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

Abstract

Contrary to other porous materials such as sandstones, bricks or porous glas, the inter-atomic bonding continuity of cement-based materials is far from obvious. When scrutinized at very microscopic level, continuity of 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 includingab initio quantum chemical methods applied toC-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, ofC-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 earthen construction materials is part of the clue of this rationale.