3CaOAl2O3

3CaOAl2O3, commonly written as 3CaO·Al2O3 or Ca3Al2O6, is the chemical formula for tricalcium aluminate, a principal aluminous phase in Portland cement clinker. In cement chemistry it is abbreviated as C3A and is one of the four major clinker phases, alongside C3S (tricalcium silicate), C2S (dicalcium silicate), and C4AF (tetracalcium aluminoferrite).

Formation and composition: C3A forms during high-temperature clinker production in cement kilns when calcium oxide from lime reacts with aluminum-bearing materials. It is a relatively reactive phase and influences the early hydration behavior of cement. Raw materials typically include limestone and sources of alumina such as bauxite or industrial byproducts containing aluminum.

Hydration and setting: When water comes into contact with C3A in the presence of gypsum (calcium sulfate), it hydrates rapidly to form ettringite, a phase with the formulation 3CaO·Al2O3·3CaSO4·32H2O. This reaction helps regulate the initial setting time of cement. If sulfate is depleted, hydration can proceed to form monosulfate and other aluminates, which can affect long-term durability and dimensional stability. Because of its high reactivity, C3A contributes to early strength but also to risks of flash setting if sulfate control is inadequate.

Industrial role and regulation: The amount of C3A in cement is tightly controlled by standards to balance early strength and resistance to sulfate attack, as excessive or insufficient C3A can influence performance in different exposure environments. In ordinary Portland cement, C3A content is a key specification parameter used by manufacturers and testing laboratories.