Abstract
Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy jumps by a finite value when touching slit walls are separated from each other. We consider a more realistic adhesion model by introducing a smoothly-varying adhesion force, which depends on the distance between the slit walls. The range of the attraction between the slit walls is found to have a pronounced influence on the shape of hysteresis loops at low vapor pressure. A large interaction range avoids an extraordinarily small relative vapor pressure necessary for pore closing, which is a precondition for low pressure hysteresis. Our extended model allows us to describe a discontinuity, which can appear in the desorption branch of swelling/shrinkage diagrams for hardened cement paste.
Highlights
Cracking of porous solids like concrete or ceramic green bodies due to capillary or solvation forces is a serious problem in many technological processes
Sorption hysteresis at moderate and low humidity can be explained by strong cohesion forces, which attract the walls of narrow slit pores [11,12,13]
The presented model suggests a possible mechanism, which leads to a sorption hysteresis loop accompanied by a related loop of the pore volume change
Summary
Cracking of porous solids like concrete or ceramic green bodies due to capillary or solvation forces is a serious problem in many technological processes. Low pressure hysteresis in some porous solids such as hardened cement paste and montmorillonite clay is thoroughly investigated experimentally and well illustrated in a large number of papers [3,4] These materials have a layered structure and a large fraction of slit-like micropores with a slit width comparable to or lower than a nanometer. We proposed a simple mathematical model to describe sorption, as well as swelling and shrinkage in porous solids with narrow slit pores [11,12,13] According to this model, narrow slits can be completely compressed by condensate tension at low relative vapor pressure of the ambient vapor (Figure 1). It will be shown that such a discontinuity is predicted by the extended adhesion model with smooth adhesion potential
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