College Park, Maryland June 6 - 10 , 2004
TP5: Hydration-kinetics and Water Content of Cement: Quasi-Elastic Neutron Spectroscopy Studies
N. M. Nemes (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), J. C. McLaughlin, D. A. Neumann (NIST Center for Neutron Research), R. A. Livingston (Office of Infrastructure R&D, Federal Highway Administration, McLean, VA 22101)
Portland cement is a complex non-homogeneous material, with many different constituent phases and structure over a wide range of length scales. The hydration of Portland cement is a complex process involving several simultaneous chemical reactions. All of these reactions involve water, and as hydration proceeds, more of the water that is initially mixed with the cement powder becomes chemically bound into reaction product phases. Quasi-elastic neutron scattering (QENS) allows for the study of the state of water in hydrating cement paste in-situ over time and over a wide range of temperatures. QENS provides a direct measure of the conversion of free water to structurally/chemically bound water and to water constrained in the pores of the cement paste. We studied the hydration kinetics of synthetic cement paste with varying alite/belite (dicalcium silicate or C2S and tricalcium silicate or C3S) and varying water/cement ratios. The time-dependent free, constrained and bound water results are analyzed in terms of the Avrami-model for the nucleation and growth regime and in terms of a diffusion-limited growth model for the later period. The results reveal the complex nature of the chemistry governing the hydration of cement as the dependence of the model parameters on the C2S/C3S ratio is highly non-linear.
The role of the water/cement ratio (w/c) on the hydration kinetics has been investigated using QENS. For a range of w/c from 0.3 to 1, the bound-water index was extracted from the neutron scattering data, and fitted to hydration kinetics model developed in previous QENS studies. The w/c ratio had no significant effect on the model parameters for the diffusion limited period, but it did affect the nucleation and growth stage.
We also studied the water content of mature samples by drying to varying degrees in saturated salt solutions. The results indicate that the free and constrained water contents are lost at a similar rate.
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