C3a

C3A refers to tricalcium aluminate, a mineral phase in ordinary Portland cement clinker. Its chemical formula is 3CaO·Al2O3. C3A is one of the four main clinker constituents, alongside C3S (tricalcium silicate), C2S (dicalcium silicate), and C4AF (calcium ferrite). In typical ordinary Portland cement, C3A comprises roughly 5–12% by weight, though cement types designed for sulfate resistance or low-C3A applications have reduced contents.

During cement hydration, C3A is highly reactive and acts in the early stages of setting. In the presence of gypsum (calcium sulfate dihydrate), C3A rapidly forms ettringite (3CaO·Al2O3·3CaSO4·32H2O), which helps regulate the setting time and contributes to early strength development. If gypsum becomes depleted or is not present in sufficient amount, C3A can react further to form monosulfate hydrates, and excessive C3A activity can lead to flash setting or expansion, compromising workability and durability.

C3A content influences sulfate resistance and early-age behavior. Cements with higher C3A content tend to be more reactive to sulfates and are less durable in sulfate-rich environments unless carefully managed. To improve sulfate resistance, manufacturers produce low-C3A cements or add supplementary cementitious materials to adjust performance. C3A is thus a critical parameter in cement chemistry, affecting setting, early strength, heat of hydration, and resistance to chemical attack.