Microstructural and phase configurational effects determining water content: Dielectric relationships of aggregated porous media

Many porous media in which we determine water content are aggregated and characterized by a dual-porosity pore network, composed of interaggregate pores and intra-aggregate pores. This paper reports sample-scale permittivity measurements made in four stable aggregate media with dual porosity. Results indicate two distinct dielectric responses depending on whether the aggregates are surrounded by water or air. We relate transitions in the permittivity response to the water retention characteristic (WRC), showing that after the interaggregate pores have drained, the slope of the water content-permittivity relationship is significantly reduced (permittivity values ranging from 5 to 7). The hydraulic critical water content (theta(hc)) is defined as the point where all the interaggregate pores are air filled and all the intra-aggregate pores are water saturated and is determined from the WRC. The dielectric critical water content (theta(dc)) is defined as the point where a slope change in the measured water content-permittivity relationship occurs. A two-step model is presented and designed to capture the physical characteristics of the permittivity response to drainage. Measurements of theta(hc) and theta(dc) reveal a separation in these two values. The difference is considered to be a function of the connectivity of the intra-aggregate pore network. A connectivity ratio is defined as 1-(theta(dc)/theta(hc)), where values close to 0 indicate low connectivity between the intra-aggregate pores and values tending to 1 indicate a high level of connectivity. Results from this work indicate that the reduced permittivity response measured in the water content-permittivity relationship is due to microstructure and phase configuration and not to "bound" water.