Assessment of the complex resistivity behavior of salt affected building materials

Assessment of the complex resistivity behavior of salt affected building materials Sabine F Kruschwitz, PhD thesis, Technical University Berlin, 2007 The ability to reliably predict the amount and type of salts within a wet masonry is one of the most challenging subjects in non-destructive combat of salt attack of building stone. To investigate the value of complex resistivity (CR) measurements for the early detection of salt and moisture related stone altering, controlled salt and saturation experiments have been performed and a new procedure of an image based effective impedance modelling developed. CR magnitude and phase measurements (1 mHz 100 Hz) were acquired on a wide variety of wholly and partially brine (NaCl,Na2SO4, CaCl2 andMgSO4) saturated building stone samples. In the case of saturated materials, and provided thorough calibration data are available, a reliable estimate of salt concentration is possible from the measured resistivity magnitude. Additional indication on the dominant cation in the solution can be obtained from the resistivity phase. The here reported results give an important guide to the limitations of CR in obtaining pore surface area and pore throat estimates. For wet porous materials, in which polarization occurs due to complex surface conduction, the dominant pore throat and amount of specific surface affect its polarizability. Materials with pore throats between 20 and 100 μm are more qualified observation objects than those with narrower pore systems, i.e. most bricks. The predicted power-law correlation between the C-C relaxation time and a characteristic length scale (pore throat size for consolidated materials) is supported. The experimental salinity study reveals how responsive polarization is to changing the pore fluid chemistry. The imaginary conductivity increased for most materials up to fluid salinities of about 1 S/m. Divalent cations in the saturating fluid reduce the imaginary conductivity compared to monovalent ones at the same fluid concentration. CR measurements on partially saturated samples demonstrate the method’s sensitivity to water content. For most materials the imaginary conductivity decreased faster than the real component. This was independent of the pore fluid. The results revealed diverse behavior such as decreases and increases in relaxation time with saturation. In some cases a suppression of a distinctive relaxation curve at low saturations was observed. The results indicate, that contrary to conclusions from recent related studies, the correlation between a C-C relaxation time and hydraulic properties may be limited. In order to infer information on the charge distributions within the electrical double layer (EDL), zeta-potentials and surface charge densities were obtained from electroacoustic measurements. The results add weight to the assumption that there is some universal positive relationship between zeta-potential and imaginary conductivity. A notable dependence of imaginary conductivity on diffuse layer surface charge was only observed in case of one sandstone. An image based effective impedance modelling approach was used to study general microstructural implications on CR responses. An increasing salt concentration (that creates a shrinking EDL) reduces the peak phase and moves it towards higher frequency. The same effect would have an increasing fluid conductivity for an otherwise fixed microstructure. For some combinations of material and salt, however, in the experimental work the peak phase was observed to shift towards lower frequency: a phenomenon that could not be explained with the modelling. Therefore, it is assumed that chemical properties, like ion mobilities or other surface chemistry properties (ion-exchange processes) must be regarded to conclusively explain surface conductivity mechanisms. Other experimental observations like the alteration of CR spectra at desaturation could likewise be modelled. If these models truly mimic the effective electrical properties, the results give new implications on the effective medium behavior. Concluding, the author values the complex resistivity method as a possible effective nondestructive testing tool for a wide range of building stones. But, a priori information and calibration data are essential; CR should not be treated as a stand-alone method.