The microwave absorption characteristics of both powdered and rectangular blocks of an electrically conductive concrete were measured and compared to a normal Portland cement concrete in a multimode cavity. The variables investigated were: irradiation time, sample mass and incident power for the powdered samples and sample orientation, water additions and multiple irradiations for the block samples. The results were quantified in terms of the microwave absorption efficiency (η_(a)). The absorption efficiency of the electrically conductive concrete was significantly higher than the Portland cement control concrete. For both of the concretes, hot spot formation occurred in the vicinity of the corner of the block. For the electrically conductive concrete this phenomenon took place close to the surface and resulted in combustion of the carbon and disintegration of the concrete. For the normal Portland cement concrete, the hot spot formed below the surface where fracturing, degradation and melting occurred.
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