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 (fusing together without melting to the point of liquefaction)

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and aluminosilicate materials such as

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

during the cement kiln stage.

Composition and preparation

The Portland clinker essentially consists of four minerals: two calcium silicates, alite (Ca₃SiO₅) and belite (Ca₂SiO₄), along with tricalcium aluminate (Ca₃Al₂O₆) and calcium aluminoferrite (Ca₂(Al,Fe)₂O₅). These main mineral phases are produced by heating at high temperature

s and

. 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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

(SiO₂) can be used. The calcium carbonate (CaCO₃) content of these limestones can be as low as 80% by weight. During the calcination process that occurs in the production of clinker, limestone is broken into lime ( calcium oxide), which is incorporated into the final clinker product, and carbon dioxide which is typically released into the atmosphere. 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, sand, iron ore, bauxite, fly ash and slag. Portland cement clinker is made by heating a homogeneous mixture of raw materials in a rotary kiln at high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about . Aluminium oxide and

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

are present only as a flux 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 clinker and its hydration reactions are characterized and studied in detail by many techniques, including calorimetry, strength development,

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

, scanning electron microscope and atomic force microscopy.

Uses

Portland cement clinker (abbreviated k in the European norms) is ground to a fine powder and used as the binder in many cement products. A small amount of gypsum (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 blast furnace cements * CEM IV: pozzolanic cements * CEM V: composite cements (with large additions of BFS, fly ashes, or silica fume) Clinker is one of the ingredients of an artificial rock imitating

and called pulhamite after its inventor, James Pulham (1820–1898). 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. 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. Young concrete is protected against desiccation (evaporation of unreacted water). Traditional methods for preventing desiccation involve covering the product with wet burlap or use of plastic sheeting.. For larger projects, such as highways, the surface is sprayed with a solution of curing compound that leaves a water-impermeable coating.

Contribution to global warming

, cement production contributed about 8% of all carbon emissions worldwide, contributing substantially to

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

. 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, publisher= BBC, access-date=2018-12-17, language=en-GB

References