4CaOFe2O3Al2O3

4CaO·Al2O3·Fe2O3, commonly written as 4CaO·Al2O3·Fe2O3 and abbreviated C4AF, is tetracalcium aluminoferrite, one of the four main mineral phases in Portland cement clinker. Its formula reflects a calcium oxide–rich oxide phase in which aluminum oxide and iron oxide are present as dopants, forming a solid solution within the clinker matrix. C4AF is typically present alongside C3S, C2S, and C3A in varying amounts depending on the raw mix and kiln conditions.

Formation and structure occur during the high-temperature processing of cement raw materials in kilns, where temperatures around 1400 to 1450 degrees Celsius promote the reaction of CaO with Al2O3 and Fe2O3 to form tetracalcium aluminoferrite. The phase is relatively less reactive than the dominant clinker minerals but remains a stable and long-lasting component of hydrated cement.

In terms of properties, C4AF contributes to the overall hydraulic activity of cement, albeit less than C3S and C3A. It influences the microstructure of hydrated cement and can affect setting heat and early strength development to a lesser extent. The iron content also gives C4AF a characteristic brownish to reddish color, which can influence the appearance of cement and concrete.

See also: Portland cement clinker, C3S, C2S, C3A, aluminates and ferrites in cement chemistry.