Recycled concrete powder (RCP) is produced via waste concrete processing. The complex sources of waste concrete form a complex and highly diverse composition in RCP. Different components in RCP affect cement hydration; however, the underlying mechanism of these influences remains unclear. The mechanism of the main components of RCP, such as unhydrated cement, C–S–H and finely ground sand, on the hydration of cement paste still lacks a comprehensive and systematic microscopic analysis. Herein, we selected three RCPs with different components: RCP1 (comprising ~11% unhydrated cement and 67% C–S–H), RCP2 (comprising ~56% C–S–H and 40% finely ground sand), and RCP3 (comprising 25% C–S–H and 75% finely ground sand). The influence of different components of RCP on cement hydration was systematically studied. Unhydrated cement in RCP is rehydrated in the mixed system, producing additional hydration products and increasing the total heat of hydration. These secondary hydration products fill the pores of RCP, enhancing the mechanical properties of the cement paste. Calcite in RCP reacts with monosulfate to form carboaluminate, and hemicarboaluminate is converted into monocarboaluminate after 28 days. Although C–S–H and finely ground sand in RCP exhibited different nucleation, water absorption, and filling effects, their total impact on the hydration heat and pore structure of the mixed system was similar. The mechanism is further verified by the strength prediction model. These findings elucidated the reaction mechanisms of RCP incement paste, which can be used for promoting resource-efficient and sustainable development.