Biochar-containing construction materials for electromagnetic shielding in the microwave frequency region: the importance of water content

Electromagnetic waves in the X-Band (8.2 – 12.4 GHz frequency) are used for radar, satellite communication, and wireless computer networks. in some countries. Shielding allows to protect humans and electronic devices from harmful effects of these waves. Cement based composites containing conductive material can be used for this purpose., and pyrolyzed carbonaceous residues (biochars) are promising in this respect. Two different biochars originated from wood (CB) and sewage sludge (SSB) pyrolysis were used as fillers in cement-based composites. The electromagnetic shielding properties of these composites have been tested vs the type and amount of biochar added. The influence of water content arising from different curing and ageing in ambient conditions has been investigated for one set of samples. Results show that CB and SSB contain 74% and 30% of graphitic carbon, respectively. In the composites, SSB particles are bulky and scarcely dispersed, while CB particles are elongated and homogenously distributed. High values of Shielding Effectiveness (SE>20 decibel, dB) are achieved at 10 GHz frequency for the composites containing 18% of CB biochar. A large role of ageing was also discovered in sample B18: increasing wet curing increases the shielding effectiveness up to 29 dB, while increasing ageing in air decreases the shielding effectiveness values: after 10 weeks the measured value is about 15 dB. The evidence suggests that the amount of physically adsorbed water is responsible for this behavior, and it should be taken into account when dealing with cement based composites used for electromagnetic shielding.