Why Does Concrete Set?: The Nature of Cohesion Forces in Hardened Cement-Based Materials
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Why Does Concrete
Set?: The Nature of Cohesion Forces in Hardened CementBased Materials
Roland J.-M. Pellenq and Henri Van Damme Abstract 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. Keywords: atomic-scale simulations, cement, cohesion forces, concrete, construction materials, porosity.
Introduction What is the difference between a onemeter-high sand castle on a beach, a sevenstory earthen building in Yemen, and a 1000-foot-high concrete and steel skyscraper in Shanghai? Their height! Indeed, but in terms of elementary granular-material mechanics going back to Coulomb in 1773,1 the difference comes from cohesion, C, that is, the internal stress that, together with intergranular friction, prevents a granular material from dividing in two and sliding along a failure plane when subjected to a force (e.g., its own weight).2 In its simplest form, the failure criterion reads C,
(1)
where and are the shear and normal stresses on the sliding plane, respectively, and is the friction coefficient.
MRS BULLETIN/MAY 2004
In dry sand, cohesion is vanishingly small, and building even the smallest vertical wall out of dry sand is virtually impossible. An avalanche starts and flows until the slope reaches equilibrium. If the surrounding air contains water vapor, some cohesion may be detected in the sand, due to microscopic capillary bridges between the asperities in contact on the rough surface of the grains.3 Then, building a vertical wall is still risky but feasible, up to moderate heights. For a small liquid bridge (as compared with the grain size), the attractive capillary force is related to the Laplace pressure across the air/water meniscus, PLaplace (2LV cos )/rK ,
(2)
(where LV is the air/water surface tension, rK is the meniscus radius, and is the
contact angle), and to the cross sectional area A of the liquid bridge. Under equilibrium conditions, rK is determined by the vapor pressure, whereas the bridge volume and area depend on the average curvature and the surface roughness of the grains. As the vapor pressure increases, the area increases, but the m
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