200px, Typical clinker nodules 200px, Hot clinker Cement clinker is a solid material produced in the manufacture of Portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually to in diameter. It is produced by

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

(fusing together without melting to the point of

liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of t ...

)

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

and aluminosilicate materials such as

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...

during the cement kiln stage.

Composition and preparation

The Portland clinker essentially consists of four mineral phases: two

calcium silicate Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite ...

s, alite (Ca₃Si) and belite (Ca₂Si), along with tricalcium aluminate (Ca₃Al) and calcium aluminoferrite (Ca₄AlFe). These main mineral phases are produced by heating at high temperature clays and

. Portland cement clinker is made by heating a homogeneous mixture of raw materials in a

rotary kiln A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: * Cement kiln, Cement * Lime kiln, Lime * Refractory, Refractories * M ...

at high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about . Aluminium oxide and

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of w ...

are present only as a

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...

to reduce the sintering temperature and contribute little to the cement strength. For special cements, such as ''low heat'' (LH) and ''sulfate resistant'' (SR) types, it is necessary to limit the amount of tricalcium aluminate formed. The major raw material for the clinker-making is usually limestone mixed with a second material containing clay as a source of alumino-silicate. An impure limestone containing clay or

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

(SiO₂) can be used. The calcium carbonate (CaCO₃) content of these limestones can be as low as 80% by weight. The second raw material (materials in the rawmix other than limestone) depend on the purity of the limestone. Some of the second raw materials used are: clay,

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(O ...

fly ash Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...

and

slag Slag is a by-product of smelting ( pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/ base metals (by ...

. The clinker and its hydration reactions are characterized and studied in detail by many techniques, including calorimetry, strength development, X-ray diffraction, scanning electron microscope and atomic force microscopy.

Uses

Portland cement clinker (abbreviated k in the European norms) is

ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...

to a fine powder and used as the binder in many cement products. A small amount of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...

(less than 5 wt.%) must be added to avoid the flash setting of the tricalcium aluminate (Ca₃Al₂O₆), the most reactive mineral phase (exothermic hydration reaction) in Portland clinker. It may also be combined with other active ingredients or cement additions to produce other types of cement including, following the European EN 197-1 standard:European cement standard EN 197-1 Cement – Part 1: “Composition, specifications and conformity criteria for common cements” which defines 27 distinct common cement products and their constituents. * CEM I: pure Portland clinker (Ordinary Portland Cement, OPC) * CEM II: composite cements with a limited addition of limestone filler or blast furnace slag (BFS) * CEM III: BFS-OPC blastfurnace cements * CEM IV: pozzolanic cements * CEM V: composite cements (with large additions of BFS,

es, or silica fume) Clinker is one of the ingredients of an artificial rock called Pulhamite after its inventor. Other ingredients were Portland cement and sand. Pulhamite can be extremely convincing and was popular in creating natural looking rock gardens in the nineteenth century. Clinker, if stored in dry conditions, can be kept for several months without appreciable loss of quality. Because of this, and because it can be easily handled by ordinary mineral handling equipment, clinker is internationally traded in large quantities. Cement manufacturers purchasing clinker usually grind it as an addition to their own clinker at their cement plants. Manufacturers also ship clinker to grinding plants in areas where cement-making raw materials are not available.

Clinker grinding aids

Gypsum is added to clinker primarily as an additive preventing the flash settings of the cement, but it is also very effective to facilitate the grinding of clinker by preventing agglomeration and coating of the powder at the surface of balls and mill wall. Organic compounds are also often added as grinding aids to avoid powder agglomeration. Triethanolamine (TEA) is commonly used at 0.1 wt. % and has proved to be very effective. Other additives are sometimes used, such as ethylene glycol, oleic acid, and dodecyl-benzene sulfonate.

Clinker minerals hydration

Upon addition of water, clinker minerals react to form different types of hydrates and "set"(harden) as the hydrated cement paste becomes

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...

. The calcium silicate hydrates (C-S-H) (hydrates of alite and belite minerals) represent the main "glue" components of the concrete. After initial setting the concrete continues to harden and to develop its mechanical strength. The first 28 days are the most critical for the hardening. The concrete does not dry but one says that it sets and hardens. The cement is a hydraulic binder whose hydration requires water. It can perfectly set under water. Water is essential to its hardening and water losses must be avoided at the young age to avoid the development of cracks. The step protecting young concrete against

desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

by water evaporation is the curing.

Contribution to global warming

, cement production contributed about 8% of all

carbon emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and l ...

worldwide, contributing substantially to

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate variability and change, Climate change in a broader sense also includes ...

. Most of those emissions were produced in the clinker manufacturing process.{{Cite news, url=https://www.bbc.com/news/science-environment-46455844, title=The massive CO2 emitter you may not know about, last=Rodgers, first=Lucy, date=2018-12-17, work=BBC News, access-date=2018-12-17, language=en-GB

References