Monitoring of Hydration in Cement Systems by Broadband Time-Domain-Reflectometry Dielectric Spectroscopy

Elizabethtown College, Elizabethtown PA

Investigators

Abstract

Prior work in our laboratory demonstrated a continuous monitoring of the dielectric relaxation spectrum in hydrating portland cement over the frequency range 10 kHz to 8 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. The proposed research will now expand this investigation to rigorously compare signal changes occurring in each component with chemical and material changes through a combination of analytical measurement, chemical variation, and experimental investigation. Measurements may include, but are not limited to: Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Thermoporometry, Quasi-Elastic Neutron Scattering (QENS), optical microscopy, and standard mechanical compressive-strength testing. Specific tasks include 1) extending measurement resolution to near 10 GHz to better resolve individual free- and bound-water components, 2) correlating the disappearance of free water near 10 GHz with the increase in hydration as determined by DSC and Thermogravimetric analysis, 3) correlating the increase in bound water near 100 MHz with the formation of reaction products as determined by QENS, microscopy, and other methods, 4) Estimating the free-water loss to evaporation by instrumenting a large test cylinder with sensors at varying depths and determining moisture gradients, 5) Examining appropriate variations in chemistry and their effect on signal evolution, and 6) Exploring the thermodynamics of the relaxation processes with respect to pore size by freezing the material during hydration. The proposed activity will expand our understanding of the chemical state of water in hydrating cement and provide a foundation for using TDR spectroscopy as a powerful tool for investigating hydration in a variety of cementitious materials. Results will have applications in a variety of inorganic/organic systems including the study of structure and dynamics in aqueous and biological systems including DNA, proteins, and micelles. The activity will impact on our collaboration with industry in process-monitoring and control and our involvement with students through undergraduate research at Elizabethtown College. It could lead to long-term collaboration with our biology department on potential biological applications and development of a business project for cement sensors through our business department.