Tricalcium aluminate (Ca 3Al 2O 6), or C 3A, is one of the primary phases in the mineral composition of Portland cement. C 3A is highly reactive with water, especially in the presence of sulfates, contributing significantly to the early heat of hydration and the initial set of cement. The rapid reaction of C 3A with water leads to the formation of ettringite when sulfates are present, initially aiding in strength development. However, the mechanism also has implications for durability under various environmental conditions, which can profoundly impact the long-term performance of concrete structures.