Concrete quality depends on various stress factors. The impedance spectroscopy method, as one of the non-destructive testing method group, was used to characterize concrete specimens and track the changes in the concrete spectrum after heat stress. The temperature range was between 0°C and 1200°C. Variances in the tan δ (f) and Im Z(f) or Re Z(f) of the specimens under investigation have been observed. The specimen quality has been described by means of the loss type prevailing in the material. The results of this study are expected to provide information about the correlation between the relative permittivity and changes in the structure for heat stress samples.

New materials based on alkali-activated slag mortars may be an important contribution of applied research, which strives to offer the use of waste materials as a full replacement for the currently used binders and explores the impact of forms of carbon incorporation on the physical and mechanical properties. The influence of graphite powder addition on the electrical properties of alkali-activated slag mortars was investigated. 1–10 wt. % of graphite powder (with 1% step) was incorporated into the mortar and the electric resistance spectra, permittivity and loss factor of the prepared prismatic samples were measured. The unique Vector analyzer R&S ZNC with a coaxial probe DAK-12 from Speag was used to determine permittivity and loss factor, electrical resistance was measured using two channel oscilloscope. Higher content of graphite powder increases the electrical conductivity, which makes the tested materials more sensitive and measurable via electromagnetic methods. The most striking change in the relative conductivity was observed at excitation frequency of the external electric field of 10 kHz. At high frequencies electrical excitation field 10 MHz to 3 GHz were determined by vector analyzer value of the real part of permittivity, which fell from the 20 to the 4. Dissipation Factor shows in his semi arc spectrum a peak at 1.2 GHz. These measurements are among the building materials still little used and open new possibilities of diagnostics.