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Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world, used as a basic ingredient of concrete, mortar, stucco, and most non-specialty grout. It usually originates from limestone. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as the European Standard EN 197-1:
Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaO·SiO2 and 2 CaO·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 oxide content (MgO) shall not exceed 5.0% by mass.
(The last two requirements were already set out in the German Standard, issued in 1909).
ASTM C150 defines Portland cement as "hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition." Clinkers are nodules (diameters, 0.2–1.0 inch [5–25 mm]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of the limestone, shales, and other naturally occurring materials make Portland cement one of the lowest-cost materials widely used over the last century throughout the world. Concrete is one of the most versatile construction materials available in the world.
Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a calcining 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 (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 rawmix 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.
Portland cement was developed from natural cements 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 from Leeds, is considered to be the originator of Portland cement. A process for the manufacture of Portland cement was patented in 1824. This cement was an artificial cement similar in properties to the material known as "Roman cement", which had been patented in 1796 by James Parker. Aspdin's process was similar to a process 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, Dave Stewart, and possibly others.
James Frost is reported to have erected a manufactory for making of an artificial cement in 1826. In 1843, Aspdin's son William improved their cement, which was initially called "Patent Portland cement", although he had no patent. In 1848, William Aspdin further improved his cement; 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). John Grant of the Metropolitan Board of Works in 1859 set out requirements for cement to be used in the London sewer project. This became a specification for Portland cement. The Hoffman "endless" kiln which gave "perfect control over combustion" was tested in 1860 and showed the process produced a better grade of cement. This cement was made at the Portland Cementfabrik Stern at Stettin, which was the first to utilize a Hoffman kiln. It is thought that the first modern Portland cement was made there. The Association of German Cement Manufacturers issued a standard on Portland cement in 1878.
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 area", 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 area. 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 industrial countries, 80% or more of cement is delivered in bulk.
|Tricalcium silicate (CaO)3 · SiO2||C3S||45–75%|
|Dicalcium silicate (CaO)2 · SiO2||C2S||7–32%|
|Tricalcium aluminate (CaO)3 · Al2O3||C3A||0–13%|
|Tetracalcium aluminoferrite (CaO)4 · Al2O3 · Fe2O3||C4AF||0–18%|
|Gypsum CaSO4 · 2 H2O||2–10%|
|Calcium oxide, CaO||C||61–67%|
|Silicon dioxide, SiO2||S||19–23%|
|Aluminum oxide, Al2O3||A||2.5–6%|
|Ferric oxide, Fe2O3||F||0–6%|
Cement sets when mixed with water by way of a complex series of chemical reactions still only partly understood. The different constituents slowly crystallise and the interlocking of their crystals gives cement its strength. Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble calcium carbonate. After the initial setting, immersion in warm water will speed up setting. Gypsum is added as an inhibitor to prevent flash setting.
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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 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.
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There are different standards for classification of Portland cement. The two major standards are the ASTM C150 used primarily in the USA 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 C150.
There are five types of Portland cements with variations of the first three according to ASTM C150.
Type I Portland cement is known as common or general purpose cement. It is generally assumed unless another type is specified. It is commonly used for general construction especially when making precast and precast-prestressed concrete that is not to be in contact with soils or ground water. The typical compound compositions of this type are:
55% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 2.8% MgO, 2.9% (SO3), 1.0% Ignition loss, and 1.0% free CaO.
A limitation on the composition is that the (C3A) shall not exceed fifteen percent.
Type II 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 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 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 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.
Types II(MH) and II(MH)a have a similar composition as types II and IIa but with a mild heat.
EN 197-1 defines 5 classes of common cement that comprise Portland cement as a main constituent. These classes differ from the ASTM classes.
|I||Portland cement||Comprising Portland cement and up to 5% of minor additional constituents|
|II||Portland-composite cement||Portland cement and up to 35% of other single constituents|
|III||Blastfurnace cement||Portland cement and higher percentages of blastfurnace slag|
|IV||Pozzolanic cement||Portland cement and up to 55% of pozzolanic constituents(volcanic ash)|
|V||Composite cement||Portland cement, blastfurnace slag or fly ash and pozzolana|
Constituents that are permitted in Portland-composite cements are artificial pozzolans (blastfurnace slag, silica fume, and fly ashes) or natural pozzolans (siliceous or siliceous aluminous materials such as volcanic ash glasses, calcined clays and shale).
White Portland cement or white ordinary Portland cement (WOPC) is similar to ordinary, grey Portland cement in all respects except for its high degree of whiteness. Obtaining this color requires some modification to the method of manufacture; because of this, it is somewhat more expensive than the grey product. The main requirement is to have low iron content which should be less than 0.5% expressed as Fe2O3 for white cement and less than 0.9% for off-white cement. It helps to have the iron oxide as ferrous oxide (FeO) which is obtained via slight reducing conditions i.e. operating with zero excess oxygen at the kiln exit. This gives the clinker and cement a green tinge. Other metals such as Cr, Mn, Ti etc. in trace content can also give color tinges so for a project it is best to use cement from a single source.
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. Cement users should wear protective clothing.
When traditional Portland cement is mixed with water the dissolution of hydroxides of calcium, sodium, and potassium produces a highly alkaline solution (pH ~13). Gloves, goggles, and a filter mask should be used for protection, and hands should be washed after contact as most cement can cause acute ulcerative damage 8–12 hours after contact if skin is not washed promptly. The reaction of cement dust with moisture in the sinuses and lungs can also cause a chemical burn as well as headaches, fatigue, and lung cancer. The development of formulations of cement that include fast-reacting pozzolans such as silica fume as well as some slow-reacting products such as fly ash have allowed for the production of comparatively low-alkalinity cements (pH<11) that are much less toxic and which have become widely commercially available, largely replacing high-pH formulations in much of the USA. After any cement sets, the hardened mass loses chemical reactivity and 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:
Source 1. CO2 derived from decarbonation of limestone,
Source 2. CO2 from kiln fuel combustion,
Source 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 worldwide. 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 practices (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 omits the CO2 associated with electric power consumption because 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 reduced to 0.7 kg 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 CO2 emitter, concrete (of which cement makes up about 15%) compares quite favorably with other building systems in this regard.
Due to the high temperatures inside cement kilns, combined with the oxidizing (oxygen-rich) atmosphere and long residence times, cement kilns are used as a processing option for various types of waste streams: indeed, they efficiently destroy many hazardous organic compounds. The waste streams also often contain combustible materials which allow the substitution of part of the fossil fuel normally used in the process.
Waste materials used in cement kilns as a fuel supplement:
Portland cement manufacture also has the potential to benefit from using industrial by-products from the waste-stream. These include in particular: