Enhancing recycled cement paste powder as a high-performance supplementary cementitious material via synergistic carbonation and ultrasonic processing

Abstract

Carbonation of recycled cement paste powder (RP) has been widely explored for CO₂ sequestration. However, the cementitious reactivity and mechanical performance of carbonated recycled cement paste powder (CRP) remain insufficient, limiting its effective utilization as a supplementary cementitious material (SCM). To address this limitation, this study proposes a two-step carbonation-ultrasonic modification strategy to transform RP into a nano-reinforced, highly active SCM. In the first step, controlled gas-solid carbonation induces the preferential formation of needle-like aragonite whiskers on RP surfaces. Subsequently, liquid-phase ultrasonic treatment detaches these whiskers while exposing the underlying silica-rich layer. The resulting liquid-solid suspension is directly incorporated into cement paste without further separation. Ultrasonic dispersion converts surface-grown aragonite into nanoscale fillers and nucleation sites, while the exposed silica-rich layer exhibits enhanced pozzolanic reactivity, collectively accelerating C-S-H nucleation and early hydration. This synergistic physical-chemical activation significantly refines pore structure. Consequently, the 28 d compressive strength increases by 37.1% relative to RP and by 26.75% compared with ordinary Portland cement (OPC). The proposed approach provides a scalable pathway for the high-value utilization of RP while simultaneously contributing to CO₂ sequestration and low-carbon cementitious systems.