructure. Consequently the mobility of the cement particles in relation to one another is only slightly affected, i. e. the consistency of the cement paste turns only slightly thicker. Setting starts after approximately one to three hours, when first calcium silicate hydrates form on the surface of the clinker particles, which are very fine-grained in the beginning. After completion of the induction period, a further intense hydration of clinker phases takes place. This third period (accelerated period) starts after approximately four hours and ends after 12 to 24 hours. During this period a basic microstructure forms, consisting of C-S-H needles and C-S-H leafs, platy calcium hydroxide and ettringite crystals growing in longitudinal shape. Due to growing crystals, the gap between the cement particles is increasingly bridged. During further hydration, the hardening steadily increases, but with decreasing speed. The density of the microstructure rises and the pores fill: the filling of pores causes strength gain.

ASTM C150

There are five types of Portland cements with variations of the first three according to ASTM C150.

Type I Portland cement is known as common or general purpose cement. It is generally assumed unless another type is specified. It is commonly used for general construction especially when making precast and precast-prestressed concrete that is not to be in contact with soils or ground water. The typical compound compositions of this type are:

55% (C₃S), 19% (C₂S), 10% (C₃A), 7% (C₄AF), 2.8% MgO, 2.9% (SO₃), 1.0% Ignition loss, and 1.0% free CaO.

A limitation on the composition is that the (C₃A) shall not exceed fifteen percent.

Type II is intended to have moderate sulfate resistance with or without moderate heat of hydration. This type of cement costs about the same as Type I. Its typical compound composition is:

51% (C₃S), 24% (C₂S), 6% (C₃A), 11% (C₄AF), 2.9% MgO, 2.5% (SO₃), 0.8% Ignition loss, and 1.0% free CaO.

A limitation on the composition is that the (C₃A) shall not exceed eight percent which reduces its vulnerability to sulfates. This type is for general construction that is exposed to moderate sulfate attack and is meant for use when concrete is in contact with soils and ground water especially in the western United States due to the high sulfur content of the soil. Because of similar price to that of Type I, Type II is much used as a general purpose cement, and the majority of Portland cement sold in North America meets this specification.

Note: Cement meeting (among others) the specifications for Type I and II has become commonly available on the world market.

Type III is has relatively high early strength. Its typical compound composition is:

57% (C₃S), 19% (C₂S), 10% (C₃A), 7% (C₄AF), 3.0% MgO, 3.1% (SO₃), 0.9% Ignition loss, and 1.3% free CaO.

This cement is similar to Type I, but ground finer. Some manufacturers make a separate clinker with higher C₃S and/or C₃A content, but this is increasingly rare, and the general purpose clinker is usually used, ground to a specific surface typically 50-80% higher. The gypsum level may also be increased a small amount. This gives the concrete using this type of cement a three day compressive strength equal to the seven day compressive strength of types I and II. Its seven day compressive strength is almost equal to types I and II 28 day compressive strengths. The only downside is that the six month strength of type III is the same or slightly less than that of types I and II. Therefore the long-term strength is sacrificed a little. It is usually used for precast concrete manufacture, where high 1-day strength allows fast turnover of molds. It may also be used in emergency construction and repairs and construction of machine bases and gate installations.

Type IV Portland cement is generally known for its low heat of hydration. Its typical compound composition is:

28% (C₃S), 49% (C₂S), 4% (C₃A), 12% (C₄AF), 1.8% MgO, 1.9% (SO₃), 0.9% Ignition loss, and 0.8% free CaO.

The percentages of (C₂S) and (C₄AF) are relatively high and (C₃S) and (C₃A) are relatively low. A limitation on this type is that the maximum percentage of (C₃A) is seven, and the maximum percentage of (C₃S) is thirty-five. This causes the heat given off by the hydration reaction to develop at a slower rate. However, as a consequence the strength of the concrete develops slowly. After one or two years the strength is higher than the other types after full curing. This cement is used for very large concrete structures, such as dams, which have a low surface to volume ratio. This type of cement is generally not stocked by manufacturers but some might consider a large special order. This type of cement has not been made for many years, because Portland-pozzolan cements and ground granulated blast furnace slag addition offer a cheaper and more reliable alternative.

