What is Cement Clinker?
Clinker is a nodular material produced in the kiln 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 1400°C-1500°C 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.
Composition of Clinker
The composition of clinker is examined by two separate approaches:
- mineralogical analysis, using petrographic microscopy and/or x-ray diffraction analysis
- chemical analysis, most accurately by x-ray fluorescence spectrometry
The four main components of clinker are:
- Alite: approximately tricalcium silicate (typically about 65% of the total)
- Belite: approximately dicalcium silicate (typically about 15% of the total)
- Aluminate: very approximately tricalcium aluminate (typically about 7% of the total)
- Ferrite: very approximately tetracalcium aluminoferrite (typically about 8% of the total)
Other substances may be present in small amounts:
- Salt phases - various combinations of sodium, potassium and calcium cations with sulfate and chloride anions, such as:
- Arcanite - K2SO4
- Calcium Langbeinite - K2Ca2(SO4)3
- Aphthitalite - K3Na(SO4)2
- Sylvite - KCl
- Low-temperature phases - various intermediate chemical species that have escaped further thermal processing, such as:
- Spurrite - Ca5(SiO4)2(CO3)
- Ternesite - Ca5(SiO4)2(SO4)
- Ellestadite - Ca10(SiO4)3(SO4)3(OH)2
- Ye'elimite - Ca4(AlO2)6(SO4)
The chemical analysis of clinker is usually given in oxide form, as follows (in oxide weight %):
Free lime= 1.0% CaO
The balance is made by addition of alkali sulphates and minor impurities, such as small amounts of oxides of titanium, manganese, phosphorus, and chromium.
The amounts of different components vary depending on the desired properties of the produced clinker.
Thermochemistry of Clinker
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:
1. 65-125°C: Free water evaporates: latent heat must be supplied. Net heat input: 2145 kJ/kg clinker.
2. 400-650°C: Clays decompose endothermically, and alkalis react with the kiln atmosphere to form liquid sulfates. Net heat input: 42.2 kJ/kg clinker.
3. 500-650°C: Dolomite decomposes endothermically. Net energy input: 19.7 kJ.
4. 650-900°C: Calcium carbonate reacts endothermically with silica to form "incipient belite". Net heat input: 722.5 kJ
5. 700-900°C: Calcium carbonate reacts endothermically with alumina and iron oxide to form incipient aluminate and ferrite. Net heat input: 207.2 kJ
6. 900-1050°C: When all available silica, alumina and iron oxide have reacted, the remaining calcium carbonate decomposes endothermically to calcium oxide. Heat input requirement: 601.9 kJ/kg clinker.
7. 1300-1425°C: Aluminate, ferrite and part of the belite melt endothermically, and belite react with calcium oxide to form alite.
8. 1425-1300°C: Having passed peak temperature, the melt re-freezes exothermically to aluminate, ferrite and belite.
Types of Clinker
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:
- Sulphate Resistant Clinker: It contains 76% alite, 5% belite, 2% tricalcium aluminate, 16 % tetracalcium aluminoferrite, and 1% free calcium oxide. Its production has decreased in recent years because sulphate resistance can easily be obtained by using granulated blast furnace slag in cement production.
- Low Heat Clinker: 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.
- White Clinker: 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.
- Low-alkali Clinker: 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.
- Belite Calciumsulfoaluminate Ternesite (BCT): 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.
Use of Clinker: Conversion to Cement
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:
- ground granulated blast furnace slag cement
- pozzolana cement
- silica fume cement
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.