Cold-sintered carbonated concrete waste composites for sustainable building materials: effect of carbonation degree on composite properties and CO2 sequestration capacity

A novel carbonated waste concrete composite (CWC) was developed by cold sintering carbonated concrete waste fines (CWFs), enabling permanent CO 2 sequestration and CWFs valorization. This study investigates the effects of the degree of carbonation (DoC) and the quality of hydrated cement paste (HCP), used as a model for the cement paste-rich fraction of CWFs, on the mechanical and microstructural properties of CWC. Results show that compressive strength of CWC increases with DoC up to ∼50%, beyond which excessive carbonation induces phase separation and stiffness mismatch, leading to weak points and strength reduction. The initial HCP quality significantly influences CWC performance. CWC’s strength development is governed by two synergistic mechanisms: mechanical interlocking during cold sintering and in-situ precipitation of a CaCO 3 layer at particle contacts from Ca(HCO 3 ) 2 decomposition. CWC reduced CO 2 emissions by 3.4-fold compared with conventional concrete of equivalent strength, demonstrating its potential as a sustainable construction material.