Tricalcium aluminate

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Tricalcium aluminate
Names
Other names
aluminate, C3A
Identifiers
3D model (JSmol)
ECHA InfoCard 100.031.744
EC Number
  • 234-932-6
PubChem CID
Properties
Ca3Al2O6, or 3CaO·Al2O3
Molar mass 270.193 g/mol
Density 3.064 g/cm3
Melting point 1,542 °C (2,808 °F; 1,815 K)(decomposes)
Hazards
GHS pictograms GHS-pictogram-exclam.svg
GHS Signal word Warning
H319
P264, P280, P305+351+338, P313
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Tricalcium aluminate Ca3Al2O6, often formulated as 3CaO·Al2O3 to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates. It does not occur in nature, but is an important mineral phase in Portland cement.

Contents

Properties

Pure tricalcium aluminate is formed when the appropriate proportions of finely divided calcium oxide and aluminium oxide are heated together above 1300 °C. The pure form is cubic, with unit cell dimension 1.5263 nm [1] and has density 3064 kg·m−3. It melts with decomposition at 1542 °C. The unit cell contains 8 cyclic Al6O1818− anions, which can be considered to consist of 6 corner sharing AlO4 tetrahedra. [2] The structure of pure liquid tricalcium aluminate contains mostly AlO4 tetrahedra in an infinite network, with a slightly higher concentration of bridging oxygens than expected from the composition and around 10% unconnected AlO4 monomers and Al2O7 dimers. [3]

In Portland cement clinker, tricalcium aluminate occurs as an "interstitial phase", crystallizing from the melt. Its presence in clinker is solely due to the need to obtain liquid at the peak kiln processing temperature (1400–1450 °C), facilitating the formation of the desired silicate phases. Apart from this benefit, its effects on cement properties are mostly undesirable. It forms an impure solid solution phase, with 15-20% of the aluminium atoms replaced by silicon and iron, and with variable amounts of alkali metal atoms replacing calcium, depending upon the availability of alkali oxides in the melt. The impure form has at least four polymorphs:

Alkali % m/mDesignationCrystal
0–1.0CICubic
1.0-2.4CIICubic
3.7-4.6OOrthorhombic
4.6-5.7MMonoclinic

Typical chemical compositions are:

OxideMass % CubicMass % Orthorhombic
SiO23.74.3
Al2O331.328.9
Fe2O35.16.6
CaO56.653.9
MgO1.41.2
Na2O1.00.6
K2O0.74.0
TiO20.20.5

Effect on cement properties

In keeping with its high basicity, tricalcium aluminate reacts most strongly with water of all the calcium aluminates, and it is also the most reactive of the Portland clinker phases. Its hydration to phases of the form Ca2AlO3(OH)·nH2O leads to the phenomenon of "flash set" (instantaneous set), and a large amount of heat is generated. To avoid this, Portland-type cements include a small addition of calcium sulfate (typically 4-8%). Sulfate ions in solution lead to the formation of an insoluble layer of ettringite (3CaO·Al2O3·3CaSO4·32H2O) over the surface of the aluminate crystals, passivating them. The aluminate then reacts slowly to form the AFm phase 3CaO·Al2O3·CaSO4·12H2O. These hydrates contribute little to strength development.

Tricalcium aluminate is associated with three important effects that can reduce the durability of concrete:

Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts (>1%) in cement make set control difficult, and the cement becomes excessively hygroscopic. The cement powder flowability is reduced, and air-set lumps tend to form. They withdraw water from gypsum on storage of the cement, leading to false set. For this reason, their formation is avoided wherever possible. It is more energetically favorable for sodium and potassium to form sulfates and chlorides in the kiln, but if insufficient sulfate ion is present, any surplus alkalis congregate in the aluminate phase. The feed and fuel in the kiln system are preferably controlled chemically to keep the sulfate and alkalis in balance. However, this stoichiometry is only maintained if there is substantial surplus oxygen in the kiln atmosphere: if "reducing conditions" set in, then sulfur is lost as SO2, and reactive aluminates start to form. This is readily monitored by tracking the clinker sulfate level on an hour-to-hour basis.

Hydration steps

Water reacts instantly with tricalcium aluminate. Hydration likely begins already during grinding of cement clinker due to residual humidity and dehydration of gypsum additives. Initial contact with water causes protonation of single bonded oxygen atoms on aluminate rings and leads to the formation of calcium hydroxide. [4] The next steps in the sequence of the hydration reaction involve the generated hydroxide ions as strong nucleophiles, which fully hydrolyze the ring structure in combination with water.

