Coal Analysis techniques are specific analytical methods designed to measure the particular

physical and chemical properties of coals. These methods are used primarily to determine the

suitability of coal for coking, power generation or for iron ore smelting in the manufacture of steel.

Chemical properties of coalCoal comes in four main types or ranks: lignite or brown coal, bituminous coal or black

coal, anthracite and graphite. Each type of coal has a certain set of physical parameters which are

mostly controlled by moisture, volatile content (in terms of aliphatic or aromatic hydrocarbons) and

carbon content.

Moisture

Moisture is an important property of coal, as all coals are mined wet. Groundwater and other

extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held

within the coal itself is known as inherent moisture and is analysed quantitatively. Moisture may

occur in four possible forms within coal:

Surface moisture: water held on the surface of coal particles or macerals

Hydroscopic moisture: water held by capillary action within the microfractures of the coal

Decomposition moisture: water held within the coal's decomposed organic compounds

Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such

as clays

Total moisture is analysed by loss of mass between an untreated sample and the sample once

analysed. This is achieved by any of the following methods :

1. Heating the coal with toluene

2. Drying in a minimum free-space oven at 150 °C (302 °F) within a nitrogen atmosphere

3. Drying in air at 100 to 105 °C (212 to 221 °F) and relative loss of mass determined

Methods 1 and 2 are suitable with low-rank coals but method 3 is only suitable for high-rank coals as

free air drying low-rank coals may promote oxidation. Inherent moisture is analysed similarly, though

it may be done in a vacuum.

Volatile matter

Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at

high temperature in the absence of air. This is usually a mixture of short and long chain

hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter of coal is determined

under rigidly controlled standards. In Australian and British laboratories this involves heating the coal

sample to 900 ± 5 °C (1650 ±10 °F) for 7 min.

Ash

Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk

mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off

during combustion. Analysis is fairly straight forward, with the coal thoroughly burnt and the ash

material expressed as a percentage of the original weight. It can also give an indication about the

quality of coal.

Fixed carbon

The fixed carbon content of the coal is the carbon found in the material which is left after volatile

materials are driven off. This differs from the ultimate carbon content of the coal because some

carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount

of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass

of volatiles determined by the volatility test, above, from the original mass of the coal sample.

Physical and mechanical propertiesRelative density

Relative density or specific gravity of the coal depends on the rank of the coal and degree of mineral

impurity. Knowledge of the density of each coal ply is necessary to determine the properties of

composites and blends. The density of the coal seam is necessary for conversion of resources into

reserves.

Relative density is normally determined by the loss of a sample's weight in water. This is best

achieved using finely ground coal, as bulk samples are quite porous. To determine in-place coal

tonnages however, it is important to preserve the void space when measuring the specific gravity.

Particle size distribution

The particle size distribution of milled coal depends partly on the rank of the coal, which determines

its brittleness, and on the handling, crushing and milling it has undergone. Generally coal is utilised

in furnaces and coking ovens at a certain size, so the crushability of the coal must be determined

and its behaviour quantified. It is necessary to know these data before coal is mined, so that suitable

crushing machinery can be designed to optimise the particle size for transport and use.

Float-sink test

Coal plies and particles have different relative densities, determined by vitrinite content, rank, ash

value/mineral content and porosity. Coal is usually washed by passing it over a bath of liquid of

known density. This removes high-ash value particles and increases the saleability of the coal as

well as its energy content per unit volume. Thus, coals must be subjected to a float-sink test in the

laboratory, which will determine the optimum particle size for washing, the density of the wash liquid

required to remove the maximum ash value with the minimum work.

Floatsink testing is achieved on crushed and pulverised coal in a process similar to metallurgical

testing on metallic ore.

Abrasion testing

Abrasion is the property of the coal which describes its propensity and ability to wear away

machinery and undergo autonomous grinding. While carbonaceous matter in coal is relatively soft,

quartz and other mineral constituents in coal are quite abrasive. This is tested in a calibrated mill,

containing four blades of known mass. The coal is agitated in the mill for 12,000 revolutions at a rate

of 1,500 revolutions per minute.(I.E 1500 revolution for 8 min.) The abrasion index is determined by

measuring the loss of mass of the four metal blades.

Special combustion testsSpecific energy

Aside from physical or chemical analyses to determine the handling and pollutant profile of a coal,

the energy output of a coal is determined using a bomb calorimeter which measures the specific

energy output of a coal during complete combustion. This is required particularly for coals used in

steam-raising.

Ash fusion test

The behaviour of the coal's ash residue at high temperature is a critical factor in selecting coals for

steam power generation. Most furnaces are designed to remove ash as a powdery residue. Coal

which has ash that fuses into a hard glassy slag known as clinker is usually unsatisfactory in

furnaces as it requires cleaning. However, furnaces can be designed to handle the clinker, generally

by removing it as a molten liquid.

Ash fusion temperatures are determined by viewing a moulded specimen of the coal ash through an

observation window in a high-temperature furnace. The ash, in the form of a cone, pyramid or cube,

is heated steadily past 1000 °C to as high a temperature as possible, preferably 1,600 °C (2,910 °F).

The following temperatures are recorded;

Deformation temperature: This is reached when the corners of the mould first become rounded

Softening (sphere) temperature: This is reached when the top of the mould takes on a spherical

shape.

Hemisphere temperature: This is reached when the entire mould takes on a hemisphere shape

Flow (fluid) temperature: This is reached when the molten ash collapses to a flattened button on

the furnace floor.

Crucible swelling index (free swelling index)

The simplest test to evaluate whether a coal is suitable for production of coke is the free swelling

index test. This involves heating a small sample of coal in a standardised crucible to around 800

degrees Celsius (1500 °F).

After heating for a specified time, or until all volatiles are driven off, a small coke button remains in

the crucible. The cross sectional profile of this coke button compared to a set of standardised

profiles determines the Free Swelling Index.

ConcreteFrom Wikipedia, the free encyclopedia

This article is about the construction material. For other uses, see Concrete (disambiguation).

Not to be confused with cement.

Outer view of the Roman Pantheon, still the largest unreinforced solid concrete dome.[1]

Inside the Pantheon dome, looking straight up. The concrete for the coffered dome was laid on moulds, probably

mounted on temporary scaffolding.

