Electrical properties of cement paste obtained from impedance spectroscopy: Fid-Bau Portal

Electrical properties of cement paste obtained from impedance spectroscopy

Kim, H.C. / Kim, S.Y. / Yoon, S.S.

The impedance spectrum of hydrating Portland cement paste was represented by an equivalent circuit involving a serial combination of R_s and capacitance C_j(f) with R_b in parallel for bulk effects and a parallel combination of R_ct and C_b(f) for the interface effects. The increase of bulk d.c. resistance, R_b, during hydration indicates that the conduction path between two electrodes becomes complex and truncated by the hydration process. The decrease of the overall resistance, R_s, of water solution in blocked pores during hydration, indicates that the blocked pores are continuously produced by hydration products. C_j(f) of the constant phase element (CPE) in the bulk is due to blocking ions by cement grains or by hydration products in the distribution of heterogeneous pores. The impedance, Z(f), of the bulk part in the equivalent circuit represents the cement paste as a heterogeneous material having a relaxation time distribution due to the various distributions of pore size and many kinds of ions in the hydrating cement paste. The R_ct and C_b(f) represent, respectively, the charge transfer resistasnce and barrier process at the cement paste/electrode interface. The four-stage variation of the heat evolution rate was explained in terms of the changes in R_ct and R_b, indicating that the resistance of the cement paste is closely related to the microstructural evolution in the hydrating cement paste. The critical relaxation time of a bulk depressed arc had a nearly constant value in spite of microstructural changes in the hydrating cement paste, indicating that the increase of bulk d.c. resistance during hydration accompanies the decrease of bulk capacitance.