Water dynamics confined in synthetic Al-substituted Tobermorite

Abstract

The main phase of the hydrated cement paste is the calcium silicate hydrate (C-S-H). Many features of such gel phase depend on the relation between its pore network and the water molecules confined inside it. However, C-S-H has a complex heterogeneous nanostructure. Therefore, tobermorite-like chains are used as a model to produce the interlayer nano-pores in molecular dynamical simulations of C-S-H. In this work, we have investigated the dynamics of water molecules confined in zeolite-like cavities of synthetic tobermorite as well as the structure of the confinement. Samples were prepared with different aluminum content in order to study the effects of Al-substitution on the structure of tobermorite and, as a consequence, on the different confinements explored by the water molecules trapped in. From the structural side, two different phases have been found in the tobermorite: an amorphous phase and a crystalline phase. Their content in the specimens depend on the Al amount remarking the important role of aluminum for the tobermorite crystallization. In addition, water dynamics have been studied by means of broadband dielectric spectroscopy in the sub-zero region (95 – 170 K) where confinement effects on the supercooled water dynamics can be observed. Two different dynamics (fast and slow) have been observed in all the samples. The slow dynamics properties depend on the type of confinement: in the Al-free samples, i.e. amorphous tobermorite, such dynamics strongly reminds that of water confined in the C-S-H framework. The fast dynamics have been related to the rotation of the OH groups in the crystalline tobemorite.