Opus caementiciumexposed in a characteristic Roman arch. In contrast to modern concrete structures, the concrete

used in Roman buildings was usually covered with brick or stone.

Concrete is a composite material composed mainly of water, aggregate, and cement. Often, additives and reinforcements are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone-like material with many uses.[2]

Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome.[3]

After the Roman Empire collapsed, use of concrete became rare until the technology was re-pioneered in the mid-18th century. Today, concrete is the most widely used man-made material (measured by tonnage).

HistoryThe word concrete comes from the Latin word "concretus" (meaning compact or condensed),

[4] the perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).

Perhaps the earliest known occurrence of cement was twelve million years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes. These ancient deposits were investigated in the 1960s and 1970s.[5]

On a human time-scale, small usages of concrete go back for thousands of years. The ancient Nabatea culture was using materials roughly analogous to concrete at least eight thousand years ago, some structures of which survive to this day.[6]

German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400-1200 BC.[7][8] Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of fully waterproof concrete.[9] Concrete was used for construction in many ancient structures.[10]

The Romans used concrete extensively from 300 BC to 476 AD, a span of more than seven hundred years.[5] During the Roman Empire, Roman concrete (or opus caementicium) was made

from quicklime, pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecturetermed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension.[11]

Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.[12]

Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm2).[13] However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application was also different:[14]

Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.[15]

Eddystone Lighthouse

The widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges such as the magnificent Pont du Gard have masonry cladding on a concrete core, as does the dome of the Pantheon.

After the Roman Empire, the use of burned lime and pozzolana was greatly reduced until the technique was all but forgotten between 500 AD and the 1300s. Between the 1300s until the mid-1700s, the use of cement gradually returned. The Canal du Midi was built using concrete in 1670,[16] and there are concrete structures in Finland that date from the 16th century.[citation needed]

Perhaps the greatest driver behind the modern usage of concrete was the third Eddystone Lighthouse in Devon, England. To create this structure, between 1756 and 1793, British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate.[17]

A method for producing Portland cement was patented by Joseph Aspdin on 1824.[18]

Reinforced concrete was invented in 1849 by Joseph Monier.[19] In 1889 the first concrete reinforced bridge was built, and the first large concrete dams were built in 1936, Hoover Dam and Grand Coulee Dam.[20]

Ancient additives

Concrete additives have been used since 6500BC by the Nabataea traders or Bedouins who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan. They later discovered the advantages of hydraulic lime—that is, cement that hardens underwater—and by 700 BC, they were building kilns to supply mortar for the construction of rubble-wall houses, concrete floors, and underground waterproof cisterns. The cisterns were kept secret and were one of the reasons the Nabataea were able to thrive in the desert.[6] In both Roman and Egyptian times it was re-discovered that addingvolcanic ash to the mix allowed it to set underwater. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant.[21]

Modern additives

In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength, electrical conductivity, or resistance to damages through spillage.[22]

Impact of modern concrete use

Concrete mixing plant in Birmingham, Alabama in 1936

Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, highways, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats. Concrete is used in large quantities almost everywhere mankind has a need for infrastructure.

The amount of concrete used worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminum combined. Concrete's use in the modern world is exceeded only by that of naturally occurring water.[23]

Concrete is also the basis of a large commercial industry. Globally, the ready-mix concrete industry, the largest segment of the concrete market, is projected to exceed $100 billion in revenue by 2015.[24] In the United States alone, concrete production is a $30-billion-per-year industry, considering only

the value of the ready-mixed concrete sold each year. [25] Given the size of the concrete industry, and the fundamental way concrete is used to shape the infrastructure of the modern world, it is difficult to overstate the role this material plays today.

Environmental and healthMain article: Environmental impact of concrete

The manufacture and use of concrete produce a wide range of environmental and social consequences. Some are harmful, some welcome, and some both, depending on circumstances.

A major component of concrete is cement, which similarly exerts environmental and social effects.[26]:142 The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas (the other two being the energy production and transportation industries). As of 2001, the production of Portland cement contributed 7% to global anthropogenic CO2 emissions, largely due to the sintering of limestone and clay at 1,500 °C (2,730 °F).[27]

Concrete is used to create hard surfaces that contribute to surface runoff, which can cause heavy soil erosion, water pollution, and flooding, but conversely can be used to divert, dam, and control flooding.

Concrete is a primary contributor to the urban heat island effect, though less so than asphalt.[citation needed]

Workers who cut, grind or polish concrete are at risk of inhaling airborne silica, which can lead to silicosis.[28] Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution.

The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and radioactivity. Wet concrete is highly alkaline and must be handled with proper protective equipment.

Recycled crushed concrete, to be reused as granular fill, is loaded into a semi-dump truck.

Concrete recyclingMain article: Concrete recycling

Concrete recycling is an increasingly common method of disposing of concrete structures. Concrete debris was once routinely shipped tolandfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.

Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through acrushing machine, often along with asphalt, bricks and rocks.

Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh

concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 March 1983, a government-funded research team (the VIRL research.codep) estimated that almost 17% of worldwide landfill was by-products of concrete based waste.

Education and researchThe National Building Museum in Washington, D.C. created an exhibition titled Liquid Stone: New Architecture in Concrete.[29] This exhibition, dedicated solely to the study of concrete as a building material, was on view for the public from June 2004 - January 2006.

Composition of concreteThere are many types of concrete available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cementitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.

"Aggregate" consists of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks such as limestone, or granite, along with finer materials such assand.

"Cement", most commonly Portland cement is associated with the general term "concrete." A range of materials can be used as the cement in concrete. One of the most familiar of these alternative cements is asphalt. Other cementitious materials such as fly ash and slag cement, are sometimes added to Portland cement and become a part of the binder for the aggregate.

Water is then mixed with this dry composite, which produces a semi-liquid that workers can shape (typically by pouring it into a form). The concrete solidifies and hardens through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust stone-like material.

"Chemical admixtures" are added to achieve varied properties. These ingredients may speed or slow down the rate at which the concrete hardens, and impart many other useful properties including increased tensile strength and water resistance.

"Reinforcements" are often added to concrete. Concrete can be formulated with high compressive strength, but always has lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension (often steel).

