Studies on hydration water dynamics and microstructure of synthetic cement

Abstract

The mechanical properties of cement pastes depend strongly on their porosities. In a wet paste, the porosity links to the free water volume after hydration. Structural water which presents in the solid phase, constrained water absorbed on the surface of the pores and free water in the center of the pores have different dynamical behavior. Hence, it should be possible to extract information on pore system by exploiting the water dynamics. We investigated the dynamics of hydration water confined in calcium- and magnesium-silicate-hydrate (C-S-H and M-S-H) gels using high-resolution quasi-elastic neutron scattering (QENS). C-S-H and M-S-H are the chemical binders present in calcium rich and magnesium rich cement. To analyze the cement QENS data, we developed a new global model which is numerically more stable than previous models for cement QENS analysis. With this model, we can correctly quantify the structural water index (SWI) and the confining radius. We also established the relation between the constrained to liquid water ratio and the temperature dependence of translational relaxation time. We analyzed two different sets of synthetic cement using this method: (1) C-S-H with different water to cement ratio (w/c) and (2) M-S-H with various additives. For the first set, SWI and confining radius are both controlled by w/c with a linear relation. For the second set, we show that by adding ASN-COOH additive, M-S-H becomes similar to C-S-H in all parameters. We also analyzed the small angle x-ray scattering data of M-S-H gel with a polydisperse cylinder model which fits better than previously published polydisperse sphere model and will be studied further in future work. The result indicates that C-SH and M-S-H have similar globule shape and fractal structure. The evidence from QENS and SAXS experiments suggest that the weak compressive strength of M-S-H compares to C-S-H is due to the high porosity rather than the globule shape.