The electromagnetic wave absorption and mechanical properties of recycled cement mortar modified by recycled concrete powder and CNTs/Nano-Fe 3 O 4

This study prepared electromagnetic wave (EMW)-absorbing recycled cement mortar using recycled concrete powder (RCP), cement, natural river sand, and a CNTs/Nano-Fe3O4 composite absorber. The effects of RCP content and CNTs/Nano-Fe3O4 dosage on phase composition, microstructure, EMW absorption performance, compressive strength, and carbon footprint were investigated. The results show that RCP and CNTs/Nano-Fe3O4 did not significantly change the main crystalline phases, but excessive dosages promoted pores, microcracks, interfacial defects, and nanoparticle agglomeration. When RCP content exceeded 15 wt%, the effective absorption bandwidth and compressive strength decreased, although peak absorption was enhanced. Excessive CNTs/Nano-Fe3O4 also reduced bandwidth and strength because of reduced workability, agglomeration, and deteriorated impedance matching. Although R20H10 exhibited a lower minimum reflection loss of −33.86 dB, R15H10 showed the best comprehensive performance, with the widest effective absorption bandwidth of 7.95 GHz, a minimum reflection loss of −23.63 dB, compressive strength of 45.98 MPa, and CO2 emission reduction of 9.59%. This work provides a feasible route for low-carbon, structure–function integrated cement-based EMW absorbing materials.