"Mineral admixtures" are becoming more popular in recent decades. The use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation, and the discovery that such materials often have complementary and valuable properties. The most conspicuous of these are fly ash, a by-product of coal-fired power plants, and silica fume, a byproduct of industrial electric arc furnaces. The use of these materials in concrete reduces the amount of resources required, as the ash and fume act as a cement replacement. This displaces some cement production, an energetically expensive and environmentally problematic process, while reducing the amount of industrial waste that must be disposed of.

The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure.

CementMain article: Cement

A few tons of bagged cement. This amount represents about two minutes of output from a 10,000 ton per day cement

kiln.

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides ofcalcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).

In modern cement kilns many advanced features are used to lower the fuel consumption per ton of clinker produced. Cement kilns are extremely large, complex, and inherently dusty industrial installations, and have emissions which must be controlled. Of the various ingredients used in concrete the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then grind it into cement. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.[30]

Water

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. [31]

A lower water-to-cement ratio yields a stronger, more durable concrete, whereas more water gives a freer-flowing concrete with a higher slump.[32] Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.[33]

Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete to form a solid mass.[34]

Reaction:[34]

Cement chemist notation: C3S + H → C-S-H + CH

Standard notation: Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2

Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2

Aggregates

Crushed stone aggregate

Main article: Construction aggregate

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

The presence of aggregate greatly increases the durability of concrete above that of cement, which is a brittle material in its pure state. Thus concrete is a true composite material.[35]

Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.[36]

Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.

In addition to being decorative, exposed aggregate adds robustness to a concrete driveway.[37]

Reinforcement

Constructing a rebar cage. This cage will be permanently embedded in poured concrete to create a reinforced

concrete structure.

Main article: reinforced concrete

Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds either steel reinforcing bars, steel fibers, glass fibers, or plastic fibers to carry tensile loads.

Chemical admixtures

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.[38] (See the section on Concrete Production, below.)The common types of admixtures[39] are as follows.

Accelerators  speed up the hydration (hardening) of the concrete. Typical materials used are CaCl2, Ca(NO3)2 and NaNO3. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.

Retarders  slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

Air entrainments  add and entrain tiny air bubbles in the concrete, which reduces damage during freeze-thaw cycles, increasing durability. However, entrained air entails a trade off with strength, as each 1% of air may decrease compressive strength 5%.[citation needed]

Plasticizers  increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.

Pigments  can be used to change the color of concrete, for aesthetics. Corrosion inhibitors  are used to minimize the corrosion of steel and steel bars in concrete. Bonding agents are used to create a bond between old and new concrete (typically a type of

polymer) with wide temperature tolerance and corrosion resistance. Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.

Mineral admixtures and blended cements

Components of CementComparison of Chemical and Physical Characteristicsa[40][41]

PropertyPortlandCement

Class FFly Ash

Class CFly Ash

SlagCement

SilicaFume

SiO2 content (%) 21 52 35 35 85–97

Al2O3 content (%) 5 23 18 12 —

Fe2O3 content (%) 3 11 6 1 —

CaO content (%) 62 5 21 40 < 1

Specific surfaceb

(m2/kg)370 420 420 400

15,000–30,000

Specific gravity 3.15 2.38 2.65 2.94 2.22

General usein concrete

Primarybinder

Cementreplacement

Cementreplacement

Cementreplacement

Propertyenhancer

aValues shown are approximate: those of a specific material may vary.

bSpecific surface measurements for silica fume by nitrogen adsorption (BET) method,others by air permeability method (Blaine).

Inorganic materials that have pozzolanic or latent hydraulic properties, these very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),[38] or as a replacement for Portland cement (blended cements). [42] Products which incorporate limestone, fly ash, blast furnace slag, and other useful materials with pozzolanic properties into the mix, are being tested and used. This development is due to cement production being one of the largest producers (at about 5 to 10%) of global greenhouse gas emissions, [43] as well as lowering costs, improving concrete properties, and recycling wastes.

Fly ash : A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareousfly ash has latent hydraulic properties.[44]

Ground granulated blast furnace slag  (GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.[45]

Silica fume : A byproduct of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface-to-volume ratio and

a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.[46]

High reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.

Concrete production

Concrete plant facility showing a Concrete mixerbeing filled from the ingredient silos.

Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a concrete plant, or often a batch plant.

In general usage, concrete plants come in two main types, ready mix plants and central mix plants. A ready mix plant mixes all the ingredients except water, while a central mix plant mixes all the ingredients including water. A central mix plant offers more accurate control of the concrete quality through better measurements of the amount of water added, but must be placed closer to the work site where the concrete will be used, since hydration begins at the plant.

A concrete plant consists of large storage hoppers for various reactive ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.

Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms, which are containers erected in the field to give the concrete its desired shape. There are many different ways in which concrete formwork can be prepared, such as Slip forming and Steel plate construction. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory settings to manufacture Precast concreteproducts.

There is a wide variety of equipment for processing concrete, from hand tools to heavy industrial machinery. Whichever equipment builders use, however, the objective is to produce the desired

building material; ingredients must be properly mixed, placed, shaped, and retained within time constraints. Once the mix is where it should be, the curing process must be controlled to ensure that the concrete attains the desired attributes. During concrete preparation, various technical details may affect the quality and nature of the product.

When initially mixed, Portland cement and water rapidly form a gel of tangled chains of interlocking crystals, and components of the gel continue to react over time. Initially the gel is fluid, which improves workability and aids in placement of the material, but as the concrete sets, the chains of crystals join into a rigid structure, counteracting the fluidity of the gel and fixing the particles of aggregate in place. During curing, the cement continues to react with the residual water in a process of hydration. In properly formulated concrete, once this curing process has terminated the product has the desired physical and chemical properties. Among the qualities typically desired, are mechanical strength, low moisture permeability, and chemical and volumetric stability.

Mixing concreteSee also: Volumetric concrete mixer and Concrete mixer

Thorough mixing is essential for the production of uniform, high-quality concrete. For this reason equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work.

Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.[47] The paste is generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.[48]

High-energy mixed (HEM) concrete is produced by means of high-speed mixing of cement, water and sand with net specific energy consumption of at least 5 kilojoules per kilogram of the mix. A plasticizer or a superplasticizer is then added to the activated mixture, which can later be mixed with aggregates in a conventional concrete mixer. In this process, sand provides dissipation of energy and creates high-shear conditions on the surface of cement particles. This results in the full volume of water interacting with cement. The liquid activated mixture can be used by itself or foamed (expanded) for lightweight concrete.[49] HEM concrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which drastically reduces capillarity in solid and porous materials.

Workability

Pouring and smoothing out concrete at Palisades Park in Washington DC.

Main article: Concrete slump test

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot readily be made more workable by addition of reasonable amounts of water.

Workability can be measured by the concrete slump test, a simplistic measure of the plasticity of a fresh batch of concrete following theASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50 mm) out of one foot (305 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the flow table test.

Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio.[50] Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.

High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.

After mixing, concrete is a fluid and can be pumped to the location where needed.

Curing

A concrete slab ponded while curing.

In all but the least critical applications, care must be taken to properly cure concrete, to achieve best strength and hardness. This happens after the concrete has been placed. Cement requires a moist, controlled environment to gain strength and harden fully. The cement paste hardens over time, initially setting and becoming rigid though very weak and gaining in strength in the weeks following. In around 4 weeks, typically over 90% of the final strength is reached, though strengthening may continue for decades.[51] The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of CO2 over several decades further strengthens the concrete and makes it more resistant to damage. However, this reaction, called carbonation, lowers the pH of the cement pore solution and can cause the reinforcement bars to corrode.

Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased

tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.[52]

During this period concrete must be kept under controlled temperature and humid atmosphere. In practice, this is achieved by spraying or ponding the concrete surface with water, thereby protecting the concrete mass from ill effects of ambient conditions. The picture to the right shows one of many ways to achieve this, ponding – submerging setting concrete in water and wrapping in plastic to contain the water in the mix. Additional common curing methods include wet burlap and/or plastic sheeting covering the fresh concrete, or by spraying on a water-impermeable temporary curing membrane.

Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.

PropertiesMain article: Properties of concrete

Concrete has relatively high compressive strength, but much lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.

Tests can be performed to ensure that the properties of concrete correspond to specifications for the application.

Different mixes of concrete ingredients produce different strengths, which are measured in psi or MPa.

Different strengths of concrete are used for different purposes. Very low-strength (2000 psi or less) concrete may be used when the concrete must be lightweight.[53] Lightweight concrete is often achieved by adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most routine uses, 3000-psi to 4000-psi concrete is often used. 5000-psi concrete is readily commercially available as a more durable, although more expensive, option. 5000-psi concrete is often used for larger civil projects.[54]Strengths above 5000 psi are often used for specific building elements. For example, the lower floor columns of high-rise concrete buildings may use concrete of 12,000 psi or more, to keep the size of the columns small. Bridges may use long beams of 10,000 psi concrete to lower the number of spans required. [55][56] Occasionally, other structural needs may require high-strength concrete. If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Strengths as high as 19,000 psi have been used commercially for these reasons.[55]

Compression testing of a concrete cylinder

Imperial Strength

Metric Equivalent

2,000 psi 14 MPa

2.500 psi 18 MPa

3,000 psi 20 MPa

3,500 psi 25 MPa

4,000 psi 30 MPa

5,000 psi 35 MPa

6,000 psi 40 MPa

7,000 psi 50 MPa

8,000 psi 55 MPa

10,000 psi 70 MPa

12,000 psi 80 MPa

19,000 psi 130 MPa

36,000 psi 250 MPa

Concrete degradation

Concrete spalling caused by thecorrosion of rebar

Main article: Concrete degradation

Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel reinforcement bars, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water).[citation needed] The micro fungiAspergillus Alternaria and Cladosporium were able to grow on samples of concrete used as a radioactive waste barrier in the Chernobylreactor; leaching aluminium, iron, calcium and silicon.[57]

Microbial concrete[edit]

Bacteria such as Bacillus pasteurii, Bacillus pseudofirmus, Bacillus cohnii, Sporosarcina pasteuri, and Arthrobacter crystallopoietesincrease the compression strength of concrete through their biomass. Not all bacteria increase the strength of concrete significantly with their biomass.[26]:143 Bacillus sp. CT-5. can reduce corrosion of reinforcement in reinforced concrete by up to four times. Sporosarcina pasteurii reduces water and chloride permeability. B. pasteurii increases resistance to acid.[26]:146 Bacillus pasteurii and B. sphaericuscan induce calcium carbonate precipitation in the surface of cracks, adding compression strength.[26]:147

Use of concrete in infrastructure

Aerial photo of reconstruction atTaum Sauk (Missouri) pumped storage facility in late November, 2009. After the

original reservoir failed, the new reservoir was made of roller-compacted concrete.

Mass concrete structuresMain article: Mass concrete

Large concrete structures such as dams, navigation locks, large mat foundations, and large breakwaters generate excessive heat during cement hydration and associated expansion. To mitigate these effects post-cooling [58]  is commonly applied during construction. An early example at Hoover Dam, installed a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating. Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.[58]

Another approach to mass concrete structures that is becoming more widespread is the use of roller-compacted concrete, which uses much lower amounts of cement and water than conventional concrete mixtures and is generally not poured into place. Instead it is placed in thick layers as a semi-dry material and compacted into a dense, strong mass with rolling compactors. Because it uses less cementitious material, roller-compacted concrete has a much lower cooling requirement than conventional concrete.

Prestressed concrete structures

40-foot cacti decorate a sound/retaining wall in Scottsdale, Arizona

Main article: Prestressed concrete

Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those experienced in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement.

For example, a horizontal beam tends to sag. Prestressed reinforcement along the bottom of the beam counteracts this. In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.

Concrete textures

When one thinks of concrete, the image of a dull, gray concrete wall often comes to mind. With the use of form liner, concrete can be cast and molded into different textures and used for decorative concrete applications. Sound/retaining walls, bridges, office buildings and more serve as the optimal canvases for concrete art. For example, the Pima Freeway/Loop 101 retaining and sound walls in Scottsdale, Arizona, feature desert flora and fauna, a 67-foot (20 m) lizard and 40-foot (12 m) cacti along the 8-mile (13 km) stretch. The project, titled "The Path Most Traveled," is one example of how concrete can be shaped using elastomeric form liner.

