Portland cement
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Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and most non-specialty grout. It is a finely-ground 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, an homogenous 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 Resistance (SR), it is necessary to limit the amount of tricalcium aluminate (3CaO.Al2O3) formed. The major raw material for the clinker-making is limestone (CaCO3). Normally, an impure limestone which contains SiO2 is used - the CaCO3 content can be as low as 80%. Secondary raw materials 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 becomes a secondary raw material.
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[edit] Setting and hardening
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. Hydration products primarily affecting the strength are calcium silicate hydrates for silicate-based cements and calcium alumino hydrates for high-alumina cements. Further hydration products are calcium hydroxide, calcium ferrite hydrate, sulfatic hydrates 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. All cements also contain aluminium oxide and iron oxide as well as sulfate as essential constituents, so the formation of calcium aluminate hydrates, calcium ferrite hydrates and compounds containing sulfate can be expected. 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 (CSH) 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 CSH needles and CSH 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 raises and the pores fill.
[edit] History
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 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 the "Roman 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. (See "The Cement Industry 1796-1914: A History," by A. J. Francis, 1977) The first Portland cement in the modern sense was probably made by the factory of "Portlandzementfabrik Stern" in Germany about 1867. This works is mentioned by Henry Reid in his 1868 book on cement manufacture which claims that the cement is superior to any other by a large margin. The German Government issued a standard on Portland cement in 1878. The Stern cement would have complied with that standard.
[edit] Production
There are three fundamental stages in the production of Portland cement:
- Preparation of the raw mixture
- Production of the clinker
- 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 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 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 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. 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).
The raw mixture is heated in a cement 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 (C2S C3S). Small amounts of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) 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. Cement manufacturing emits 0.2 Pg C/yr as CO2. (1 Pg = 1 thousand million metric tons.)
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 pulverized. The powder is now ready for use, and will react with the addition of water.
| Clinker | Mass% | Cement | Mass% |
| Tricalcium silicate (CaO)3.SiO2, C3S | 45-75% | Calcium oxide, CaO, C | 61-67% |
| Dicalcium silicate (CaO)2.SiO2, C2S | 7-32% | Silicon oxide, SiO2, S | 19-23% |
| Tricalcium aluminate (CaO)3.Al2O3, C3A | 0-13% | Aluminium oxide, Al2O3, A | 2.5-6% |
| Tetracalcium aluminoferrite (CaO)4.Al2O3.Fe2O3, C4AF | 0-18% | Ferric oxide, Fe2O3, F | 0-6% |
| Gypsum CaSO4 | 2-10% | Sulfate |
[edit] Use
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.
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 (C3A), 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 (C3S), 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 (C2S) proceeds more slowly than that of the above compounds slowly increasing later-age strength. All three reactions mentioned above release heat.
[edit] 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
[edit] Types of Portland cement
[edit] General
There 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.
[edit] ASTM C150
There are five types of Portland cements with variations of the first three according to ASTM C150. 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 meeting the specifications for Type I and II has become commonly available 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 stocked by manufacturers 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. This type of cement has a higher cost than most other types. Portland-pozzolan cements can often be used in place of Type IV cement at a significant cost savings.
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. 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 their 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.
In many countries this type of cement is no longer made. It has been replaced by binary blended cements containing more than 60% ground granulated blast furnace 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 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.
[edit] EN 197
EN 197-1 defines 5 classes of common cement that comprise Portland cement as a main constituent. These classes 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 | Portland cement and up to 55% of pozzolanic constituents |
| V | Composite cement | Portland cement, blastfurnace slag and pozzolana or fly ash |
Constituents that are permitted in Portland-composite cements are blastfurnace slag, silica fume, natural and industrial pozzolans, silicious and calcareous fly ash, burnt shale and limestone.
[edit] Safety and environmental effects
[edit] Safety
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 thought to be 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.
[edit] Environmental effects
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." [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]
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. [5]
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 40-45 kWh per tonne cement. 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.
[edit] Cement plants as alternatives to conventional waste disposal or processing
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: [6]
- Car and truck tires; steel belts are easily tolerated in the kilns
- Waste solvents and lubricants.
- Hazardous waste; cement kilns completely destroy hazardous organic compounds
- Bone meal; slaughter house waste due to bovine spongiform encephalopathy contamination concerns
- Waste plastics
- Sewage sludge
- Rice shells
- Sugar cane waste
Industrial by-products used as a raw material:
- Blastfurnace slag (granulated, water quenched)
- Fly ash (from power plants)
- Silica fume (from steel mills)
- Synthetic gypsum (from desulphurisation)
[edit] External links
- World Production of Hydraulic Cement, by Country
- PCA - The Portland Cement Association
- PPC Limited - Pretoria Portland Cement Limited, South Africa
- A cracking alternative to cement
- What is the Difference Between Cement, Portland Cement & Concrete?
- Cement Sustainability Initiative
- www.cimento.org / The World of the Cement
