4. Conclusion
In this study, carbon fiber and graphite are mixed into foam concrete, and the impact of single carbon fiber and compound graphite on the mechanical strength, hydration process, electrical conductivity and electromagnetic parameters of foam concrete are studied. In addition, the microstructure of foam concrete is characterized by SEM and XCT tests. The results indicate that:
(1) The specimen achieves its maximum compressive strength and flexural strength when the carbon fiber content is 0.3wt.% and 0.6wt.% respectively. The compressive strength decreases after adding graphite, and changing the graphite content has only a small influence on strength. When the graphite content is 2wt%, the flexural strength is greatest.
(2) Carbon fiber serves as an obstacle in the cement slurry to restrict the free movement of early water molecules and delays the hydration process of cement at early stages. Graphite can promote the hydration of sulfoaluminate cement at early stages. The hydration product of group C2S1, ettringite, has the strongest diffraction peak.
(3) The carbon fiber in the slurry can effectively split the foam and increase the pore size ratio within 400 µm in the foam concrete. When graphite is further added, it increases the pore size of carbon fiber specimens, and most of small pores have the size of 200 to 400 µm. The pore size distribution results obtained from XCT tests are similar to those obtained from SEM tests.
(4) At 0.6wt.% of carbon fiber doping, the percolation threshold is reached and the conductivity of the specimen is 0.00590 S/m, increased by 114.55% than that of group O. For the group with the addition of carbon fiber and graphite, the percolation threshold is reached when the carbon fiber content is 0.6wt.% and the graphite content is 2wt.%.
(5) The foam concrete without wave absorbents has poor electromagnetic wave loss capability, minimum reflection loss of only -3.58 dB. After adding 0.6wt.% carbon fiber, the electromagnetic wave loss capability is increased and effective bandwidth with reflection loss less than -10 dB at a thickness of 8 mm reaches 1.7 GHz, representing 35.42% of the tested band. After compounding with graphite, the absorption peak of the foam concrete occurs earlier and the bandwidth with larger attenuation constant is wider. Effective bandwidth with reflection loss of less than -10 dB at a thickness of 6 mm reaches 2.5 GHz, representing 52.08% of the tested frequency band.