Changes in pore structure and mercury contact angle of hardened cement paste depending on relative humidity

Abstract

R SUMEHardened cement paste (hcp) is a porous heterogeneous material consisting of dispersed particles like Calcium Silicate Hydrates (C-S-H). These are of micron to nanometer size forming pores on a nanometer scale. Thus, hcp can be regarded as a colloidal system. Surface forces play a dominant role. Adsorbed water molecules interact with the surface. Capillary condensation occurs in the pores below bulk conditions acting in form of capillary and disjoining forces. All these forces are able to alter the structure and properties of the hardened cement paste depending on the moisture content. Pore size distributions were measured with mercury intrusion porosimetry on hcp specimens, which had been prestored over the entire range of relative humidity. Swelling and shrinkage of hcp tubes were also determined. The pore size distribution is corrected for each humidity step regarding the particular porosity, contact angle and volume change. The pore size distribution as a function of relative humidity is nonlinear and characterized by an extreme value in the medium range of humidity.

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

“…As introduced in Part I [1], a long-standing puzzle in the thermodynamics of concrete or cement paste and other nanoporous solids is the pronounced hysteresis of the sorption/desorption isotherm at low vapor pressure [36,26,27,38,39,31,3,25]. Typical experimental data for wetting/drying cycles in concrete is shown in Fig 1, and similar behavior can be observed (or expected) in many other important situations, such as water sorption in dry soils or wood, carbon sequestration in porous absorbents, and natural gas recovery from nanoporous shales.…”