Surface characterization of carbonated recycled concrete fines and its effect on the rheology, hydration and strength development of cement paste

Carbonation treatment can effectively improve the performance of recycled concrete aggregate and fines due to the reactions of CO₂ with CH and C–S–H gel of cement paste. To better understand the mechanisms involved in the performance improvement, the surface properties of carbonated recycled cement paste powder (CRP) and its effect on the rheology, hydration and strength development of cement paste was studied. The results showed that during the carbonation, the surface of CRP was covered by a layer of amorphous silica gel. The generated CaCO₃ was wrapt by the silica gel and seldom exposed. The silica layer led to the poor flowability of CRP-cement paste due to that the silica gel on the surface of CRP has a strong affinity for H₂O. During the very early hydration, the silica gel dissolved and then CaCO₃ was exposed. CaCO₃ is capable of chemically absorbing Ca²⁺, which facilitated the nucleation of C–S–H nuclei and stabilized the C–S–H phase. As a result, the C–S–H grew densely, uniformly and perpendicularly on the surface of CRP. In addition, the chemically absorbing Ca²⁺ enabled the chemical bond to be formed between CaCO₃ and C–S–H. Due to increased C–S–H resulted from reactions of silica gel with CH at the interface and the stronger bond formed between CaCO₃ and C–S–H, the interface between CRP and hydration products was much stronger than that between recycled cement paste powder (RP) and hydration products.