Multi-GHz Monitoring of Cement Hydration using Time Domain Reflectometry Dielectric Spectroscopy

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cover page paper 3.1.jpg

Multi-GHz Monitoring of Cement Hydration using Time Domain Reflectometry Dielectric Spectroscopy

25.00

Authors

N. E. Hager III,1,2 R.C. Domszy,1,2 and M. R. Tofighi3

Abstract

The compressive strength of concrete is directly related to the degree of hydration in the cement paste. Our prior work demonstrated a continuous monitoring of the physical state of water in hydrating cement paste from around 10 kHz to several GHz throughout the cure process. The broadband complex permittivity was monitored as a function of cure time using Time-Domain-Reflectometry (TDR) Dielectric Spectroscopy and an embedded capacitance sensor. Current work now focuses on extending the frequency range to around 12-15 GHz, to more fully capture the free-water relaxation occurring in this range and separate it from the bound-water relaxation occurring at lower frequencies. New methods being developed include a TDR Smith chart, which displays the TDR transient in a complex reflection-coefficient plane, accentuating differences between expected sensor loading and unwanted signal artifacts. Such artifacts may include reflections from sample boundaries, resonance in the sensing pin, reflections from shielding arrangements, and errors in numerical transforms. Other methods include improved calibration using reference liquids which closely match the real and imaginary permittivity of hydrating cement at various stages of cure. In fresh cement paste, where water-loading and ion conductivity is high, a mixture of saline and Poly(methyl methacrylate) (PMMA) powder provides a reference with a relaxation time similar to pure water, but at a reduced permittivity and increased conductivity comparable to fresh cement paste. At longer cure times, where water-loading and ion conductivity are minimal, references such as tetrahydrofuran with added electrolyte provide a comparable relaxation reference and reduced conductivity comparable to cured paste. These two references set a high and low "calibration window" through which the real and imaginary permittivity can be monitored over a wide range in cure. To verify dispersion in the captured TDR transient directly, transients are compared in the time domain with constant-permittivity references to ensure expected stretched exponential behavior is present. Methods should have broad applications in a variety of inorganic/organic materials and aqueous systems of interest in groundwater characterization.

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