Portland cement

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Sampling fast set Portland cement
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Sampling fast set Portland cement

Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete and mortar. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (as source of calcium) with clay or sand (as source of silicon) and grinding the product. The resulting powder, when mixed with water, will become a hydrated solid over time.

Portland cement was first manufactured in Britain in the early part of the 19th 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. The patent for Portland cement was issued to Joseph Aspdin, a British bricklayer, in 1824.

Contents

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 molecules found in cement.

The raw materials for Portland cement production are a mixture (as fine dust in the 'Dry process' or in the form of a slurry in the 'Wet process') of 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 raw mixture is heated in a kiln, a gigantic slowly rotating and sloped cylinder, with temperatures increasing over the length of the cylinder up to ~1480°C. The temperature is regulated so that the product contains sintered but not fused lumps. Too low a temperature causes insufficient sintering, but too high a temperature results in a molten mass or glass. In the lower-temperature part of the kiln, calcium carbonate (limestone) turns into calcium oxide (lime) and carbon dioxide. In the high-temperature part, calcium oxides and silicates react to form dicalcium and tricalcium silicates (Ca2Si Ca3Si). Small amounts of tricalcium aluminate (Ca3Al) and tetracalcium aluminoferrite (Ca4AlFe)are also formed. The resulting material is clinker, and can be stored for a number of years before use. Prolonged exposure to water decreases the reactivity of cement produced from weathered clinker.

The energy required to produce clinker is ~1700 J/g. 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.

In order to achieve the desired setting qualities in the finished product, about 2% gypsum is added to the clinker and the mixture is pulverized very finely. The powder is now ready for use, and will react with the addition of water.

Typical constitutents for portland clinker and portland cement:
Clinker Weight% Cement Weight%
Tricalcium silicate C3S 45-65% C - Calcium oxide, CaO 62-67%
Dicalcium silicate C2S 15-30% S - Silicon oxide, SiO2 20-25%
Tricalcium aluminate C3A 1-8% A - Aluminium oxide, Al2O3 3-7%
Tetracalcium aluminoferrite C4AF 8-15% F - Ferro oxide, Fe2O3 2-5%
Gypsum 1-3% Sulfate


Use

The most common use for portland cement is the production of concrete. Concrete is a composite material consisting of aggregate, 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.

When water is mixed with Portland cement, the product sets in a few hours and hardens over a period of weeks. The initial setting is caused by a reaction between the water, gypsum, and tricalcium aluminate (Ca3Al), forming the crystalline hydration products calcium-alumino-hydrate (CAH), ettringite (Aft), and monosulfate (Afm). The later hardening and the development of cohesive strength is due to the reaction of water and tricalcium silicate (Ca3Si), forming an amorphous hydrated product called calcium-silicate-hydrate(CSH gel). In each case the hydration products surround and cement together the individual grains. The hydration of dicalcium silicate (Ca2Si) proceeds more slowly than that of the above compounds slowly increasing later-age strength. The ultimate cementing agent is probably gelatinous silica (SiO2). All three reactions mentioned above release heat.

Plastic cement is a type of Portland cement with the addition of a plasticizing material (limestone or hydrated lime), as well as other materials to reduce setting time and facilitate workability (see superplasticizer). Plastic cement is used primarily for spreading onto walls to make exterior stucco, as Portland cement (used primarily for concrete) would have poor spreadability. In this usage, the term "plastic" does not refer to the addition of an organic polymer. Rather, it refers to the addition of a substance to increase the workability of the mixture.

Portland cement business

In 2002 the world production of hydraulic cement was 1,800 million metric tons. The top three producers were China with 704, India with 100, and the United States with 91 million metric tons for a combined total of about half the world total by the world's three most populous states. [1]

"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." Jan 7, 2004

"Demand for cement in China is expected to advance 5.4% annually and exceed 1 billion metric tons in 2008, driven by slowing but healthy growth in construction expenditures. Cement consumed in China will amount to 44% of global demand, and China will remain the world's largest national consumer of cement by a large margin." Nov 1, 2004

Types of Portland Cement

General

There are different standards for classification of portland cement. The two major standards are the American ASTM C150 and European EN-197. EN 197 cement Types CEM I, II, III, IV, and V do not correspond to the cement types in ASTM C 150, nor can ASTM cements be substituted for EN specified cement, or vice a versa, without the designer’s approval.

