Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, and then grinding the clinker with the addition of several percent (often around 5%) gypsum. Several types of portland cement are available. The most common, historically called ordinary portland cement (OPC), is grey, but white portland cement is also available.
The cement was so named by Joseph Aspdin, who obtained a patent for it in 1824, because, once hardened, it resembled the fine, pale limestone known as Portland stone, quarried from the windswept cliffs of the Isle of Portland in Dorset. Portland stone was prized for centuries in British architecture and used in iconic structures such as St Paul's Cathedral and the British Museum.
His son William Aspdin is regarded as the inventor of "modern" portland cement due to his developments in the 1840s.
The low cost and widespread availability of the limestone, , and other naturally occurring materials used in portland cement make it a relatively cheap building material. At 4.4 billion tons manufactured (in 2023), Portland cement ranks third in the list (by mass) of manufactured materials, outranked only by sand and gravel. These two are combined, with water, to make the most manufactured material, concrete. This is Portland cement's most common use.
William Aspdin had left his father's company, to form his own cement manufactury. In the 1840s William Aspdin, apparently accidentally, produced which are a middle step in the development of portland cement. In 1848, William Aspdin further improved his cement. Then, in 1853, he moved to Germany, where he was involved in cement making. William Aspdin made what could be called "meso-portland cement" (a mix of portland cement and hydraulic lime). 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.Hahn, Thomas F., and Emory Leland Kemp. Cement mills along the Potomac River. Morgantown, WV: West Virginia University Press, 1994. 16. Print.
In 1859, John Grant of the Metropolitan Board of Works, set out requirements for cement to be used in the London sewer project. This became a specification for portland cement. The next development in the manufacture of portland cement was the introduction of the rotary kiln, patented by Frederick Ransome in 1885 (U.K.) and 1886 (U.S.); which allowed a stronger, more homogeneous mixture and a continuous manufacturing process. The Hoffmann "endless" kiln which was said to give "perfect control over combustion" was tested in 1860 and shown to produce a superior grade of cement. This cement was made at the Portland Cementfabrik Stern at Szczecin, which was the first to use a Hoffmann kiln. The Association of German Cement Manufacturers issued a standard on portland cement in 1878.
Portland cement had been imported into the United States from England and Germany, and in the 1870s and 1880s, it was being produced by Eagle Portland cement near Kalamazoo, Michigan. In 1875, the first portland cement was produced in the Coplay Cement Company Kilns under the direction of David O. Saylor in Coplay, Pennsylvania, US.Meade, Richard Kidder. Portland cement: its composition, raw materials, manufacture, testing and analysis. Easton, PA: 1906. The Chemical Publishing Co. 4–14. Print. By the early 20th century, American-made portland cement had displaced most of the imported portland cement.
(The last two requirements were already set out in the German Standard, issued in 1909).
Clinkers make up more than 90% of the cement, along with a limited amount of calcium sulphate (CaSO4, which controls the set time), and up to 5% minor constituents (fillers) as allowed by various standards. Clinkers are nodules (diameters, ) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The key chemical reaction distinguishing portland cement from other hydraulic limes occurs at these high temperatures (>) as belite (Ca2SiO4) combines with calcium oxide (CaO) to form alite (Ca3SiO5).
The materials in cement clinker are alite, belite, tricalcium aluminate and tetracalcium alumino ferrite. The aluminium, iron and magnesium oxides are present as a flux allowing the calcium silicates to form at a lower temperature,McArthur, Hugh, and Duncan Spalding. Engineering materials science: properties, uses, degradation and remediation. Chichester, U.K.: Horwood Pub., 2004. 217. Print. and contribute little to the strength. For special cements, such as low heat (LH) and sulphate resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3 CaO·Al2O3) formed.
The major raw material for the clinker-making is usually limestone (CaCO3) mixed with a second material containing clay as source of alumino-silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of these limestones can be as low as 80%. Secondary raw materials (materials in the raw mix other than limestone) depend on the purity of the limestone. Some of the 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.
The clinker phases—calcium silicates and aluminates—dissolve into the water that is mixed with the cement, which results in a fluid containing relatively high concentrations of dissolved ions. This reaches supersaturation with respect to specific mineral phases: usually first ettringite, and then calcium silicate hydrate (C-S-H)—which precipitate as newly formed solids. The interlocking of the C-S-H (which is crystallographically disordered, and can take on needle or crumpled-foil morphologies) and the ettringite crystals gives cement its initial setting, converting the fluid into a solid, and chemically incorporating much of the water into these new phases.
Gypsum is included in the cement as an inhibitor to prevent flash (or quick) setting; if gypsum is not present, the initial formation of (needle-shaped) ettringite is not possible, and so (plate-shaped) hydrocalumite-group ("AFm") calcium aluminate phases form instead. This premature formation of AFm phases causes a rapid loss of flowability, which is generally not desirable because it means that placement of the cement or concrete is very difficult.
Hardening of the cement then proceeds through further C-S-H formation, as this fills in the spaces between the (still-dissolving) cement grains with newly formed solid phases. Portlandite also precipitates from the pore solution to form part of the solid microstructure, and some of the initially-formed ettringite may be converted to AFm phases, releasing part of the sulfate from its structure to continue reacting with any remaining tricalcium aluminate
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 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 1 week, 35 MPa at 4 weeks, and 41 MPa at 3 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.
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 (utilizing cement chemist notation).
A limitation on the composition is that the (C3A) shall not exceed 15%.
Type II provides moderate sulphate resistance, and gives off less heat during 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 8%, which reduces its vulnerability to sulphates. This type is for general construction exposed to moderate sulphate 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 sulphur content of the soils. 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 types I and II has become commonly available on the world market.
Type III 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 area 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 28-day compressive strengths of types I and II. 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. It is usually used for precast concrete manufacture, where high one-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. Consequently, 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 sulphate 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 sulphate resistance. The maximum content of (C3A) allowed is 5% for type V portland cement. Another limitation is that the (C4AF) + 2(C3A) composition cannot exceed 20%. This type is used in concrete to be exposed to alkali soil and ground water sulphates 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, only on a limited basis. They are a poor approach to air-entrainment which improves resistance to freezing under low temperatures.
Types II(MH) and II(MH)a have a similar composition as types II and IIa, but with a mild heat.
Constituents that are permitted in portland-composite cements are artificial pozzolans (blast furnace slag (in fact a latent hydraulic binder), silica fume, and fly ashes), or natural pozzolans (siliceous or siliceous aluminous materials such as volcanic ash glasses, calcined clays and shale).
The production of comparatively low-alkalinity cements (pH<11) is an area of ongoing investigation.
In Scandinavia, France, and the United Kingdom, the level of chromium(VI), which is considered to be toxic and a major skin irritant, may not exceed 2 parts per million (ppm).
In the US, the Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for portland cement exposure in the workplace as 50 mppcf (million particles per cubic foot) over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. At levels of 5000 mg/m3, portland cement is IDLH.
Portland cement is caustic, so it can cause chemical burns. The powder can cause irritation or, with severe exposure, lung cancer, and can contain a number of hazardous components, including crystalline silica and hexavalent chromium. Environmental concerns are the high energy consumption required to mine, manufacture, and transport the cement, and the related air pollution, including the release of the greenhouse gas carbon dioxide, dioxin, , , and particulates. Production of portland cement contributes about 10% of world carbon dioxide emissions. The International Energy Agency has estimated that cement production will increase by between 12 and 23% by 2050 to meet the needs of the world's growing population. There are several ongoing researches targeting a suitable replacement of portland cement by supplementary cementitious materials.
Epidemiologic Notes and Reports Sulfur Dioxide Exposure in Portland Cement Plants, from the Centers for Disease Control, states:
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, , , and particulates), accidents, and worker exposure to dust.
The associated with portland cement manufacture comes mainly from four sources:
Overall, with nuclear or hydroelectric power, and efficient manufacturing, generation can be reduced to per kg cement, but can be twice as high. 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 emitter, concrete (of which cement makes up about 15%) compares quite favourably with other modern building systems in this regard.. Traditional materials such as lime based mortars as well as timber and earth based construction methods emit significantly less .
Waste materials used in cement kilns as a fuel supplement:
Portland cement manufacture also has the potential to benefit from using industrial byproducts from the waste stream.
Composition
The European Standard EN 197-1 uses the following definition:
Manufacturing
Cement grinding
+ Typical constituents of portland clinker plus gypsum
showing cement chemist notation (CCN)
! Clinker
! CCN
! Mass25–50% 20–45% 5–12% 6–12% 2–10% + Typical constituents of portland cement
showing cement chemist notation
! Cement
! CCN
! Mass61–67% 19–23% 2.5–6% 0–6% 1.5–4.5%
Setting and hardening
Use
Types
General
ASTM C150
EN 197 norm
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 blast furnace slag Portland cement and up to 55% of pozzolan Portland cement, blast furnace slag or fly ash and pozzolana
CSA A3000-08
General use cement Moderate sulphate resistant cement Moderate heat cement High early strength cement Low heat cement High sulphate resistant; generally develops strength less rapidly than the other types.
White portland cement
Safety issues
Environmental effects
Decarbonation of limestone Fairly constant: minimum around per kg of cement, maximum 0.54, typical value around 0.50 worldwide. Kiln fuel combustion Varies with plant efficiency: efficient precalciner plant per kg cement, low-efficiency wet process as high as 0.65, typical modern practices (e.g. UK) averaging around 0.30. Produced by vehicles in cement plants and distribution Almost insignificant at 0.002–0.005. So typical total is around per kg finished cement. Electrical power generation Varies with local power source. Typical electrical energy consumption is on the order of 90–150 kWh per tonne cement, equivalent to per kg finished cement if the electricity is coal-generated.
Cement plants used for waste disposal or processing
See also
External links
Further reading
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