Effect of CO2 curing on mechanical and physical properties of the recycled aggregates containing silica fume

CO2 curing is a promising method for enhancing the properties of the recycled concrete aggregates (RCAs) due to its economic and environmental benefits. However, the knowledge about its effectiveness in improving the properties of aggregates derived from blended supplementary cementitious materials (SCMs) remains limited. This study explores the impact of CO2 curing on the mechanical and physical properties, mineralogical composition, and microstructural changes of recycled aggregates incorporating varying amounts of silica fume (SF). The results showed that upon the incorporation with 5 wt% SF, CO2 curing increased the compressive strength of the aggregate samples by 17.9 %. The microhardness improved from 50 to 56 HV with the addition of 10 wt% SF. Moreover, CO2 curing modified the physical characteristics of the samples regardless of SF content. However, it caused mechanical degradation in samples containing 20 wt% SF, at both marco- and micro-scale. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) revealed that higher SF dosages led to the formation of poorly crystallised CaCO3 during CO2 curing. Additionally, Low-field H-1 nuclear magnetic resonance (LF NMR), N-2 adsorption/desorption, scanning electron microscopy (SEM), and thermodynamic models showed that the carbonation products enlarged the capillary pores, and reduced the solid volume of the hydration products. Those findings underscore the importance of calcium hydroxide in protecting against mechanical degradation in SF-blended recycled aggregates during CO2 curing. This study provides an insight of the carbonation of SF-blended composites, potentially accelerating the application of CO2 curing on RCAs.