Cement clinker is a solid material produced in the manufacture of Portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) 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.
The Portland clinker essentially consists of four minerals: two calcium silicates, alite (Ca3Si) and belite (Ca2Si), along with tricalcium aluminate (Ca3Al) and calcium aluminoferrite (Ca4AlFe) (Ca3Si), Ca2Si, Ca3Al, Ca4AlFe are abbreviations reflecting the stoichiometry of key elements). These main mineral phases are produced by heating at high temperature clays and limestone. 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.
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 1,450 C (2,640 F). 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 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.
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:
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.
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.
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.
As of 2018[update], 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.
Clinker is a nodular material produced in the kilning stage during the production of cement and is used as the binder in many cement products. The lumps or nodules of clinker are usually of diameter 3-25 mm and dark grey in color. It is produced by heating limestone and clay to the point of liquefaction at about 1400C-1500C in the rotary kiln. Clinker, when added with gypsum (to control the setting properties of cement and ensure compressive strength) and ground finely, produces cement. Clinker can be stored for long periods of time in a dry condition without degradation of quality, hence it is traded internationally and used by cement manufacturers when raw materials are found to be scarce or unavailable.
The raw materials entered into the kiln are taken at room temperature. Inside the kiln, the temperature continues to rise and when it reaches its peak, clinker is produced by rapid cooling. Though the reaction stages often overlap, they can be expressed in a sharply-defined sequence as follows:
The most common type of clinker is produced for Portland cement and its blends. The types of clinker vary depending on the type of cement for which the clinker is produced. Aside from the Portland cement blends, some special types of cement clinker are listed below:
It contains 29% alite, 54% belite, 2% tricalcium aluminate and 15 % tetracalcium aluminoferrite, with very little free lime. It is no longer produced because cement produced from ordinary clinker and ground granulated blast furnace slag has excellent low heat properties.
It contains 76% alite, 15% belite, 7% tricalcium aluminate, no tetracalcium aluminoferrite, and 2% free lime, but the composition may vary widely. White clinker produces white cement which is used for aesthetic purposes in construction. The majority of white cement goes into factory-made pre-cast concrete applications.
Reduction of alkali content in clinker is done by either replacing the raw-mix alumina source with another component (thus obtaining a more expensive material from a more distant source), or installing an "alkali bleed", which involves removing some of the kiln system's high temperature gases (which contain the alkalis as fume), resulting in some heat wastage.
This concept is used in producing a type of clinker with up to 30% less carbon dioxide emission. Energy efficiency improves and the electricity costs for the manufacturing process are about 15% lower as well.
Clinker, combined with additives and ground into a fine powder, is used as a binder in cement products. Different substances are added to achieve specific properties in the produced cement. Gypsum added to and ground with clinker regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid, and dodecyl-benzene sulphonate. The most notable type of cement produced is Portland cement, but certain active ingredients of chemical admixtures may be added to clinker to produce other types of cement, such as:
Clinker is primarily used to produce cement. Since it can be stored in dry condition for several months without noticeable deterioration, it is traded internationally in large amounts. Cement manufacturers buy clinker for their cement plants in areas where raw materials for cement are scarce or unavailable.
Availability and implementation: Source code and documentation for clinker and clustermap.js is available on GitHub (github.com/gamcil/clinker and github.com/gamcil/clustermap.js, respectively) under the MIT license. clinker can be installed directly from the Python Package Index via pip.
Portland cement clinker is a dark grey nodular material made by heating ground limestone and clay at a temperature of about 1400 C - 1500 C. The nodules are ground up to a fine powder to produce cement, with a small amount of gypsum added to control the setting properties.
Nodules range in size from 1mm to 25mm or more and are composed mainly of calcium silicates, typically 70%-80%. The strength of concrete is mainly due to the reaction of these calcium silicates with water.Portland cement clinker contains four principal minerals:
Typical clinker analysis (oxide weight%).SiO2Al2O3Fe2O3CaOMgOK2ONa2OSO3LOIIRTotal220.127.116.116.61.00.60.21.01.50.598.9Free lime = 1.0% CaO
When limestone is converted to clinker, CO2 is released (also known as process emissions). A substantial amount of heat is needed to start and sustain the chemical reaction, leading to further CO2 emissions (also known as combustion emissions). The clinker is finely ground and mixed with gypsum and often with alternative raw materials to make cement.Over the past decades, the cement industry has invested heavily in four main levers to reduce direct CO2 emissions:Thermal efficiencyCement kilns have become highly energy efficient as older plants are being upgraded or replaced.The energy intensity of cement manufacturing is influenced by regional characteristics such as raw material moisture content and burnability, fuel types, plant size distribution, and cement standards. Because of these variables, especially a higher thermal substitution rate, CEMBUREAU has projected an energy consumption of 3.300 MJ/tonne clinker by 2050.