ASTM C150

There are five types of Portland cements with variations of the first three according to ASTM C150. ASTM stands for the American Society of Testing Materials and is basically a manual for all materials and their properties and proper uses. In addition, pozzolanic ash or other pozzolans are often added to cement to improve its properties and lower its cost.

Type I Portland cement is known as common 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. This type is the most basic and common type of Portland cement.

Type II is known 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. This 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. Another limitation is the percentage of (C3S) + (C3A) shall not exceed 58. The two limitations are meant to minimize cracking caused by temperature gradients.

Note: Cement is increasingly sold as a blend of Type I/II on the world market.

Type III is known for its 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 produced grinding clinker, bonded cement chunks, with a high percentage of (C3A) and (C3S) into a finer texture. The gypsum level is also 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. The highly early strength is gained by increasing the tricalcium silicate, (C3S), in the mix. This increased amount of tricalcium silicate brings the danger of free lime in the cement and high volume changes after setting. Type III can also be used in concrete that comes in contact with soil and ground water. It is usually used for 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. 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 in stock and has to be special ordered in large quantities. A limitation on this type is that the maximum percentage of (C3A) is seven, and the maximum percentage of (C3S) is thirty-five. Another negative about this type of cement is its higher cost. Recently mix designs using pozzolans and water-reducing admixtures have been developed to decrease the cement content which has allowed for Type II Portland cement to be substituted in for Type IV in the production of dams. This helps lower the cost of the dam.

Note: Type IV cement is not really used any more in industry.

Type V is known for its extreme sulfate resistance. 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. This type is used in concrete that has a tendency to be exposed to alkali soil and ground water sulfates. It is generally not meant for use around seawater, but it can be done as long as the (C3A) composition is above two percent. It usually requires an advance order and is generally available to the western United States and Canada. Another limitation is that the (C4AF) + 2(C3A) composition cannot exceed twenty percent. This type of cement is essential in the construction of canal linings, culverts, and siphons because of there contact with ground waters containing sulfates. This is required because sulfates cause serious deterioration and swelling to the other types of Portland cement. The serious deterioration will eventually cause the concrete to fail. Type V Portland cement is a very uncommon type used in everyday construction but is routinely used in harsh marine environments.

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 and minimum and maximum optional specification found in the ASTM manual. These types are only available in the eastern United States and Canada but can only be found on a limited basis. They are a poor approach to air-entrainment which improves resistance to freezing under low temperatures.

EN 197

EN 197-1 classify portland cement in 5 classes that differ from ASTM.

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 Comprising Portland cement and higher percentages of pozzolana
V Composite cement Comprising Portland cement and higher percentages of blastfurnace slag and pozzolana or fly ash

Safety and enviromental effects

Stop! The neutrality of this section is disputed.


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

In Scandinavia and France, the level of chrome VI, which is toxic and a major skin irritant, may not exceed 2 ppm (parts per million), which corresponds to a maximum chromium level of 3.3 micrograms per gram.

"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 SO((2)), and peak and full-shift concentrations of SO((2)) should be periodically measured." [2]

"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." [3] Lest one feel this is an isolated case, Cement Reviews' "Environmental News" web page details case after case of environmental problems with cement manufacturing. [4]

In light of safety, environmental, and other concerns, "[t]he Cement Sustainability Initiative (CSI) was formed to help the cement industry to address the challenges of sustainable development. [Areas of concern include:]

  1. Climate protection and CO2 management,
  2. Responsible use of fuel and materials,
  3. Employee health and safety,
  4. Emission monitoring and reporting,
  5. Local impacts on land and communities, and
  6. Reporting and Communication" [5]

An independent research effort of AEA Technology to identify critical issues for the cement industry today concluded the most important environment, health and safety performance issues facing the cement industry are atmospheric releases (including greenhouse gas emissions, dioxin, NO(X), SO(2), and particulates), accidents and worker exposure to dust. [6]

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