4CaOAl2O3Fe2O3

4CaOAl2O3Fe2O3, commonly abbreviated as C4AF, is a major phase present in Portland cement clinker. It is known as tetracalcium aluminoferrite, a solid solution composed of four calcium oxide units with aluminum oxide and ferric oxide in the lattice. The formula is often written as 4CaO·Al2O3·Fe2O3, but the exact proportions of Al2O3 and Fe2O3 can vary, giving a range of ferrite compositions within the C4AF family.

Formation and occurrence: C4AF forms during the high-temperature clinkering of cement raw materials, typically in the range of about 1350–1450°C. Its presence reflects the combined availability of lime (CaO), alumina (Al2O3), and ferric oxide (Fe2O3) in the raw mix and the conditions of the clinker furnace. It is one of the four primary clinker phases alongside C3S, C2S, and C3A.

Properties and hydration: C4AF is brown to dark brown in color and is less reactive than the silicate phases C3S and C2S. Its contribution to early strength development is modest, and it participates in cement hydration by forming ferrite hydrates during curing. The ferrite phase influences the microstructure of the cement paste and can affect the color of the cement, particularly in unground clinker or low-bleed cements.

Industrial relevance: The C4AF content of clinker is controlled by the raw material composition and burning conditions and typically constitutes a minority share of the mass. While not the primary driver of early strength, C4AF contributes to the overall hydration behavior, color, and heat evolution of cement. It is commonly referenced alongside C3S, C2S, and C3A in discussions of cement chemistry.