Monitoring of cement hydration by broadband time-domain-reflectometry dielectric spectroscopy
The broadband complex permittivity is monitored continuously in hydrating cement paste over the frequency range of 10 kHz-8 GHz and from initial mixing to several weeks of cure. Measurements are made by time domain reflectometry (TDR) dielectric spectroscopy, using an adjustable capacitance sensor, which can be embedded in the material in situ. The results are fit to a relaxation model, which includes terms representing (1) a Cole-Davidson relaxation near 1 MHz, which grows initially and then decreases with an advancing cure; (2) a Debye relaxation near 100 MHz, which grows initially and then decreases with an advancing cure; (3) a free-water relaxation near 10 GHz, which decreases with an advancing cure; and (4) an ion conductivity and electrode polarization, which decreases with an advancing cure. The model is fit continuously as function of cure time extracting parameters for the relaxation amplitudes, relaxation frequencies, and distribution parameters as a function of cure time. The results are contrasted with measurements in tricalcium silicate and tricalcium silicate with varying ion content, revealing differences that may indicate the nature of the processes involved. Alternative methods for extracting reaction-rate information directly from the TDR transient are presented, providing a robust monitoring procedure usable in the field. Such methods are demonstrated using a variation in temperature and comparison with measurements of reaction heat obtained by calorimetry. © 2004 American Institute of Physics.