Influence of micronized magnetite (Fe 3 O 4 ) on mechanical, microstructural, and electromagnetic shielding performance of carbon fiber-reinforced geopolymers

Electromagnetic (EM) pollution has increased in parallel with rapid technological development, underscoring the need for eco-friendly building materials capable of effective shielding. This study investigates carbon fiber-reinforced micronized calcite/rhyolitic tuff–based geopolymer composites incorporating 2%, 4%, and 8% by volume of micronized magnetite (Fe 3 O 4 ) powder. Physical, mechanical, and microstructural properties were evaluated at 2, 28, and 90 days, while EM shielding performance was measured over 900–6000 MHz at 100 MHz intervals. The findings demonstrated that incorporating 2% magnetite resulted in the highest compressive strength, whereas higher dosages reduced strength. Flexural strength decreased with increasing magnetite content; however, carbon fibers contributed to flexural performance through their crack-bridging effect. Magnetite incorporation reduced shrinkage by up to 66%, thereby improving dimensional stability. The EM results demonstrated that micronized magnetite modified the frequency-dependent transmission and shielding responses yet did not enhance the overall total shielding effectiveness. The magnetite-free M0 tile exhibited the highest EM shielding effectiveness, with a measurement of ∼67 dB at 5500 MHz. This result indicates that the semi-conductive carbon-fiber network was the dominant contributor to absorption-based shielding behavior.