Enhancing the reactivity of mixed recycled aggregate powder and kaolin via mechanochemical activation: synergistic effects of co-milling and impacts on cement hydration

Using construction and demolition waste (C&DW) as an alternative supplementary cementitious material (SCM) is a promising option both for the upcycling of C&DW and decarbonation of cement industry. Mixed recycled aggregate (MRA) is a processed C&DW recycled product with a high potential for large-scale applications as an SCM when processed into powder. However, its relatively low reactivity remains a major technical challenge. This study investigates the effectiveness of beneficiating MRA powder through mechanochemical activation with kaolin and evaluates its performance as an SCM in cement. By leveraging MRA powder’s quartz/feldspar particles as natural milling aids, the co-milling process simultaneously enhances kaolinite amorphization, prevents particle agglomeration and modifies calcite in MRA powder, resulting in a highly reactive material that matches fly ash reactivity with only 14 % kaolinite content. When blended with cement, the milled MRA-kaolin enhances early age hydration, stabilizes ettringite by forming AFm-CO₃ phases, increasing the Ca/Al ratio in C-A-S-H by providing additional calcium and carbonate source and refines the pore structure. Unlike milled kaolin clay, co-milled MRA-kaolin improves strength at both early and late ages at a 25 % replacement level. These results provide fundamental insights into the activation mechanisms of kaolin with MRA powder and establish this approach as a promising strategy for developing new SCMs for sustainable concrete applications.