Electrochemical Impedance Spectroscopy study on the absorption and evaporation processes in natural stones

Abstract

Efforts have been made to create new procedures based on Electrochemical Impedance Spectroscopy (EIS) to study several characteristics of cementitious materials with practical interest. Similar approaches can be followed to study other materials, in which the comprehension of their physical, transport-related and durability properties are also needful. Natural stones are one of these materials, wherein such properties play a fundamental role on their performance when in service, in both monumental and non-monumental constructions.This work aims at presenting a first approach on the application of EIS to study electrolyte absorption and evaporation processes in stones. Stones with different porosities and different fluid-transport behaviors were monitored by EIS using a setup of 4 or 2 electrodes assemblies.During absorption, EIS spectra presented two distinct responses according to the frequency, a resistive behaviour and a capacitive slope. The evolution of the spectra showed that the absorption process can be accurately monitored in less porous stones, whereas in more porous stones the procedure still requires some improvements.In what concerns evaporation tests, EIS spectra were more complex, especially for longer periods. At early stages two time constants were poorly detectable, but the evolution of evaporation produced progressive modifications on the spectra and a second time constant became well-resolved, as result of two effects: a gradual loss of contact efficiency and a decrease of moisture within the pores.In both cases EIS enabled to distinguish between different fluid-transport behaviors and showed to be dependent upon the moisture content within the pores and open porosity of each stone variety.