Investigation of the State of Water in Hydrating Layered Sodium Disilicate in Crystalline and Amorphous Forms by Quasi-Elastic Neutron Scattering

The structure and state of water within hydrating crystalline 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 h 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 allowed the QENS signal from water to be associated with two states that included 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 modeled with first-order reaction kinetics to quantify the hydration of the layered silicate structure within kanemite. First-order reaction kinetics was also used to model the reaction using amorphous starting material, which may also suggest the presence of hydrated layered structure in the reaction product. The presence of the hydrated layered structure in amorphous material was confirmed by XRD analyses. The consequences of this for the alkali−silica reaction (ASR) and its swelling mechanism are discussed. On the basis of these results, a standard test for ASR reactivity of aggregates could be developed using natrosilite as a reagent, and the kinetic parameters as a scale of reactivity.