<p>Steam curing has been proved to cause damage to the later properties of concrete members. The supplementary curing after initial steam curing, however, is critical for improving the performance of steam-cured concrete. In this study, the hydration process of cement paste under different supplementary curing conditions (i.e., standard curing, saturated calcium hydroxide solution, and drying and freezing conditions), the content and morphology of hydration products, as well as the pore structure were deeply investigated for seeking the hydration- and microstructural-related mechanism affected the service performance of steam-cured concrete. In addition, the influence of replacing cement with 20% fly ash (FA) and 10% ground granulated blast furnace slag (GGBFS) on the hydration and microstructure development of steam-cured cement paste was also discussed. The results indicate that supplementary curing in saturated calcium hydroxide solution promotes the continuing hydration of steam-cured cement paste, while supplementary curing in freezing and drying condition inhibits the further hydration. The addition of FA and GGBFS has a significant promoting effect on the later development of the microstructure of the cement paste. Furthermore, the cumulative pore content of the steam-cured cement paste decreases with the curing age increasing, among which the supplementary curing in saturated calcium hydroxide solution has the most remarkable decreasing effect.</p>