Why does concrete set?: The nature of cohesion forces in hardened cement-based materials

Unlike other porous materials such as sandstone, brick, or porous glass, the interatomic bonding continuity of cement-based materials like concrete is far from obvious. When scrutinized at the micro- or nanoscopic level, the continuity of the ionic-covalent bonding in the solid phase is interrupted almost everywhere by water molecules or liquid water films. The same situation is found in set plaster. Yet, plaster and cementitious materials are able to withstand stresses of the same order of magnitude as rocks. Molecular simulation studies and direct-force measurements by atomic force microscopy provide strong arguments for predicting that short- and medium-range surface forces mediated by partially or totally hydrated calcium ions are the essential components of cement strength, with additional contributions from van der Waals and capillary forces. This provides a clue for understanding the nano- and mesostructure of cement-based materials and new levers for improving their properties.