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SOIL PHYSICS

The Effects of the pH-influenced Structure on the Dielectric Properties of Kaolinite–Water Mixtures

^a Hangzhou Metro Group Co. Ltd., Hangzhou, China
^b Dep. of Civil Engineering, Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

^* Corresponding author (ceyhwang@ust.hk).

The properties of clay–water mixtures are reflected in their wide-frequency dielectric responses that result from different polarization mechanisms. This feature is useful in various soil science applications but has not been systematically investigated. The objective of this study was to characterize the broadband dielectric spectrum of kaolinite sediments with different charge properties and structures using a slim-form open-ended coaxial probe. The sediment structure in this study was manipulated by changing the pore-fluid pH. When the pH was below the isoelectrical point of the edge surface, IEP_edge, the edge-to-face flocculation structure was formed in voluminous sediments with an average porosity of 0.87 (Group A samples). A higher dielectric constant due to bulk water polarization (an average of 65.47) was measured because of the higher water content. As the pH was increased to greater than the IEP_edge, a dense sediment with face-to-face aggregation was produced in the Group B samples, and a lower dielectric constant (an average of 60.11) was obtained. In bound water polarization, a higher relaxation strength (2.7 times higher on average) and a longer relaxation time (1.5 times longer on average) were observed in the Group B samples compared with those of the Group A samples. Similar trends can also be found in the results of spatial polarization. These findings can be attributed to more negatively charged surfaces and denser packing. Fluid conductivity dominated the global conductivity of the sediment in the Group A samples so that the β value, i.e., the ratio between the conductivity of the sediment and the fluid, was <1. The β value was >1 in the Group B samples owing to an overcompensation of surface conduction.

Abbreviations: DC, direct current • IEP_edge, isoelectric point of the edge surface