Type V is used where sulfate resistance is important. Its typical compound composition is:

38% (C₃S), 43% (C₂S), 4% (C₃A), 9% (C₄AF), 1.9% MgO, 1.8% (SO₃), 0.9% Ignition loss, and 0.8% free CaO.

This cement has a very low (C₃A) composition which accounts for its high sulfate resistance. The maximum content of (C₃A) allowed is five percent for Type V Portland cement. Another limitation is that the (C₄AF) + 2(C₃A) composition cannot exceed twenty percent. This type is used in concrete that is to be exposed to alkali soil and ground water sulfates which react with (C₃A) causing disruptive expansion. It is unavailable in many places although its use is common in the western United States and Canada. As with Type IV, Type V Portland cement has mainly been supplanted by the use of ordinary cement with added ground granulated blast furnace slag or tertiary blended cements containing slag and fly ash.

Types Ia, IIa, and IIIa have the same composition as types I, II, and III. The only difference is that in Ia, IIa, and IIIa an air-entraining agent is ground into the mix. The air-entrainment must meet the minimum and maximum optional specification found in the ASTM manual. These types are only available in the eastern United States and Canada but can only be found on a limited basis. They are a poor approach to air-entrainment which improves resistance to freezing under low temperatures.

EN 197

EN 197-1 defines 5 classes of common cement that comprise Portland cement as a main constituent. These classes differ from the ASTM classes.

Constituents that are permitted in Portland-composite cements are blastfurnace slag, silica fume, natural and industrial pozzolans, silicious and calcareous fly ash, burnt shale and limestone.

White Portland cement

White Portland cement differs physically from the gray form only in its color, and as such can fall into many of the above categories (e.g. ASTM Type I, II and/or III). However, its manufacture is significantly different from that of the gray product, and is treated separately.

Safety and environmental effects Safety

When cement is mixed with water a highly alkaline solution (pH ~13) is produced by the dissolution of calcium, sodium and potassium hydroxides. Gloves, goggles and a filter mask should be used for protection. Hands should be washed after contact. Cement can cause serious burns if contact is prolonged or if skin is not washed promptly. Once the cement hydrates, the hardened mass can be safely touched without gloves.

In Scandinavia, France and the UK, the level of chromium(VI), which is thought to be toxic and a major skin irritant, may not exceed 2 ppm (parts per million).

Environmental effects

Portland cement manufacture can cause 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, consumption of large quantities of fuel during manufacture, release of CO₂ from the raw materials during manufacture, 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.

Epidemiologic Notes and Reports Sulfur Dioxide Exposure in Portland Cement Plants, from the Centers for Disease Control states "Workers at Portland cement facilities, particularly those burning fuel containing sulfur, should be aware of the acute and chronic effects of exposure to SO₂ [sulfur dioxide], and peak and full-shift concentrations of SO₂ should be periodically measured." ^[4] "The Arizona Department of Environmental Quality was informed this week that the Arizona Portland Cement Co. failed a second round of testing for emissions of hazardous air pollutants at the company's Rillito plant near Tucson. The latest round of testing, performed in January 2003 by the company, is designed to ensure that the facility complies with federal standards governing the emissions of dioxins and furans, which are byproducts of the manufacturing process." ^[5] Cement Reviews' "Environmental News" web page details case after case of environmental problems with cement manufacturing. ^[6]

An independent research effort of AEA Technology to identify critical issues for the cement industry today concluded the most important environment, health and safety performance issues facing the cement industry are atmospheric releases (including greenhouse gas emissions, dioxin, NO_x, SO₂, and particulates), accidents and worker exposure to dust. ^[7]

The CO₂ associated with Portland cement manufacture falls into 3 categories:

(1) CO₂ derived from decarbonation of limestone,

(2) CO₂ from kiln fuel combustion,

(3) CO₂ produced by vehicles in cement plants and distribution.

