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, in some countries, for wireless computer networks. 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 pyrolysed 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 are analysed vs the type and amount of biochar added. The influence of water content arising from different curing and ageing in ambient conditions is 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. Values of Shielding Effectiveness (SE) > 20 decibel at normal incidence are achieved for the composites containing 18% of CB biochar. The influence of ageing was also investigated for a sample with 18% of wood commercial biochar: increasing wet curing increases the shielding effectiveness, while increasing ageing in air decreases the shielding effectiveness values: after 10 weeks, the measured value is about 5 dB less. The evidence suggests that the amount of physically adsorbed water is responsible for this behaviour, and it should be taken into account when dealing with cement based composites used for electromagnetic shielding.