If a dry porous body is kept in exterior conditions, its mass will increase, once it has reached a first end value. The mass will increase as a function of relative humidity (RH). When the RH rises the mass increases, when the RH drops, the mass decreases. The reason is the moisture absorption or release from the ambient air. Water molecules are deposited on the interior surfaces. Especially in very fine capillaries, micropores with a diameter of 0.1µm , this process may cause saturation (capillary condensation). Apart from the properties of the material, the salt content of the sample is important for water vapor sorption.
| Vapor sorption is the absorption of moisture from the gas phase |
In order to characterize building materials with respect to their hygrothermal behavior, samples are stored at a constant temperature (i.e. isotthermal conditions) but at different humidities. These experiments provide sorption isotherms. The sorption isotherms are the graphical representation of the sorption behavior of a material. It represents the relationship between the moisture content and the RH of the ambient air at a specific temperature (equilibrium).
Hygroscopic moisture absorption
In many cases the moisture content of a building material is determined by the content of hygroscopic salts. The hygroscopic moisture absorption is the moisture absorption from the ambient air into a building material. The material absorbs more moisture the more salt it contains and the higher the relative humidity (RH) rises. A salt contaminated masonry wall can absorb many times more moisture from the ambient air than a salt free building material, because it has a lower equilibrium moisture or moisture sorption.
The hygroscopic moisture absorption refers to the absorption of moisture from the gas phase of the air due to a hygroscopic salt.
| The sorption isotherm is a graphical representation of the sorption behavior of a material as a function of the relative humidity at a constant temperature. |
The sorption isotherm graph is recorded from values taken of a sample at different temperatures. The sample must then be dried.
At a specific moisture content value, the sample is saturated and cannot hold any further moisture. It has reached equilibrium moisture of 100% and even at a relative humidity of 99% RH the sample dries out. If the moisture content decreases, the remaining moisture in the sample is bound more firmly. In order to dry the sample, a low RH is necessary, e.g. 50% RH. According to the graph, at 50%RH the sample can only dry out to up to 3.5 g of moisture content. A further release of moisture is only possible if the RH decreases or the temperature rises. At 80° C the sample will only hold 2g of moisture.
These relationships are valid for desorption (the release of moisture). Similar considerations apply for absorption. However, the isotherm should then be interpreted in such a manner that relative humidity or temperature allows for a specific humidification ??Für die Wasseraufnahme (Absorption) gelten analoge Überlegungen. Hier muss die Isotherme in der Art gedeutet werden, bei welcher Luftfeuchte- bzw. Temperatur ist eine bestimmte Befeuchtung möglich. Absorption and desorption isotherm graphs have the same course. This is due to the energy, which bonds the moisture inside the sample. The energy is released when moisture is absorbed. At the same moisture content, water is absorbed more easily than it is released.
--SLeithaeuser 14:03, 20 January 2013 (CET)