Tricalcium aluminate

Properties:
– Tricalcium aluminate forms from a mixture of calcium oxide and aluminum oxide at temperatures above 1300°C.
– The crystals are cubic with a unit cell dimension of 1.5263nm.
– It has a density of 3064kg/m³ and melts at 1542°C.
– The structure of pure liquid tricalcium aluminate consists mostly of AlO tetrahedra in an infinite network.
– In Portland cement clinker, it occurs as an interstitial phase and has several polymorphs.

Effect on cement properties:
– Tricalcium aluminate reacts strongly with water and generates a large amount of heat, leading to flash set.
– It forms AFm phases contributing little to strength development.
– It can cause heat release and sulfate attack in concrete, reducing durability.
– Alkali-loaded polymorphs are more reactive and can make set control difficult.
– The presence of tricalcium aluminate in concrete can lead to delayed ettringite formation.

Hydration:
– Tricalcium aluminate hydrates in the formation of Portland cement.
– Two hydrates form during hydration: C3AH6 and C4AH19.
– In the presence of gypsum, the sulfate forms C4ASH12.

References:
– Kosmatka, Steven (2012). Cement.
– Kirk-Othmer Encyclopedia of Chemical Technology.
– Mondal, P.; Jeffery, J. W. (1975). The Crystal Structure of Tricalcium Aluminate.
– Drewitt, James W. E.; et al. (2017). Structure of liquid tricalcium aluminate.https://en.wikipedia.org/wiki/Tricalcium_aluminate