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

Abstract

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

Hysteresis from Multiscale Porosity: Modeling Water Sorption and Shrinkage in Cement Paste

“…The presence of dissolved ions produces a repulsive osmotic pressure, but this is small: a mean field treatment with walls separated by 0.5 nm gives a pressure around 1 MPa or less, regardless of ion concentration [71]. More important than osmotic pressure are effects arising due to the discreteness of ions: correlation effects can even result in a force that is joining instead of disjoining [53,72].…”

“…Additionally, molecular models can be used to calculate the pressure exerted by the fluid [21,72], which is dominated by strong electrostatic interactions among water molecules, dissolved ions, and the charged C─S─H surfaces. For example, molecular models treating these effects have explained the observed hysteresis in the drying shrinkage of swelling clays, layered materials that share many features with cement paste [26,90,91].…”

Dimensional change and elastic behavior of layered silicates and Portland cement paste

“…In nanoindentation studies, it has been shown that decalcification of C-S-H reduces the modulus of elasticity [8,9]. The modulus values computed from the dynamic molecular modeling and free energy minimization methods for various layered silicate systems are significantly higher than those obtained from experimental techniques [10][11][12]. These measurements do not follow any specific trend regarding their dependence on the C/S ratio of the material.…”

Modelos estructurales del empaquetamiento aleatorio de partículas esféricas de Tobermorita: una aproximación computacional sencilla

“…Some attempts have been already made, but either they have paid attention to the computational justification of the jennite and tobermorite crystals as appropriate structural models (13)(14) or they have determined different structural and dynamical properties by assuming these crystalline models (15)(16)(17)(18)(19). Unfortunately, all of them have centred their attention to the short-ordering of C-S-H, being the linkage with the colloidal scale not covered.…”

“…Varios trabajos de simulación ya han sido descritos en la literatura, pero o bien han prestado atención a la justificación computacional de los cristales de tobermorita y jennita como modelos estructurales (13)(14) o bien han determinado diferentes propiedades dinámicas y estructurales asumiendo estos modelos cristalinos (15)(16)(17)(18)(19). Desafortunadamente, todos ellos han centrado su atención en el orden a corto alcance del gel C-S-H sin tener en cuenta la conexión a escala coloidal.…”