Source 1 is fairly constant: minimum around 0.47 kg CO₂ per kg of cement, maximum 0.54, typical value around 0.50 world-wide. Source 2 varies with plant efficiency: efficient precalciner plant 0.24 kg CO₂ per kg cement, low-efficiency wet process as high as 0.65, typical modern proactice (e.g UK) averaging around 0.30. Source 3 is almost insignificant at 0.002-0.005. So typical total CO₂ is around 0.80 kg CO₂ per kg finished cement. This leaves aside the CO₂ associated with electric power consumption, since this varies according to the local generation type and efficiency. Typical electrical energy consumption is of the order of 90-150 kWh per tonne cement, equivalent to 0.09-0.15 kg CO₂ per kg finished cement if the electricity is coal-generated.

Overall, with nuclear- or hydroelectric power and efficient manufacturing, CO₂ generation can be as little as 0.7 kg per kg cement, but can be as high as twice this amount. The thrust of innovation for the future is to reduce sources 1 and 2 by modification of the chemistry of cement, by the use of wastes, and by adopting more efficient processes. Although cement manufacturing is clearly a very large CO₂ emitter, concrete (of which cement makes up about 15%) compares quite favorably with other building systems in this regard. See also cement kiln emissions.

Cement plants as alternatives to conventional waste disposal or processing

 

Used tyres being fed to a pair of cement kilns

Due to the high temperatures inside cement kilns, combined with the oxidizing (oxygen-rich) atmosphere and long residence times, cement kilns have been used as a processing option for various types of waste streams. The waste streams often contain combustible material which allows the substitution of part of the fossil fuel normally used in the process.

Waste materials used in cement kilns as a fuel supplement:

Car and truck tires; steel belts are easily tolerated in the kilns

1.                Waste solvents and lubricants.

2.                Hazardous waste; cement kilns completely destroy hazardous organic compounds

3.          Bone meal; slaughter house waste due to bovine spongiform encephalopathy contamination concerns

4.          Waste plastics

5.          Sewage sludge

6.          Rice shells

7.          Sugar cane waste

8.                Portland cement manufacture also has the potential to remove industrial byproducts from the waste-stream, effectively sequestering some environmentally damaging wastes. These include:

1.                Slag

2.                Fly ash (from power plants)

3.                Silica fume (from steel mills)

4.                Synthetic gypsum (from desulphurisation)       

 

Rawmix blending

The rawmix is formulated to a very tight chemical specification. Typically, the content of individual components in the rawmix must be controlled within 0.1% or better. Calcium and silicon are present in order to form the strength-producing calcium silicates. Aluminium and iron are used in order to produce liquid ("flux") in the kiln burning zone. The liquid acts as a solvent for the silicate-forming reactions, and allows these to occur at an economically low temperature. Insufficient aluminium and iron lead to difficult burning of the clinker, while excessive amounts lead to low strength due to dilution of the silicates by aluminates and ferrites. Very small changes in calcium content lead to large changes in the ratio of alite to belite in the clinker, and to corresponding changes in the cement's strength-growth characteristics. The relative amounts of each oxide are therefore kept constant in order to maintain steady conditions in the kiln, and to maintain constant product properties. In practice, the rawmix is controlled by frequent chemical analysis (hourly by X-Ray fluorescence analysis, or every 3 minutes by prompt gamma neutron activation analysis). The analysis data is used to make automatic adjustments to raw material feed rates. Remaining chemical variation is minimized by passing the raw mix through a blending system that homogenizes up to a day's supply of rawmix (15,000 tonnes in the case of a large kiln).

 

 

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مقاله در مورد سیمان :

 

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

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