Microscopic physical basis of the poromechanical behavior of cement-based materials

Contrary to other porous materials Such as sandstones, bricks or porous glass, the inter-atomic bonding continuity of cement-based materials is tar from obvious. When scrutinized at very microscopic level, continuity or the ionic-covalent bonding in the solid phase is almost everywhere interrupted by water molecules or liquid water films of variable thickness. Yet, concrete and cement pastes are able to withstand stresses of the same magnitude as rocks. The purpose of this paper to explore the possible reasons for such a high cohesion in terms of inter-particle forces using general arguments and molecular simulation computations including ab initio quantum chemical methods applied to C-S-H. As it will be discussed, molecular simulation studies provide strong arguments for predicting that short- and medium-range attractive electrostatic forces are the essential components of the cohesion of C-S-H with, at short distance (sub-nm), a significant iono-covalent contribution involving strongly localized calcium ions and water molecules and, at larger distance (a few nm), ionic correlation forces involving hydrated and mobile calcium ions in liquid water films. Only a marginal contribution is expected from van der Waals attraction whereas capillary forces might contribute at a level comparable to that of correlation forces in unsaturated conditions. The parallel with clay-based carthen construction materials is part of the clue or this rationale.