Monitoring of Hydration in Cement Systems by Broadband Time-Domain-Reflectometry Dielectric Spectroscopy
N.E. Hager III, R.C. Domzsy
In previous work we demonstrated a continuous monitoring of the dielectric relaxation spectrum in hydrating portland cement over the frequency range 10 kHz to 3 GHz from initial mixing to several weeks cure. Measurements were made using broadband Time-Domain-Reflectometry Dielectric Spectroscopy with a 35 ps stimulus and an embedded capacitance sensor. Three fundamental signals were identified, corresponding to unreacted free water, bound-water attaching to developing microstructure, and grain polarization. The three signal components were fit to appropriate molecular models as a function of cure time and monitored throughout the process. The result is 1) a free-water relaxation which monitors the disappearance of water into hydration and thus follows percent hydration, and 2) a bound-water relaxation which monitors water attaching to developing microstructure and thus monitors formation of this microstructure, and 3) a grain-polarization relaxation which monitors development of this microstructure.
Our current work involves extending the measurement bandwidth to more fully capture the free-water relaxation and comparing the improved model-fit amplitudes with analytical measurements of the cement hydration process. An open-face 9 mm diameter capacitance sensor interrogates a larger sample volume than before, with procedures developed for controlling radiation losses from the sensor and eliminating computational poles in the calibration. Model calibration systems with known relaxation parameters are used to calibrate and verify the free-water relaxation at amplitudes comparable to hydrating cement, with multiple systems such as water/acetone solutions and 1,2 dichloroethane providing references at different levels of hydration. Results show continuous dielectric spectra to between 10 and 15 GHz, with a free-water relaxation resolvable from bound water relaxation at advanced stages of hydration. The free-water relaxation and bound water relaxation is monitored as a function of cure time and compared to calcium hydroxide formation as measured by differential scanning calorimetry.
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