Building with concrete

The Buffalo City Court Building in Buffalo, NY.

Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life. Concrete is used more than any other manmade material in the world. [59] As of 2006, about 7.5 billion cubic meters of concrete are made each year, more than one cubic meter for every person on Earth.[60]

More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete, prestressed concreteand precast concrete are the most widely used types of concrete functional extensions in modern days. See Brutalism.

Energy efficiency

Energy requirements for transportation of concrete are low because it is produced locally from local resources, typically manufactured within 100 kilometers of the job site. Similarly, relatively little energy is used in producing and combining the raw materials (although large amounts of CO2are

produced by the chemical reactions in cement manufacture).[citation needed] The overall embodied energy of concrete is therefore lower than for most structural materials other than wood.[citation needed]

Once in place, concrete offers great energy efficiency over the lifetime of a building. [61] Concrete walls leak air far less than those made of wood frames[citation needed]. Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs. [62] While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.

Pervious concreteMain article: Pervious concrete

Pervious concrete is a mix of specially graded coarse aggregate, cement, water and little-to-no fine aggregates. This concrete is also known as "no-fines" or porous concrete. Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totalling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze–thaw climates to minimize the possibility of frost damage.

Nano concrete

Concrete is the most widely manufactured construction material. The addition of carbon nanofibres to concrete has many advantages in terms of mechanical and electrical properties (e.g. higher strength and higher Young’s modulus) and self-monitoring behavior due to the high tensile strength and high conductivity. Mullapudi [63] used the pulse velocity method to characterize the properties of concrete containing carbon nanofibres. The test results indicate that the compressive strength and percentage reduction in electrical resistance while loading concrete containing carbon nanofibres differ from those of plain concrete. A reasonable concentration of carbon nanofibres need to be determined for use in concrete, which not only enhances compressive strength, but also improves the electrical properties required for strain monitoring, damage evaluation and self-health monitoring of concrete.

Fire safety

A modern building: Boston City Hall(completed 1968) is constructed largely of concrete, both precast and poured in

place. Of Brutalist architecture, it was voted "The World's Ugliest Building" in 2008.

Concrete buildings are more resistant to fire than those constructed using steel frames, since concrete has lower heat conductivity than steel and can thus last longer under the same fire conditions. Concrete is sometimes used as a fire protection for steel frames, for the same effect as

above. Concrete as a fire shield, for example Fondu fyre, can also be used in extreme environments like a missile launch pad.

Options for non-combustible construction include floors, ceilings and roofs made of cast-in-place and hollow-core precast concrete. For walls, concrete masonry technology and Insulating Concrete Forms (ICFs) are additional options. ICFs are hollow blocks or panels made of fireproof insulating foam that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.

Concrete also provides good resistance against externally applied forces such as high winds, hurricanes, and tornadoes owing to its lateral stiffness, which results in minimal horizontal movement. However this stiffness can work against certain types of concrete structures, particularly where a relatively higher flexing structure is require to resist more extreme forces.

Earthquake safety

As discussed above, concrete is very strong in compression, but weak in tension. Larger earthquakes can generate very large shear loads on structures. These shear loads subject the structure to both tensile and compressional loads. Concrete structures without reinforcement, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally.[64] These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey [65]).

Useful life

The Tunkhannock Viaduct was begun in 1912 and is still in regular service as of 2014.

Concrete can be viewed as a form of artificial sedimentary rock. As a type of mineral, the compounds of which it is composed are extremely stable. [66] Many concrete structures are built with an expected lifetime of approximately 100 years, [67] but researchers have suggested that adding silica fume could extend the useful life of bridges and other concrete uses to as long as 16,000 years.[68] Coatings are also available to protect concrete from damage, and extend the useful life. Epoxy coatings may be applied only to interior surfaces, though, as they would otherwise trap moisture in the concrete.[69]

A self-healing concrete has been developed that can also last longer than conventional concrete.[70]

Large dams, such as the Hoover Dam, and the Three Gorges Dam are intended to last "forever", a period that is not quantified.[71]

World recordsThe world record for the largest concrete pour in a single project is the Three Gorges Dam in Hubei Province, China by the Three Gorges Corporation. The amount of concrete used in the construction

of the dam is estimated at 16 million cubic meters over 17 years. The previous record was 12.3 million cubic meters held by Itaipu hydropower station in Brazil.[72][73][73][74]

The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of 715 m (2,346 ft).[75][76]

The world record for the largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture and the concrete supplier is Unibeton Ready Mix.[77][78] The pour (a part of the foundation for the Abu Dhabi's Landmark Tower) was 16,000 cubic meters of concrete poured within a two-day period.[79] The previous record, 13,200 cubic meters poured in 54 hours despite a severe tropical storm requiring the site to be covered with tarpaulins to allow work to continue, was achieved in 1992 by joint Japanese and South Korean consortiums Hazama Corporation and the Samsung C&T Corporation for the construction of the Petronas Towers inKuala Lumpur, Malaysia.[80]

The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 225,000 square feet (20,900 m2) of concrete placed within a 30-hour period, finished to a flatness tolerance of FF 54.60 and a levelness tolerance of FL 43.83. This surpassed the previous record by 50% in total volume and 7.5% in total area.[81][82]

The record for the largest continuously placed underwater concrete pour was completed 18 October 2010, in New Orleans, Louisiana by contractor C. J. Mahan Construction Company, LLC of Grove City, Ohio. The placement consisted of 10,251 cubic yards of concrete placed in a 58.5 hour period using two concrete pumps and two dedicated concrete batch plants. Upon curing, this placement allows the 50,180-square-foot (4,662 m2) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the construction of the Inner Harbor Navigation Canal Sill & Monolith Project to be completed in the dry.[83]

CementFrom Wikipedia, the free encyclopedia

For other uses, see Cement (disambiguation).

Not to be confused with Concrete.

Cement is usually a grey powder before being mixed with other materials and water. Cement powder causes allergic

reactions at skin contact and is biohazardous to skin, eyes and lungs, so handlers should wear a dust mask, goggles

and protective gloves.[1][2][3]

A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" traces to theRomans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that 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 can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to be used in the presence of water (see hydraulic and non-hydraulic lime plaster).