Related Research Articles

Cement hydraulic binder used in the composition of mortar and concrete

A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is only behind water as the planet's most-consumed resource.

Portland cement binder used as basic ingredient of concrete

Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the late 19th century by Joseph Aspdin, and usually originates from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum. Several types of Portland cement are available. The most common, called ordinary Portland cement (OPC), is grey, but white Portland cement is also available. Its name is derived from its similarity to Portland stone which was quarried on the Isle of Portland in Dorset, England. It was named by Joseph Aspdin who obtained a patent for it in 1824. However, his son William Aspdin is regarded as the inventor of "modern" Portland cement due to his developments in the 1840s.

Calcium oxide Chemical compound of calcium

Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "lime" connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the single chemical compound calcium oxide. Calcium oxide that survives processing without reacting in building products such as cement is called free lime.

Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.

In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminum atom.

Ettringite sulfate mineral

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.

Yeelimite sulfate mineral

Ye'elimite is the naturally occurring form of calcium sulfoaluminate, Ca4(AlO2)6SO3. It gets its name from Har Ye'elim in Israel in the Hatrurim Basin west of the Dead Sea where it was first found in nature by Shulamit Gross.

Alite is an impure form of tricalcium silicate, Ca3SiO5, sometimes formulated as 3CaO·SiO2 (C3S in cement chemist notation, CCN) with typically 3-4% of substituent oxides. It is the major, and characteristic, mineral phase in Portland cement. The name was given by Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement. Alite is a name in common use in the cement industry, but it is very rare in nature (known as hatrurite).

Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation).

Clinker (cement) cement type

In the manufacture of Portland cement, clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) in diameter, produced by sintering limestone and aluminosilicate materials such as clay during the cement kiln stage.

Dodecacalcium hepta-aluminate

Dodecacalcium hepta-aluminate (12CaO·7Al2O3, Ca12Al14O33 or C12A7) is an inorganic solid that occurs rarely in nature as the mineral mayenite. It is an important phase in calcium aluminate cements and is an intermediate in the manufacture of Portland cement. Its composition and properties have been the subject of much debate, because of variations in composition that can arise during its high-temperature formation.

Calcium aluminate cements

Calcium aluminate cements are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement" and "Ciment fondu" in French. They are used in a number of small-scale, specialized applications.

Monocalcium aluminate (CaAl2O4) is one of the series of calcium aluminates. It does occur in nature, although only very rarely, as two polymorphs known as krotite and dmitryivanovite, both from meteorites. It is important in the composition of calcium aluminate cements.

Calcium aluminoferrite

Calcium aluminoferrite (Ca2(Al,Fe)2O5) is a dark brown crystalline phase commonly found in cements. In the cement industry it is termed ferrite. It also exists in nature as the rare mineral brownmillerite.

An AFm phase is an "alumina, ferric oxide, monosulfate" phase, or aluminate ferrite monosulfate, or Al2O3, Fe2O3 mono, in cement chemist notation. AFm phases are important in the hydration of hydraulic cements.

Cement mill

A cement mill is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical roller mills which are more effective than ball mills.

Friedel's salt is an anion exchanger mineral belonging to the family of the layered double hydroxides (LDHs). It has affinity for anions as chloride and iodide and is capable of retaining them to a certain extent in its crystallographical structure.

The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.

Energetically modified cements (EMC) are a class of cementitious materials made from pozzolans, silica sand, blast furnace slag, or Portland cement.

Sulfate attack in cement, mortar and concrete.

References

  1. H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN   0-12-683900-X, pp 23
  2. P. Mondal and J. W. Jeffery, The crystal structure of tricalcium aluminate, Ca3Al2O6, Acta Crystallogr. (1975). B31, 689-697,doi : 10.1107/S0567740875003639
  3. Drewitt, James W. E.; et al. (2017). "Structure of liquid tricalcium aluminate". Physical Review B. 95: 064203. doi:10.1103/PhysRevB.95.064203. hdl:1983/2dd23037-2924-4b98-a6a1-69c2393cb7f1 . Retrieved 2018-07-12.
  4. R. K. Mishra, L. Fernández-Carrasco, R. J. Flatt, H. Heinz, A Force Field for Tricalcium Aluminate to Characterize Surface Properties, Initial Hydration, and Organically Modified Interfaces in Atomic Resolution, Dalton Trans. (2014). 43, 10602–10616,doi : 10.1039/C4DT00438H