Quasi-Elastic Neutron Scattering Study of Hydration Water in Synthetic Cement: An Improved Analysis Method Based on a New Global Model

Quasi-elastic neutron scattering (QENS) measuring the energy transfer (E) and momentum exchange (Q) between a neutron and a sample target is often used to study hydration water in cement. A global model introducing a Q-independent elastic component, a Q-dependent elastic component, and a quasi-elastic component with explicit translational and rotational intermediate scattering functions has been developed to analyze the quasi-elastic neutron scattering (QENS) spectra from water confined in cement pastes. To demonstrate the advantage of this model, it has been used to analyze four sets of cement data: calcium-silicate-hydrate (CSH), pure magnesium-silicate-hydrate (MSH), MSH with aluminum-silicate nanotubes (ASN), and MSH with carboxyl-functionalized ASN (ASN-COOH), previously studied with a much simpler model. It is shown that, with this new model, several important parameters that describe the structure of the cementitious materials can be extracted from the QENS data. Particularly, the structural water index, a quantity crucial for the hydration mechanism, can be accurately determined by simultaneously analyzing the Q range from 0.5 to 1.3 Å^-1. Three other parameters, the confining radius a, which characterizes the size of the pore where the free water molecules reside, and the translational relaxation time τ_T0 and the rotational relaxation time τ_R of the nonstructural water molecule, can also be extracted. This global model is more realistic than previous model(s) and can be used as a practical method for extracting both dynamical as well as structural information from QENS spectra of hydrating cement pastes measured at Q values even higher than 1 Å^-1.