Tricalcium aluminate (C3A) is the compound with the highest CaO content among the aluminate phases. C3A contains the aluminum oxide that is not combined in calcium aluminoferrite. It melts incongruently, releasing calcium oxide. Pure C3A crystallizes in cubic form. The lattice can incorporate different ions in solid solution, e.g., Fe3+, Mg2+, Si4+, or K+ and Na+. Due to the incorporation of alkali metals, the symmetry of crystallization changes from cubic to orthorhombic and monoclinic, and this results in a higher hydraulic reactivity as compared to cubic C3A. Consequently, the technical properties of Portland cement clinker are changed. Altogether, C3A reacts rapidly with water, but its hydraulic properties are not very pronounced. However, together with silicates, it improves the initial strength of cement.
