College Park, Maryland      June 6 - 10 , 2004

TP8: Investigation of the State of Water in Hydrating Layered and Amorphous Sodium Disilicate (Alkali-Silica Reactants) by Quasi-elastic Neutron Scattering.

J.W. Phair, R.A. Livingston (Office of Infrastructure Research and Development, Federal Highway Administration, McLean, VA), C.M. Brown (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), A.J. Benesi (Department of Chemistry, The Pennsylvania State University, University Park, PA)

The structure and state of water within hydrating layered and amorphous sodium-disilicate were monitored for the first time using quasi-elastic neutron scattering (QENS) and x-ray diffraction (XRD). The QENS kinetic data collected for the first 12 hours of the reaction, were fitted to a model consisting of a Lorentzian and a Gaussian function each convoluted with the energy resolution of the instrument. This allows the QENS signal from water to be associated with two states that include bound and free water, as confirmed by thermogravimetric analyses and 2H NMR. In situ QENS data were collected for a set of sodium disilicate and silica mixtures at a series of discrete temperatures between 20 and 40 C. The bound water was successfully modelled with Langmuir adsorption kinetics to explain the formation of a layered silicate structure within kanemite. Langmuir adsorption kinetics were also used to model the reaction using amorphous starting material suggesting the presence of layered structure in the reaction product. The presence of the layered structure in amorphous material was confirmed by XRD analyses. The consequences this has for alkali-silica reaction (ASR) gel reaction and its swelling mechanism are discussed. Based on these results, a standard test for ASR reactivity of aggregates could be developed using natrosilite as standard reagent, and the kinetic parameters as a scale of reactivity.

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