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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.
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. 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. The German Government issued a standard on Portland cement in 1878.
Types of Portland cement
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.
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.
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.
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.
Typical constituents of Portland clinker and Portland cement. Cement industry style notation in italics:
Tricalcium silicate (CaO)3.SiO2, C3S
Calcium oxide, CaO, C
Dicalcium silicate (CaO)2.SiO2, C2S
Silicon oxide, SiO2, S
Tricalcium aluminate (CaO)3.Al2O3, C3A
Aluminium oxide, Al2O3, A
Tetracalcium aluminoferrite (CaO)4.Al2O3.Fe2O3, C4AF
Ferric oxide, Fe2O3, F
Decorative use of Portland cement panels on London’s Grosvenor estate
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).
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.
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 197-1 defines 5 classes of common cement that comprise Portland cement as a main constituent. These classes differ from the ASTM classes.
Comprising Portland cement and up to 5% of minor additional constituents
Portland cement and up to 35% of other single constituents
Portland cement and higher percentages of blastfurnace slag
Portland cement and up to 55% of pozzolanic constituents
Portland cement, blastfurnace slag and pozzolana or fly ash
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.
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.
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."  "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."  Cement Reviews' "Environmental News" web page details case after case of environmental problems with cement manufacturing. 
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, NOx, SO2, and particulates), accidents and worker exposure to dust. 
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.
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:
2. Fly ash (from power plants)
3. Silica fume (from steel mills)
4. Synthetic gypsum (from desulphurisation)
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).
مقاله در مورد سیمان :
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
The most important use of cement is the production of
aggregates to form a strong building material which is
durable in the face of normal environmental effects.
The earliest construction cements are as old as
construction, 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
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
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
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
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
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
· 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
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.
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 of both his concrete and raw cement have
shown that William Aspdin's product made at
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
Portland cement is the most common type of cement in
general usage, as it is a basic ingredient of concrete,
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.
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 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
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-
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".
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
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
“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.
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
silica fume from silicon and ferrosilicon manufacturing,
and sometimes recycled concrete from demolition of
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,
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
"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
In 2006 is was estimated that China manufactured 1.235
billion metric tons of cement, which is 44% of the world
total cement production