Moisture retention in porous materials

Author: Dr. Andreas Nicolai
English version by Sandra Leithäuser

linked articles

• Modeling of Salt and Humidity Transport
• Moisture transport mechanisms

This article describes the mechanisms of moisture retention and common approaches for models and material functions, especially the sorption isotherms and the moisture retention capacity (MRC)

Introduction

For porous building materials, a correlation between the relative humidity of the ambient air and the weight of a material sample can be determined using gravimetric analysis. This correlation is attributed to moisture retention in the material. Vapour penetrates the material, is deposited on the surface, condensates inside the capillaries or is chemically bound. The following discussion will solely consider the physical effects of the capillary condensation.

Definitions

In the discussion about moisture retention processes a differentiation between the hygroskopic and super-hygroscopic region is made. Materials within the hygroscopic region have low moisture content while materials in the super-hygroscopic region display higher moisture content. It is not possible to clearly separate the two regions, for many building materials it lies between 92% and 95% RH. A possibility for separating the regions is in describing the dominant [[ |moisture transport mechanisms]]. In the hygroscopic region vapor diffusion, in the super-hygroscopic region the capillary liquid water transport prevails. The latter is particularly of interest for the redistribution and transport of salt solution.

The equilibrium weight context between gravimetrically determined humid mass/ moisture content of the building material and a moisture potential can be determined experimentally. Moisture potentials here are the relative humidity and the liquid water pressure and accordingly capillary pressure.

Models for describing moisture retention

Sorption isotherms

Within the hygroscopic region, usually the relationship between relative humidity and moisture content is shown at constant temperature. This representation is called the sorption isotherm, because it is determined experimentally through a sorption process. In a standard experimental setup a material is oven dried, weighed and then, for a certain time left in a place that has a constant defined relative humidity (e.g. in a desiccators). The sample is weighed in regular time intervals, until a constant weight occurs. The equilibrium moisture content Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta_\ell} together with the constant relative humidity Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \phi} inside the desiccators, equals a point Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta_\ell\left(\phi\right)} on the sorption isotherm.

Moisture retention curve

The super-hygroscopic region starts mostly at 92% RH. In the representation of the sorption isotherm no details about the gradient of the function Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta_\ell\left(\phi\right)} can be deducted anymore. This is why another representation is chosen, the moisture retention curve (MRC). With the moisture retention curve, the moisture content is applied above the capillary pressure of the pore solution. On saturation, it reaches 0 Pa, which corresponds to 100% RH. If the material dehumidifies, the capillary pressure increases rapidly (with negative sign). The relationship between capillary pressure and relative humidity is given by the Kelvin- equation:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \phi = e^{ p_c / \rho_w R_w T} }

With this equation, the two representations of humidity retention behavior of a material can be transferred into one another and converted.

Literature

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relative humidity