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

Since porous building materials are capable of adsorbing water vapor present in the air, a correlation between the relative humidity in ambient air and the weight of a material sample can be determined by gravimetric analysis. The correlation corresponds to the moisture retention by the material as water vapor penetrates into it and is adsorbed on the pore surface, can condensate in capillaries or can be chemically bound. The following discussion will solely consider the physical effects of capillary condensation.

Definitions

In discussing moisture retention processes, a differentiation between hygroscopic and super-hygroscopic region is made. Materials have low moisture content in the hygroscopic region, while they display higher moisture content in the super-hygroscopic region. It is not possible to clearly separate the two regions, for many building materials this lies between 92% and 95% RH.

A possibility for separating the regions is by describing the dominant moisture transport mechanisms. In the hygroscopic region this is vapor diffusion, while in the super-hygroscopic region the capillary liquid water transport prevails. The latter is of particular interest for the redistribution and transport of salt solutions.

The equilibrium weight determined gravimetrically from the moisture content of the building material and its correlation to the moisture potential(s) (such as Relative Humidity or the liquid water pressure, i.e., capillary pressure) can be determined experimentally.

Models for describing moisture retention

Sorption isotherms

Within the hygroscopic region, the relationship between relative humidity and moisture content is usually given 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 an environment with a defined, constant relative humidity (e.g., in a desiccator). The sample is weighed at regular time intervals, until it reaches a constant weight. The equilibrium moisture content $\theta _{\ell }$ at the given relative humidity $\phi$, gives one point $\theta _{\ell }\left(\phi \right)$ of the sorption isotherm. Several such points have to be determined to obtain the complete 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 $\theta _{\ell }\left(\phi \right)$ can be obtained from it. 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:

$\phi =e^{p_{c}/\rho _{w}R_{w}T}$

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

Literature

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relative humidity

see exsiccator