Microwave absorption properties of cementitious composites containing carbonyl iron powder (CIP) and fly ash: Formation and effect of CIP core–shell structure

The superior microwave absorption of cementitious composites is beneficial for reducing buildings’ microwave pollution. This study prepared cementitious composites and characterized their microwave absorption, electromagnetic properties, hydration products, and microstructures. The results confirmed that carbonyl iron powder (CIP)–calcium particles with a core–shell structure formed during the hydration of the composites without fly ash. The core–shell structure could suppress the skin effect and the dielectric loss of the cementitious composites, enhancing their impedance matching and microwave absorption. The formation of the calcium compound shell resulted in the relative lack of calcium during the hydration and enhanced the polymerization of C(-A)-S-H gels in the samples without fly ash. Increasing the carbonyl iron powder content of the composites containing high-volume fly ash improved the Debye relaxation processes and the conductivity due to a lack of a shell structure around the CIP, which enhanced their dielectric loss and magnetic loss.