Theory of sorption hysteresis in nanoporous solids: Part II Molecular condensation

Abstract

a b s t r a c tMotivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple ''hierarchical wetting'' model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.

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

“…We believe that pore blocking does not influence the removal of water from the interlayer space, as this space is surrounded by gel pores, which will already be empty at the RH at which the interlayer space begins to empty. Hysteresis in the interlayer space can, instead, arise from single-pore effects due to the molecular nature of water [48,49], strong electrostatic interactions among dissolved ions and the C─S─H surface [21,50], and the collapse of the space.…”

Quantitative discrimination of the nano-pore-structure of cement paste during drying: New insights from water sorption isotherms

“…Research on this topic has been active for decades [1][2][3][4][5][6][7][8], with special attention being paid to: (1) determining the specific surface area of the C-S-H [9][10][11] and (2) establishing relationships between the hysteresis (i.e., path dependence) that separates the adsorption and desorption curves and pore structure [12][13][14][15][16]. The various interpretations have been summarized in several papers [17,18], but only recently has progress been made to interpret the full desorption/ adsorption isotherms.…”

“…The results of an study conducted by Neubauer et al show that the very large volume changes that are associated with emptying the interlayer spaces, and the reduction in the bulk volume, are in part compensated by fissure-like networks that form throughout the microstructure [51,52]. Thus the fundamental objection that emptying the interlayer space over estimates the bulk shrinkage can be tolerated because of the expansions caused throughout the microstructure due to fissure formation [12,13]. Chappuis [53] postulated that internal shrinkage is prevented due to the rigid structure of the cement paste; a rather compact entanglement of C-S-H and other crystals and highlighted that drying causes an increase in the sizes of capillary pores, in relation to severity of drying.…”

Simulation of Humidity Fields in Concrete: Experimental Validation and Parameter Estimation

“…It can be seen that the adsorption and desorption curves are different, highlighting the hysteretic behavior of concrete in regard to water retention (Azenha 2009;ACI 2006). This hysteresis is usually explained with the socalled ink-bottle effect (Brunauer 1943;Bazant and Bažant 2012).…”