Patrick Bonnaud

Effects of elevated temperature on the structure and properties of calcium–silicate–hydrate gels: the role of confined water

The binding phase of cementitious materials, calcium–silicate–hydrates, can be described as nanogranular and as an inorganic hydrogel. Similar to other hydrated “soft matter,” the water confined within the nano- to microscale pores of such cementitious materials plays a crucial role in the structure and properties of cement pastes. When compared to organic hydrogels, non-stoichiometric calcium–silicate–hydrates (C–S–H) are relatively robust against changes in humidity and temperature. However, under extreme physical environments, changes in the amount, and location, and physical state of water can limit damage tolerance and sustainability of otherwise stiff and strong cementitious macrostructures. Here, we employed Grand Canonical Monte-Carlo and Molecular Dynamics simulations to investigate the effect of temperature on the water content within and between C–S–H grains constituting the cement microstructure, and on the associated physical and mechanical properties of this material. We found water content within grains decreased with increasing relative temperature up to T/T* = 2 (where T* is the transition temperature at which the bulk liquid and gas are in equilibrium for a given pressure), and that C–S–H grains densified with attendant increases in heat capacity, stiffness, and hardness. Although intragranular cohesion increased monotonically with increasing relative temperature over this range, intergranular cohesion increased up to a relative temperature of T/T* = 1.1 and then decreased at higher relative temperatures. This finding suggests a rationale for the decreased mechanical performance of cement paste and concrete at high relative temperatures, and supports previous claims of peak hardness in C–S–H at an intermediate relative temperature between 1 and 2.61. Further, these atomistic simulations underscore the important role of confined water in modulating the structure and properties of calcium–silicate–hydrates upon exposure to extreme environments.

Confined water in nanoporous silica: Application to humidity sensors

Because of their strong hydrophilicity, nanoporous silica materials are good candidates for humidity sensor applications. We employed molecular modeling to investigate the water behavior when confined in such materials in order to refine the design of the next generation of devices. We focused in this work on the mechanical behavior of those porous silica materials.

Effect of the Titanium Nanoparticle on the Quantum Chemical Characterization of the Liquid Sodium Nanofluid

Suspension state of a titanium nanoparticle in the liquid sodium was quantum chemically characterized by comparing physical characteristics, viz., electronic state, viscosity, and surface tension, with those of liquid sodium. The exterior titanium atoms on the topmost facet of the nanoparticle were found to constitute a stable Na-Ti layer, and the Brownian motion of a titanium nanoparticle could be seen in tandem with the surrounding sodium atoms. An electrochemical gradient due to the differences in electronegativity of both titanium and sodium causes electron flow from liquid sodium atoms to a titanium nanoparticle, Ti + Na → Ti(δ-) + Na(δ+), making the exothermic reaction possible. In other words, the titanium nanoparticle takes a role as electron-reservoir by withdrawing free electrons from sodium atoms and makes liquid sodium electropositive. The remaining electrons in the liquid sodium still make Na-Na bonds and become more stabilized. With increasing size of the titanium nanoparticle, the deeper electrostatic potential, the steeper electric field, and the larger Debye atmosphere are created in the electric double layer shell. Owing to electropositive sodium-to-sodium electrostatic repulsion between the external shells, naked titanium nanoparticles cannot approach each other, thus preventing the agglomeration.