Although the C3A content in clinker is relatively low, its behavior is crucial because it controls the setting time of cement.The presence of minor elements, mainly alkalis (including Na+ and K+) during the clinkerization modifies the structure of C3A and different polymorphs structures can be obtained.Previous reports have shown that the hydration of cubic C3A and Na-doped C3A, which exhibits an orthorhombic structure, follow different paths, forming hydration products with distinct morphologies.The two minerals are known to have different hydration mechanisms.Therefore, it is essential to understand the C3A reactivity during the early hydration of Portland cement to achieve an acceptable workability of the fresh mixes and to reduce or eliminate problems related to abnormal setting times.The present paper presents a detailed assessment of the hydration of cubic and orthorhombic C3A in a calcium sulfate solutions using scanning transmission X-ray microscopy (STXM).