Ca3Al2O6

Ca3Al2O6, commonly written as 3CaO·Al2O3, is the chemical formula for tricalcium aluminate (C3A), a calcium aluminate phase that is a key component of cement clinker. It forms when calcium oxide from lime and aluminum oxide sources are fused at high temperatures in cement kilns, producing one of the principal aluminous phases in ordinary Portland cement.

Formation and properties: Ca3Al2O6 is produced by high-temperature reaction of CaO and Al2O3 during clinkering, typically at temperatures around 1350–1450 °C. In cement chemistry, C3A is notable for its rapid hydration and its strong interaction with sulfate-containing species. Its behavior strongly influences early strength development and setting time of cementitious systems.

Hydration behavior: When Ca3Al2O6 hydrates in water, it reacts with calcium sulfate (gypsum) to form ettringite (3CaO·Al2O3·3CaSO4·32H2O) in the early stages, contributing to initial strength. If sulfate supply is limited or depleted, monosulfoaluminate can form instead, and the absence of sulfate can lead to flash setting unless controlled. The quantity of C3A in cement is therefore a critical parameter for sulfate resistance and early-age performance.

Industrial relevance and variants: In ordinary Portland cement, C3A is present as a reactive aluminous phase and is carefully regulated to balance early strength with sulfate resistance. High-alumina cements and specialized calcium aluminate cements rely more heavily on calcium aluminate phases, including Ca3Al2O6, to achieve different hydration characteristics and high-temperature stability.

Crystal chemistry: Ca3Al2O6 has a complex crystal structure with several polymorphic forms depending on temperature and hydration state, and it has been studied extensively in cement and refractory contexts.