Monitoring of hydration processes in cement materials by broadband Time-Domain-Reflectometry Dielectric Spectroscopy
Prior work in our laboratory demonstrated a continuous monitoring of the chemical state of water in hydrating cement paste, over the frequency range 10 kHz to 8 GHz and from initial mixing to several weeks cure. The broadband complex permittivity is obtained over the range using Time-Domain-Reflectometry (TDR) Dielectric Spectroscopy, using Fourier transform methods and an embedded capacitance sensor. Three fundamental signals are identified, corresponding to unreacted free water appearing near 10 GHz, bound-water attaching to developing microstructure near 100 MHz, and grain polarization occurring around 1 MHz. The three signal components are 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 component which monitors developing microstructure. Our current work focuses on investigating changes in this relaxation spectrum and its cure evolution with changes in chemistry and processing conditions. Using the full frequency transform, we explore relative changes in various signal components with changes in ionic strength and backfilling of the pore space with excess free water. Monitoring the transient amplitude directly, we follow specific frequency components by monitoring corresponding delay times and follow reaction rates in the time domain. Changes in reaction rate are compared for changes in temperature, addition of accelerants and retarders, and other factors. Reaction rates obtained by TDR are also compared with rates obtained by other measures of cement hydration; such as heat evolution by isothermal calorimetry, bound-water formation by thermogravimetric analysis, and calcium hydroxide production by differential scanning calorimetry. The transient analysis can be integrated with a small portable TDR sampler to form a robust cure-monitoring system usable in the field. Copyright © 2007 MS&T'07®.