Non-hydraulic cement will not set in wet conditions or underwater, it sets as the cement dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.

Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, pozzolanas, such as fly ash. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., Portland cement).

The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates). Presently cheaper than volcanic ash, fly ash from power stations, recovered as a pollution control measure, or other waste or by products are used as pozzolanas with plain cement to produce hydraulic cement. Pozzolanas can constitute up to 40% of Portland cement.

Hydraulic cement can harden underwater or when constantly exposed to wet weather. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack.

The most important uses of cement are as a component in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.

ChemistryNon-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), harden by carbonation in presence of the carbon dioxide naturally present in the air. Firstcalcium oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure:

CaCO3 → CaO + CO2

The calcium oxide is then spent (slaked) mixing it with water to make slaked lime:

CaO + H2O → Ca(OH)2

Once the water in excess from the slaked lime is completely evaporated (this process is technically called setting), the carbonation starts:

Ca(OH)2 + CO2 → CaCO3 + H2O

This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed to air, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the lime cycle.

Conversely, the chemistry ruling the action of the hydraulic cement is hydration. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being:

rotary Kiln

Belite (2CaO·SiO2);Alite (3CaO·SiO2);Celite (3CaO·Al2O3);Brownmillerite (4CaO·Al2O3·Fe2O3).

The silicates are responsible of the mechanical properties of the cement, the celite and the browmillerite are essential to allow the formation of the liquid phase during the kiln sintering (firing). The chemistry of the above listed reactions is not completely clear and is still the object of research.[4]

History of the origin of cementCements before the 18th century

An early version of cement made with lime, sand, and gravel was used in Mesopotamia in the third millennium B.C. and later in Egypt. It is uncertain where it was first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture (see also: Pozzolanic reaction), but concrete made from such mixtures was used by the Ancient Macedonians [5] [6]  and three centuries later on a large scale by Roman engineers.[7] They used both natural pozzolans (trass or pumice) and artificial pozzolans (ground brick or pottery) in these concretes.[citation needed] The huge dome of the Pantheon in Rome and the massive Baths of Caracalla are examples of ancient structures made from these concretes, many of which are still standing. [8] The vast system of Roman aqueducts also made extensive use of hydraulic cement. [9] Although any preservation of this knowledge in literary sources from the Middle Ages is unknown, medieval masons and some military engineers maintained an active tradition of using hydraulic cement in structures such as canals, fortresses, harbors, and shipbuilding facilities.[10][11] This technical knowledge of making hydraulic cement was later formalized by French and British engineers in the 18th century.[10]

Cements in the 18th, 19th, and 20th centuries[edit]

John Smeaton made an important contribution to the development of cements when he was planning the construction of the third Eddystone Lighthouse (1755–59) in theEnglish Channel now known as Smeaton's Tower. He needed a hydraulic mortar that would set and develop some strength in the twelve hour period between successive high tides. He performed experiments with combinations of different limestones and additives including trass and pozzolanas [12]  and did 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.

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 was Parker's "Roman cement".[13] This was developed by James Parker in the 1780s, and finally patented in 1796. It was, in fact, nothing like 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 hydraulic lime cements of clay and chalk. Roman cement quickly became popular but was largely replaced by Portland cement in the 1850s.[12]

In Russia, Egor Cheliev created a new binder by mixing lime and clay. His results were published in 1822 in his book A Treatise on the Art to Prepare a Good Mortar published in St. Petersburg. A few years later in 1825, he published another book, which described the various methods of making cement and concrete, as well as the benefits of cement in the construction of buildings and embankments.[14][15]

Apparently unaware of Smeaton's work, the same principle was identified by Frenchman 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[16] considered the "principal forerunner"[12] of Portland cement and "...Edgar Dobbs of Southwark patented a cement of this kind in 1811."[12]

James Frost,[17] 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. However, Aspdins' cement was nothing like modern Portland cement but was a first step in its development, called a proto-Portland cement.[12] Joseph Aspdins' son William Aspdin had left his fathers company and in his cement manufacturing apparently accidentally produced calcium silicates in the 1840s, a middle step in the development of Portland cement. William Aspdin's innovation was counterintuitive for manufacturers of "artificial cements", because they required more lime in the mix (a problem for his father), a much higher kiln temperature (and therefore more fuel), and 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 onward, and was soon the dominant use for cements. Thus Portland cement began its predominant role.

Isaac Charles Johnson further refined the production of meso-Portland cement (middle stage of development) and claimed to be the real father of Portland cement.[18]

Setting time and "early strength" are important characteristics of cements. Hydraulic limes, "natural" cements, and "artificial" cements all rely upon their belite content for strengthdevelopment. Belite develops strength slowly. Because they were burned at temperatures below 1,250 °C (2,280 °F), they contained no alite, which is responsible for early strength in modern cements. The first cement to consistently contain alite was made by William Aspdin 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 Johnson) have claimed precedence in this invention, but recent analysis [19] 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.

Sorel cement was patented in 1867 by Frenchman Stanislas Sorel and was stronger than Portland cement but its poor water restive and corrosive qualities limited its use in building construction. The next development with the manufacture of Portland cement was the introduction of the rotary kiln which allowed a stronger, more homogeneous mixture and a continuous manufacturing process.[12]

Also, tabby, a wall building method using lime, sand and oyster shells to form a concrete, was introduced to the Americas by the Spanish in the sixteenth century. [20] The lime may have been made from burned oyster shells which were available in some coastal areas in the form of shell middens. Calcium aluminate cements were patented in 1908 in France by Jules Bied for better resistance to sulfates.

The National Cement Share Company of Ethiopia's new plant inDire Dawa.

