Phase separation of recycled concrete powder during grinding and consequent influences on its hydration behaviors in cement paste

Recycled concrete powder (RCP) collected from waste concrete recycling presents significant variation in both composition and hydration behavior, while available literatures were mainly carried out on RCPs derived from target concrete, mortar, or cement paste, RCPs collected from actual waste concrete is totally different from RCPs reported, their roles has not been clarified yet due to its complexity in components, leading to substantial deviation in understanding the roles of RCP in cement-based materials. In this study, RCP collected from a waste concrete reutilization plant was classified into five fractions, then effects of RCP fractions on heat evolution and mechanical properties of cement pastes were investigated. The results showed that hydration products and un-hydrated clinker were predominantly concentrated in fine RCP fraction, while higher proportion of aggregate grains (mainly quartz, anorthite, and dolomite) in coarse RCP fraction, indicating that cement paste was pulverized or milled into finer particles prior to fine and coarse aggregates due to its diminished elastic modulus. Compared with coarse RCP fraction, fine RCP fraction showed significant acceleration of cement hydration and higher strength contribution, which can be attributed to the remarkable pore refinement of fine particle and conspicuous seeding effect of C–S–H in RCP. Furthermore, increased dissolution of Si4+ and Al3+ from the fine RCP fraction was observed, then more hydration products were generated to densify the microstructure of cement paste. The results provided a deeper insight into the roles of RCP in cement-based materials, and then facilitate the efficient utilization of RCP to achieve resource-conservation development.