cement clinker

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 and aluminosilicate materials such as clay during the cement kiln stage.

Composition and preparation

The Portland clinker essentially consists of four mineral phases: two calcium silicates, 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 limestone.

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 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 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 (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, sand, iron ore, bauxite, fly ash and slag.
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 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 blastfurnace 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 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. 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 by water evaporation is the curing.

Contribution to global warming

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

See also

* Environmental impact of concrete

References

Cement
Concrete
Limestone
Climate forcing