Con~ary to other porous materials such as smadstones, bricks or porous glass, the inter-atomic bonding continuity of cement-based materials is fax from obvious. When scrutin~ed at very microscopic level, continuity of the ioniccovalent bonding in the solid phase is almost everywhere intemapted 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 argtmaents 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 mediumrange 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 tbrces in unsaturated conditions. The parallel with clay-based earthen construction materials is part of the clue of this rationale. RESUME Contrairement gz d~utres matdriaw; por~ux comme les gr~s, les briques ou eertains verres, les mat~riau~ cimentaires ne possbdent t'x~ un schdma simple de liaisons int~atomiques. s continuit~ du rd~eau de liaisons iono-covalentes y est

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