What Is C3A (Aluminate Phase) in Cement?
C3A (tricalcium aluminate) is 5-10% of Portland cement clinker, the fastest-reacting phase. Without gypsum it flash sets; low-C3A resists sulfate.
C3A (tricalcium aluminate, 3CaO·Al2O3) is the fastest-reacting phase in Portland cement clinker, typically 5-10% by mass, and is the phase that gypsum is added at grinding to control: without sulfate regulation, C3A causes flash set within minutes of water contact [1]. It is also the phase whose content most directly governs sulfate resistance, which is why structural specifications in aggressive ground conditions specify C3A limits rather than cement types alone. For the full four-phase context, see what the chemical composition of clinker is.
Common questions about this topic
C3A is tricalcium aluminate, written 3CaO·Al2O3 in oxide notation and abbreviated C3A in cement-chemist shorthand (where C = CaO, A = Al2O3). It forms during sintering in the rotary kiln and constitutes 5-10% of ordinary Portland cement clinker by mass (Taylor, *Cement Chemistry*, 2nd ed., Thomas Telford, 1997) [1]. C3A and aluminate phase are interchangeable terms in plant and laboratory documentation. The phase crystallises in a cubic form for most commercial clinkers; orthorhombic C3A occurs in high-alkali clinkers where Na2O exceeds roughly 1%.
Without sulfate control, C3A reacts almost instantaneously with water, forming dense calcium aluminate hydrates (C3AH6, katoite) that rigidly set the paste before workability is established. This is flash set. During cement grinding, gypsum (CaSO4·2H2O) is interground with clinker, typically at 3-5% by mass (regulated as SO3 in EN 197-1 and ASTM C150) [3]. Dissolved sulfate ions react preferentially with C3A to form ettringite (C6AS3H32), a needle-shaped crystal coating that passivates the C3A surface and slows the reaction to the target setting time window:
C3A contributes modestly to 1-3 day compressive strength; its primary effect is on setting time rather than long-term load-bearing capacity. Its heat of hydration is the highest of the four main clinker phases, approximately 840-870 kJ/kg (Taylor, 1997) [1], substantially above [C3S (alite)](/en/blog/c3s-alite-cement) at ~500 kJ/kg and [C4AF (ferrite)](/en/blog/c4af-ferrite-phase-cement) at ~420 kJ/kg. High-C3A cements therefore carry elevated thermal risk for mass concrete.
Sulfate ions in soil or groundwater react with hydrated C3A to form expansive ettringite (late-stage, after the normal ettringite has converted to monosulfate), causing swelling and disruption of the hardened paste. This is sulfate attack. ASTM C150 limits C3A content to achieve sulfate resistance [4]:
C3A content is set by controlling Al2O3 in the raw mix (typically from clay, shale, or bauxite additions) relative to Fe2O3. A higher alumina modulus gives more C3A and less C4AF; a lower AM gives more C4AF and less C3A. For sulphate-resistant clinker production, AM is held at 0.6-1.0 versus 1.5-2.5 for standard OPC. This is a kiln-feed chemistry decision made at [raw-meal preparation](/en/blog/raw-meal-preparation-cement-plant).
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