What Is C4AF (Ferrite Phase) in Cement?

C4AF is tetracalcium aluminoferrite, written 4CaO·Al2O3·Fe2O3 in oxide notation and abbreviated C4AF in cement-chemist shorthand (where C = CaO, A = Al2O3, F = Fe2O3). It constitutes 8-15% of ordinary Portland cement clinker by mass and crystallises from the melt phase during kiln cooling (Taylor, *Cement Chemistry*, 2nd ed., Thomas Telford, 1997) [1]. In the crystallographic literature it is also called brownmillerite. C4AF in clinker is technically a mid-point in a solid-solution series ranging from C2F (dicalcium ferrite) to C2A (dicalcium aluminate); the C4AF stoichiometry is the standard approximation used in the Bogue calculation.

C4AF and [C3A (the aluminate phase)](/en/blog/c3a-aluminate-phase-cement) are together called the flux phases. They begin to melt at approximately 1,280-1,300 °C, forming a liquid phase that reaches roughly 20-30% of the clinker charge at peak burning temperature (~1,450 °C) [2]. This liquid phase is essential: alite (C3S) and belite (C2S) form primarily through solid-state dissolution and re-precipitation reactions that require the liquid medium to proceed at commercially viable rates. Without the ferrite and aluminate flux, achieving equivalent clinker quality would demand materially higher kiln temperatures, raising fuel consumption and refractory wear.

The iron oxide (Fe2O3) in C4AF is the chromophore that gives Portland cement and concrete their characteristic grey to grey-brown colour. White cement is produced by reducing Fe2O3 in the raw mix to below 0.5% and using a reducing atmosphere or rapid water quench at the kiln exit to prevent iron from re-oxidising; the result is a clinker with essentially no C4AF [1]. White cement production is consequently more energy-intensive: without the ferrite-aluminate flux, the kiln must run hotter to achieve comparable clinker quality, and the raw mix must be sourced from very low-iron materials, which limits raw material geography.

C4AF hydrates more slowly than C3A and with a substantially lower heat of hydration (approximately 420 kJ/kg vs 840-870 kJ/kg for C3A, per Taylor, 1997) [1]. In sulfate-bearing environments, C4AF-derived hydrates are less expansive than C3A-derived ettringite, making high-C4AF clinkers intrinsically more sulfate-tolerant. ASTM C150 Type V (sulfate-resistant) cement caps C3A at 5% and effectively increases C4AF by requiring a higher Fe2O3 content in the raw mix [3].

C4AF content is governed by the alumina modulus (AM = Al2O3/Fe2O3). Lowering AM (by increasing Fe2O3 via iron ore or mill-scale additions to the raw mix) shifts the C3A-C4AF balance toward ferrite. Standard OPC targets AM at 1.5-2.5; sulphate-resistant grades hold AM at 0.6-1.0. This is a kiln-feed chemistry decision coordinated during [raw-meal preparation](/en/blog/raw-meal-preparation-cement-plant). Reducing C4AF below a practical minimum is counterproductive: the flux phase is load-bearing for clinkering efficiency, and its elimination (as in white cement) carries the fuel-cost penalty described above.