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.
Composition and preparation
The Portland clinker essentially consists of four mineral phases: two calcium silicates, alite (C3S) and belite (C2S), along with tricalcium aluminate (C3A) and calcium aluminoferrite (C4AFe). 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 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 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 wt.%. 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, a.o., 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 little gypsum (less than 5 wt.%) must be added to avoid the flash setting of the tricalcium aluminate (C3A), 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:
- 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, 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.
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.
Upon addition of water, clinker minerals react to form different types of hydrates and set as hydrated cement paste. The calcium silicate hydrates (C-S-H) (hydrates of alite and belite minerals) represent the main "glue" components of the hardened cement paste. After initial setting the hydrated paste continues to harden and to develop its mechanical strength. The first 28 days are the most critical for the cement hardening. The cement does not dry but one says that the cement sets and hardens. The cement is a hydraulic binder whose hydration requires water. It perfectly set under water. Water is essential to its hardening and water losses must be avoided at the concrete young age to avoid the development of cracks. The step protecting young concrete against dessiccation by water evaporation is the curing.
Contribution to global warming
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.
- Siegbert Sprung "Cement" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. doi:10.1002/14356007.a05_489.pub2
- Ferrari, L. (2012). "Reaction of clinker surfaces investigated with atomic force microscopy". Construction and Building Materials. 35: 92–96. doi:10.1016/j.conbuildmat.2012.02.089.
- 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.
- Sohoni, S.; R. Sridhar; G. Mandal (1991). "Effect of grinding aids on the fine grinding of limestone, quartz and portland cement clinker". Powder Technology. 67 (3): 277–286. doi:10.1016/0032-5910(91)80109-V.
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