The early hydration of cement in situ was investigated by laser scanning confocal microscopy (LSCM). The cement compact with the size of 35 mm×25 mm×5 mm was prepared. The hydration processes of cement were observed after adding water in the compacted sample. The results show that cement compact hydration has an important reference effect on the normal cement paste hydration. The hydration products appear when the sample contacts with water for 8 h. After 16-h hydration, the most pores are filled in by the hydration products. After 1-d hydration, the hydration process of cement reaches a stabilized stage when the microstructure becomes denser.