In the US the first large-scale use of cement was Rosendale cement, a natural cement mined from a massive deposit of a largedolostone rock deposit discovered in the early 19th century near Rosendale, New York. Rosendale cement was extremely popular for the foundation of buildings (e.g., Statue of Liberty, Capitol Building, Brooklyn Bridge) and lining water pipes. But its long curing time of at least a month made it unpopular after World War One in the construction of highways and bridges and many states and construction firms turned to the use of Portland cement. Because of the switch to Portland cement, by the end of the 1920s of the 15 Rosendale cement companies, only one had survived. But in the early 1930s it was discovered that, while Portland cement had a faster setting time it was not as durable, especially for highways, to the point that some states stopped building highways and roads with cement. Bertrain H. Wait, an engineer whose company had worked on the construction of the New York City's Catskill Aqueduct, was impressed with the durability of Rosendale cement, and came up with a blend of both Rosendale and synthetic cements which had the good attributes of both: it was highly durable and had a much faster setting time. Mr. Wait convinced the New York Commissioner of Highways to construct an experimental section of highway near New Paltz, New York, using one sack of Rosendale to six sacks of synthetic cement. It was proved a success and for decades the Rosendale-synthetic cement blend became common use in highway and bridge construction.[21]

Modern cements[edit]

Modern hydraulic cements began to be developed from the start of the Industrial Revolution (around 1800), driven by three main needs:

Hydraulic cement render (stucco) 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.

Types of modern cement[edit]

Components of CementComparison of Chemical and Physical Characteristicsa[22][23]

PropertyPortlandCement

Class FFly Ash

Class CFly Ash

SlagCement

SilicaFume

SiO2 content (%)

21 52 35 35 85–97

Al2O3 content (%)

5 23 18 12 —

Fe2O3 content (%)

3 11 6 1 —

CaO   content (%)

62 5 21 40 < 1

Specific surfaceb

(m2/kg)370 420 420 400

15,000–30,000

Specific gravity

3.15 2.38 2.65 2.94 2.22

General   usein concrete

Primarybinder

Cementreplacement

Cementreplacement

Cementreplacement

Propertyenhancer

aValues shown are approximate: those of a specific material may vary.

bSpecific   surface  measurements   for   silica   fume by  nitrogen  adsorption (BET)   method,others by air permeability method (Blaine).

Portland cement[edit]Main article: Portland cement

Portland cement is by far the most common type of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known ascalcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement', the most commonly used type of cement (often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar and most non-specialty 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 grey or white.

Energetically modified cement[edit]Main article: Energetically modified cement

The grinding process to produce energetically modified cement (EMC) yields materials made from pozzolanic minerals that have been treated using a patented milling process ("EMC Activation").[24] This yields a high-level replacement of Portland cement in concrete with lower costs, performance and durability improvements, with significant energy and carbon dioxide savings. [25] The resultant concretes can have the same, if not improved, physical characteristics as "normal" concretes, at a fraction of the cost of using Portland cement.[26]

Portland cement blends[edit]

Portland cement blends are often available as inter-ground mixtures from cement producers, but similar formulations are often also mixed from the ground components at the concrete mixing plant. [27]

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. [28]

Portland flyash cement contains up to 35% fly ash. The fly ash is pozzolanic, so that ultimate strength is maintained. Because fly ash addition allows a lower concrete water content, early strength can also be maintained. Where good quality cheap fly ash is available, this can be an economic alternative to ordinary Portland cement.[29]

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 wherevolcanic 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.[30]

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".

Very finely ground cements are made from mixtures of cement with sand or with slag or other pozzolan type minerals that are extremely finely ground together. Such cements can have the same physical characteristics as normal cement but with 50% less cement particularly due to their increased surface area for the chemical reaction. Even with intensive grinding they can use up to 50% less energy to fabricate than ordinary Portland cements.[26]

Pozzolan-lime cements. Mixtures of ground pozzolan and lime are the cements used by the Romans, and can 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 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 (CaO · Al2O3 or CA in Cement chemist notation, CCN) and mayenite Ca12Al14O33 (12 CaO · 7 Al2O3, or 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 C4A3S 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. [31][32] 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 of free lime. As with any natural material, such cements have highly variable properties.

Geopolymer cements are made from mixtures of water-soluble alkali metal silicates and aluminosilicate mineral powders such as fly ash and metakaolin.

Curing (setting)[edit]

Cement sets or cures when mixed with water which causes a series of hydration chemical reactions. The constituents slowly hydrate and crystallize; the interlocking of the crystals gives cement its strength. Maintaining a high moisture content in cement during curing increases both the speed of curing, and its final strength. Gypsum is often added to Portland cement to prevent early hardening or "flash setting", allowing a longer working time. The time it takes for cement to cure varies depending on the mixture and environmental conditions; initial hardening can occur in as little as twenty minutes, while full cure can take over a month. Cement typically cures to the extent that it can be put into service within 24 hours to a week.

Safety issues[edit]

Bags of cement routinely have health and safety warnings printed on them because not only is cement highly alkaline, but the setting process is exothermic. As a result, wet cement is strongly caustic and can easily cause severe skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation. Some ingredients can be specifically allergenic and may cause allergic dermatitis.[1] Reducing agents are sometimes added to cement to prevent the formation of carcinogenic chromate in cement. Cement users should wear protective clothing.[33][34][35]

Cement industry in the world[edit]

Global Cement Production in 2010

Global Cement Capacity in 2010

See also: List of countries by cement production

In 2010, the world production of hydraulic cement was 3,300 million tonnes. The top three producers were China with 1,800, India with 220, and USA with 63.5 million tonnes for a combined total of over half the world total by the world's three most populated states.[36]

For the world capacity to produce cement in 2010, the situation was similar with the top three states (China, India, and USA) accounting for just under half the world total capacity.[37]

Over 2011 and 2012, global consumption continued to climb, rising to 3585 Mt in 2011 and 3736 Mt in 2012, while annual growth rates eased to 8.3% and 4.2%, respectively.

China, representing an increasing share of world cement consumption, continued to be the main engine of global growth. By 2012, Chinese demand was recorded at 2160 Mt, representing 58% of world consumption. Annual growth rates, which reached 16% in 2010, appear to have softened, slowing to 5–6% over 2011 and 2012, as China’s economy targets a more sustainable growth rate.

Outside of China, worldwide consumption climbed by 4.4% to 1462 Mt in 2010, 5% to 1535 Mt in 2011, and finally 2.7% to 1576 Mt in 2012.

