Cohesion: meaning and origins

Abstract

Cohesion is a key property of materials. It equals the energy stored in the bonds uniting two surfaces of the material. The determination of surface energy thus deserves appropriate attention. Mechanical probing is difficult to execute in an unambiguous way, and alternative methods must be employed. The surface forces responsible for cohesion are characteristic for each class of materials. Recently the investigations on cohesion-controlled granular materials are providing a unifying view. We will present two cases: hydraulic binders and clay mixtures for terracotta manufacture. Cohesion in hydraulic binders appears as soon as the dry powder is mixed with water. It can be measured with small amplitude oscillation rheology. It originates from both (i) an aging process where the attracting particles rearrange towards the minimum energy configuration and (ii) the rigidifying of the particle contacts due to C-S-H precipitation active even during the dormant period. The details of the shear modulus growth after shear rejuvenation permit to discriminate between the two. Concerning clay mixtures, cohesion determines the pressure-forming processability