Tricalcium aluminate

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.

Properties

Tricalcium aluminate forms upon heating a 3:1 mixture of calcium oxide and aluminium oxide above 1300 °C. The crystals are cubic, with unit cell dimension 1.5263 nm 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. 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.
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, 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/m	Designation	Crystal
0–1.0	C_I	Cubic
1.0-2.4	C_II	Cubic
3.7-4.6	O	Orthorhombic
4.6-5.7	M	Monoclinic

Typical chemical compositions are:

Oxide	Mass % Cubic	Mass % Orthorhombic
SiO₂	3.7	4.3
Al₂O₃	31.3	28.9
Fe₂O₃	5.1	6.6
CaO	56.6	53.9
MgO	1.4	1.2
Na₂O	1.0	0.6
K₂O	0.7	4.0
TiO₂	0.2	0.5

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 leads to the phenomenon of "flash set", and a large amount of heat is generated. To avoid this, Portland-type cements include a small addition of calcium sulfate. Sulfate ions in solution lead to the formation of an insoluble layer of ettringite is used, or slag is added to the cement or to the concrete mix. The slag contributes sufficient aluminium to suppress formation of ettringite.

delayed ettringite formation, where concrete is cured at temperatures above the decomposition temperature of ettringite. On cooling, expansive ettringite formation takes place.

Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts 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

Tricalcium aluminate hydrates in the process of forming Portland cement. Using X-ray diffraction, two hydrates form, the hexahydrate abbreviated C3AH6 and a nonadecahydrate abbreviated C4AH19. In the presence of gypsum, a component of typical clinkers, the sulfate forms with the formula C4ASH12.