A review: Enhanced performance of recycled cement and CO2 emission reduction effects through thermal activation and nanosilica incorporation

Reducing carbon dioxide (CO2) emissions in the cement industry is a key challenge in achieving sustainable development within the context of a dual-carbon economy. Therefore, this study focuses on cement thermal activation and nanosilica-modified recycled cement (RC). Based on recent domestic and international literature, this review summarizes and analyzes the current research status and trends in three aspects: the hydration characteristics of RC, improvement in microstructure, and enhancement of mechanical properties. Additionally, by comparing the CO2 emissions during the production processes of conventional cement and RC, a systematic analysis of the energy-saving and emission reduction effects of using waste cementitious materials to produce RC is conducted. The results indicate that thermal-activated recycled cement exhibits strong initial hydration capacity, an increase in the quantity of C -S -H and CH, a reduction in pore structure, and improved structural denseness, resulting in enhanced mechanical strength. When nanosilica is incorporated, its volcanic ash activity and nucleation effect reduce the setting time of RC, increase its water absorption capacity, and improve the capillary absorption coefficient. However, an excess of nanosilica can lead to strength reduction, with the optimal performance achieved at a nanosilica content of 2%. In the production process of RC, compared to ordinary cement, it can reduce carbon emissions by up to 53%. Therefore, using waste cementitious materials to produce RC has a significant energy-saving and emission reduction effect. Finally, the study summarizes the shortcomings in the current research on RC and proposes future research directions, providing references for the future application and promotion of RC.