Multiscale microstructure and reactivity evolution of recycled concrete fines under gas-solid carbonation

To promote the application of carbonated recycled concrete powder (CRP), it is vital to thoroughly understand the performance of recycled concrete powder (RP) during the carbonation process. This paper presents an experimental study on the multiscale microstructure evolution of CRP and its chemical reactivity development during gas-solid carbonation. The phase transformation, nanostructure and reactivity evolution were investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ²⁹Si nuclear magnetic resonance (NMR) and zeta potential test. Scanning electron microscope and energy-dispersive spectroscopy (SEM-EDS), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) were employed to study the microstructural characteristics. Results indicate that portlandite, ettringite, and unhydrated clinker were carbonated into CaCO₃ and alumina gel within 1d, while the C-S-H subsequently underwent decalcification, yielding silica gel and nano CaCO₃. Regarding microstructure, calcium redistributes during carbonation, and silica phase undergoes polymerization from a nanoscale point of view. The CaCO₃ derived from portlandite firstly formed and refine the pores, followed by the outward distribution of later-generated silica gel and nano calcium carbonate from C-S-H due to space limitations within the particle. The initially formed CaCO₃ can chemically absorb Ca²⁺ in cement paste to facilitate the nucleation and growth of C-S-H, while the highly reactive silica gel obtained in later stage can further promote the formation of C-S-H. This study provides theoretical and technological support to improve the efficiency of carbonation processes and advance their engineering applications.