Cement clinker
Cement clinkeris a solid material produced in the manufacture ofportland cementas 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 bysintering(fusing together without melting to the point ofliquefaction)limestoneandaluminosilicatematerials such asclayduring thecement kilnstage.[1][2]
Composition and preparation[edit]
The Portland clinker essentially consists of four minerals: twocalcium silicates,alite(Ca3SiO5) andbelite(Ca2SiO4), along withtricalcium aluminate(Ca3Al2O6) andcalcium aluminoferrite(Ca2(Al,Fe)2O5). These main mineral phases are produced by heating at high temperatureclaysandlimestone.[3]
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 orsilicon dioxide(SiO2) can be used. Thecalcium 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 therawmixother than limestone) depend on the purity of the limestone. Some of the second raw materials used are: clay,shale,sand,iron ore,bauxite,fly ashandslag.
Portland cementclinker is made by heating a homogeneous mixture of raw materials in arotary kilnat high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about 1,450 °C (2,640 °F).Aluminium oxideandiron oxideare present only as afluxto reduce the sintering temperature and contribute little to the cementstrength.
For special cements, such aslow heat(LH) andsulfate resistant(SR) types, it is necessary to limit the amount oftricalcium aluminateformed.
The clinker and its hydration reactions are characterized and studied in detail by many techniques, includingcalorimetry,strengthdevelopment,X-ray diffraction,scanning electron microscopeandatomic force microscopy.[4]
Uses[edit]
Portland cement clinker (abbreviated k in the European norms) isgroundto a fine powder and used as the binder in many cement products. A small amount ofgypsum(less than 5 wt.%) must be added to avoid the flash setting of thetricalcium 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 produceother types of cementincluding, following the European EN 197-1 standard:[5]
- CEM I: pure Portland clinker (Ordinary Portland Cement, OPC)
- CEM II: composite cements with a limited addition of limestone filler orblast furnace slag(BFS)
- CEM III:BFS-OPCblast furnace cements
- CEM IV:pozzolaniccements
- CEM V: composite cements (with large additions of BFS,fly ashes,orsilica fume)
Clinker is one of the ingredients of an artificial rock imitatinglimestoneand calledpulhamiteafter 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[edit]
Gypsum is added to clinker primarily as an additive preventing the flash settings of the cement, but it is also very effective to facilitate thegrindingof clinker by preventing agglomeration and coating of the powder at the surface of balls andmillwall.[citation needed]
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 asethylene glycol,oleic acid,and dodecyl-benzene sulfonate.[6]
Clinker minerals hydration[edit]
Upon addition of water, clinker minerals react to form different types of hydrates and "set" (harden) as the hydrated cement paste becomesconcrete.Thecalcium silicate hydrates(C-S-H) (hydrates ofaliteandbeliteminerals) represent the main "glue" components of the concrete. After initial setting the concrete continues to harden and to develop itsmechanical 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 againstdesiccation(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.[7]
Contribution to global warming[edit]
As of 2018[update],cement production contributed about 8% of allcarbon emissionsworldwide, contributing substantially toglobal warming.Most of those emissions were produced in the clinker manufacturing process.[8]
See also[edit]
References[edit]
- ^Berger, Ernest Elmer (1929).Calcium Sulphate Retarders for Portland Cement Clinker.United States:U.S. Government Printing Office.p. 29.
- ^"Apparatus for Burning and Grinding Cement".Stone, an Illustrated Magazine: Devoted to the Quarrying and Cutting of Stone for Architectural Uses.New York, New York, United States: Frank W. Hoyt: 55–56. December 1898.
- ^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.
- ^Whiting, Nancy M.; Snyder, Mark B. (2003). "Effectiveness of Portland Cement Concrete Curing Compounds".Transportation Research Record: Journal of the Transportation Research Board.1834:59–68.doi:10.3141/1834-08.S2CID110538072.
- ^Rodgers, Lucy (2018-12-17)."The massive CO2emitter you may not know about ".BBC News.BBC.Retrieved2018-12-17.