විකිපීඩියා, නිදහස් විශ්වකෝෂය වෙතින්
වෙත පනින්න: සංචලනය, සොයන්න
Lafarge cement plant in Contes, (France).


Cement used in construction is characterized as hydraulic or non-hydraulic. Hydraulic cements (e.g. Portland cement) harden because of hydration chemical reactions that occur independently of the admixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g. lime and gypsum plaster) must be kept dry in order to gain strength.


Concrete should not be confused with cement because the term cement refers only to the anhydrous powder substance (ground clinker) used to bind the aggregate materials of concrete. Upon the addition of water and/or additives the cement mixture is referred to as concrete, especially if aggregates have been added.

History of the origin of cement[සංස්කරණය]

පූර්ව භාවිත[සංස්කරණය]

It is uncertain where it was first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture (see also: Pozzolanic reaction), but concrete made from such mixtures was first used on a large scale by Roman engineers.[1] 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 and the massive Baths of Caracalla.[2] The vast system of Roman aqueducts also made extensive use of hydraulic cement.[3] 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 columns.

නූතන සිමෙන්ති[සංස්කරණය]


  • Hydraulic render (stucco) for finishing brick buildings in wet climates.
  • 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 was Parker's "Roman cement".[4] 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 කැල්සියම් කාබනේට්. 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 chalk.




Types of modern cement[සංස්කරණය]

පෝර්ට්ලන්ඩ් සිමෙන්ති[සංස්කරණය]

Cement is made by heating limestone (calcium carbonate), with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix . The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement', the most commonly used type of cement (often referred to as OPC).


Portland cement blends[සංස්කරණය]





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.[10]






Non-Portland hydraulic cements[සංස්කරණය]

Pozzolan-lime cements. Mixtures of ground pozzolan and lime are the cements used by the Romans, and can 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.




Calcium sulfoaluminate cements are made from clinkers that include ye'elimite (Ca4(AlO2)6SO4 or C4A3\bar \mathrm{S} 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.[12][13] 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 significantly higher.



The setting of cement[සංස්කරණය]

Cement sets when mixed with water by way of a complex series of hydration chemical reactions still only partly understood. The different constituents slowly hydrate and crystallise while the interlocking of their crystals gives to cement its strength. Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble කැල්සියම් කාබනේට්. After the initial setting, immersion in warm water will speed up setting. In Portland cement, gypsum is added as a compound preventing cement flash setting.

Environmental impacts[සංස්කරණය]


CO2 emissions[සංස්කරණය]

Cement manufacturing releases CO2 in the atmosphere both directly when කැල්සියම් කාබනේට් is heated, producing lime and carbon dioxide,[14] and also indirectly through the use of energy, particularly if the energy is sourced from fossil fuels. The cement industry is the second largest CO2 emitting industry behind power generation. The cement industry produces about 5% of global man-made CO2 emissions, of which 50% is from the chemical process, and 40% from burning fuel.[15] The amount of CO2 emitted by the cement industry is nearly 900 kg of CO2 for every 1000 kg of cement produced. [16]

In certain applications, lime mortar, reabsorbs the CO2 chemically released in its manufacture, and has a lower energy requirement in production. Newly developed cement types from Novacem[17] and Eco-cement can absorb carbon dioxide from ambient air during hardening.[18]

Heavy metal emissions in the air[සංස්කරණය]

In some circumstances, mainly depending on the origin and the composition of the raw materials used, the high-temperature calcination process of limestone and clay minerals can release in the atmosphere gases and dust rich in volatile heavy metals, a.o, thallium,[19] cadmium and mercury are the most toxic. Heavy metals (Tl, Cd, Hg, ...) are often found as trace elements in common metal sulfides (pyrite (FeS2), zinc blende (ZnS), galena (PbS), ...) present as secondary minerals in most of the raw materials. Environmental regulations exist in many countries to limit these emissions.

Heavy metals present in the clinker[සංස්කරණය]


Use of alternative fuels and by-products materials[සංස්කරණය]


ලෝකයේ සිමෙන්ති නිපැයුම[සංස්කරණය]




In 2006 it was estimated that China manufactured 1.235 billion metric tons of cement, which was 44% of the world total cement production.[23] "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."[24]

Cement Production.JPG


මේවාත් බලන්න[සංස්කරණය]


  1. Hill, Donald: A History of Engineering in Classical and Medieval Times, Routledge 1984, p. 106.
  2. Pure natural pozzolan cement.
  3. Aqueduct Architecture: Moving Water to the Masses in Ancient Rome.
  4. A.J. Francis, The Cement Industry 1796-1914: A History, David & Charles, 1977, ISBN 0-7153-7386-2, Chap. 2.
  5. Francis op. cit., Chap. 5
  6. Hewlett op. cit., Chap. 1
  8. U.S. Federal Highway Administration. "Ground Granulated Blast-Furnace Slag". http://www.fhwa.dot.gov/infrastructure/materialsgrp/ggbfs.htm. සම්ප්‍රවේශය කෙරුණු දිනය 2007-01-24. 
  9. U.S. Federal Highway Administration. "Fly Ash". http://www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm. සම්ප්‍රවේශය කෙරුණු දිනය 2007-01-24. 
  10. U.S. Federal Highway Administration. "Silica Fume". http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm. සම්ප්‍රවේශය කෙරුණු දිනය 2007-01-24. 
  11. SINTEF Civil and Environmental Engineering, Cement and Concrete, N-7465 Trondheim, Norway. "Mechanism for performance of energetically modified cement EMC". http://www.emccement.com/Articles/EMC%20mechanism%20paper.pdf. 
  14. EIA - Emissions of Greenhouse Gases in the U.S. 2006-Carbon Dioxide Emissions
  16. Mahasenan, Natesan; Steve Smith, Kenneth Humphreys, Y. Kaya (2003). "The Cement Industry and Global Climate Change: Current and Potential Future Cement Industry CO2 Emissions". Greenhouse Gas Control Technologies - 6th International Conference. Oxford: Pergamon. pp. 995–1000. ISBN 9780080442761. http://www.sciencedirect.com/science/article/B873D-4P9MYFN-BK/2/c58323fdf4cbc244856fe80c96447f44. Retrieved 2008-04-09. 
  17. Novacem | Imperial Innovations
  18. Jha, Alok (2008-12-31). "Revealed: The cement that eats carbon dioxide
    . The Guardian (London). http://www.guardian.co.uk/environment/2008/dec/31/cement-carbon-emissions. Retrieved 2010-04-28.
  19. "Factsheet on: Thallium". http://www.epa.gov/safewater/pdfs/factsheets/ioc/thallium.pdf. සම්ප්‍රවේශය කෙරුණු දිනය 2009-09-15. 
  20. Guidelines for the Selection and Use of Fuels and Raw Materials in the Cement Manufacturing Process, World Business Council for Sustainable Development, published 2005-06-01.
  21. http://web.archive.org/web/20051221043539/http://www.indexmundi.com/en/commodities/minerals/cement/cement_table23.html
  22. http://www.atimes.com/atimes/China/FA07Ad02.html Asia Times January 7, 2004
  23. China now no. 1 in CO2 emissions; USA in second position: more info, NEAA, published 2007-06-19. Retrieved 2007-07-20.
  24. Nov 1, 2004

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බාහිර සබැඳුම්[සංස්කරණය]

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