Iran is now the 3rd largest cement producer in the world and has increased its output by over 10% from 2008 to 2011.[38] Due to climbing energy costs in Pakistan and other major cement-producing countries, Iran is a unique position as a trading partner, utilizing its own surplus petroleum to power clinker plants. Now a top producer in the Middle-East, Iran is further increasing its dominant position in local markets and abroad.[39]

The performance in North America and Europe over the 2010–12 period contrasted strikingly with that of China, as the global financial crisis evolved into a sovereign debt crisis for many economies in this region and recession. Cement consumption levels for this region fell by 1.9% in 2010 to 445 Mt, recovered by 4.9% in 2011, then dipped again by 1.1% in 2012.

The performance in the rest of the world, which includes many emerging economies in Asia, Africa and Latin America and representing some 1020 Mt cement demand in 2010, was positive and more than offset the declines in North America and Europe. Annual consumption growth was recorded at 7.4% in 2010, moderating to 5.1% and 4.3% in 2011 and 2012, respectively.

As at year-end 2012, the global cement industry consisted of 5673 cement production facilities, including both integrated and grinding, of which 3900 were located in China and 1773 in the rest of the world.

Total cement capacity worldwide was recorded at 5245 Mt in 2012, with 2950 Mt located in China and 2295 Mt in the rest of the world.[40]

China[edit]

"For the past 18 years, China consistently has produced more cement than any other country in the world. [...] (However,) China's cement export peaked in 1994 with 11 million tonnes shipped out and has been in steady decline ever since. Only 5.18 million tonnes 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."[41]

In 2006, it was estimated that China manufactured 1.235 billion tonnes of cement, which was 44% of the world total cement production.[42] "Demand for cement in China is expected to advance 5.4% annually and exceed 1 billion tonnes 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."[43]

In 2010, 3.3 billion tonnes of cement was consumed globally. Of this, China accounted for 1.8 billion tonnes.[44]

Africa[edit]See also: Cement in Africa

Environmental impacts[edit]

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.

CO2 emissions[edit]

Global carbon emission by type to 2004. Attribution: Mak Thorpe

Carbon concentration in cement spans from ≈5% in cement structures to ≈8% in the case of roads in cement.[45] Cement manufacturing releases CO2 in the atmosphere both directly when calcium carbonate is heated, producing lime and carbon dioxide,[46] and also indirectly through the use of energy if its production involves the emission of CO2. The cement industry produces about 5% of global man-made CO2 emissions, of which 50% is from the chemical process, and 40% from burning fuel.[47]

The amount of CO2 emitted by the cement industry is nearly 900 kg of CO2 for every 1000 kg of cement produced. In the European union the specific energy consumption for the production of cement clinker has been reduced by approximately 30% since the 1970s. This reduction in primary energy requirements is equivalent to approximately 11 million tonnes of coal per year with corresponding benefits in reduction of CO2 emissions. This accounts for approximately 5% of anthropogenic CO2.[48]

The high proportion of carbon dioxide produced in the chemical reaction leads to a large decrease in mass in the conversion from limestone to cement. So, to reduce the transport of heavier raw materials and to minimize the associated costs, it is more economical for cement plants to be closer to the limestone quarries rather than to the consumer centers.[49]

In certain applications, lime mortar reabsorbs the same amount of CO2 as was released in its manufacture, and has a lower energy requirement in production than mainstream cement. Newly developed cement types from Novacem[50] and Eco-cement can absorb carbon dioxide from ambient air during hardening.[51] Use of the Kalina cycle during production can also increase energy efficiency.

Heavy metal emissions in the air[edit]

In some circumstances, mainly depending on the origin and the composition of the raw materials used, the high-temperature calcination process of limestone and clay minerals can release in the atmosphere gases and dust rich in volatile heavy metals, a.o, thallium,[52] cadmium and mercury are the most toxic. Heavy metals (Tl, Cd, Hg, ...) are often found as trace elements in common metal sulfides (pyrite (FeS2), zinc blende (ZnS), galena (PbS), ...) present as secondary minerals in most of the raw materials. Environmental regulations exist in many countries to limit these emissions. As of 2011 in the United States, cement kilns are "legally allowed to pump more toxins into the air than are hazardous-waste incinerators."[53]

Heavy metals present in the clinker[edit]

The presence of heavy metals in the clinker arises both from the natural raw materials and from the use of recycled by-products or alternative fuels. The high pH prevailing in the cement porewater (12.5 < pH < 13.5) limits the mobility of many heavy metals by decreasing their solubility and increasing their sorption onto the cement mineral phases.Nickel, zinc and lead are commonly found in cement in non-negligible concentrations.

Use of alternative fuels and by-products materials[edit]

A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln (referred to as co-processing), replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale, and limestone. Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value, and burns to give ash containing minerals useful in the clinker matrix.[54]

Normal operation of cement kilns provides combustion conditions which are more than adequate for the destruction of even the most difficult to destroy organic substances. This is primarily due to the very high temperatures of the kiln gases (2000 °C in the combustion gas from the main burners and 1100 °C in the gas from the burners in the precalciner). The gas residence time at high temperature in the rotary kiln is of the order of 5–10 seconds and in the precalciner more than 3 seconds.[55]

Due to bovine spongiform encephalopathy (BSE) in the European beef industry, the use of animal-derived products to feed cattle is now severely restricted. Large quantities of waste animal meat and bone meal (MBM), also known as animal flour, have to be safely disposed of or transformed. The production of cement kilns, together with the incineration, is to date one of the two main ways to treat this solid effluent of the food industry.

Green cement[edit]

Green cement is a cementitious material that meets or exceeds the functional performance capabilities of ordinary Portland cement by incorporating and optimizing recycled materials, thereby reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable construction material.

The manufacturing process for green cement succeeds in reducing, and even eliminating, the production and release of damaging pollutants and greenhouse gasses, particularly CO2.

Growing environmental concerns and increasing cost of fuels of fossil origin have resulted in many countries in sharp reduction of the resources needed to produce cement and effluents (dust and exhaust gases).[55]

Peter Trimble, a design student at the University of Edinburgh has proposed 'DUPE'based on sporosarcina pasteurii, a bacterium with binding qualities which, when mixed withsand and urine produces a concrete said to be 70% as strong as conventional materials. [56] The idea has been commercialized in the USA