Moisture transport, condensation and dehydration
of gypsum under fire conditions

    Introduction
Gypsum is widely used as a building material in di erent applications such as plasterboard or building blocks. The large latent heat stored in free and chemically bound water, combined with a low thermal conductivity make it an excellent insulating and re retarding material. Gypsum plasterboard acts as a protective barrier for the load bearing wood or steel frame it is attached to. As gypsum is a very clean and well defined material we have also used is as a benchmark material to study the processes like interpret moisture and/or dehydration at high temperatures. Gypsum is also from an NMR point of view a nice material  because it does not contain any paramagnetic impurities. Depending on the vapour pressure the dehydration will take place in one ore two steps as can be seen in figure 1. Using NMR one can also seen these processes and If the NMR signal can be related to the the amount of chemically bound water and hence the NMR signal can be used to obtain the degree of hydration of gypsum.




Figure 1: (left) Mass vs. temperature curves of gypsum during heating as measured by TGA. Two experiments were performed: open and closed. The one step process under open conditions is replaced by a two step dehydration under closed conditions, with a mass loss ratio of 3:1.
(right) Measured NMR signal of dry gypsum during slow dehydration. The signal is shown as a function of the relative mass of the sample.
A relative mass of 1 corresponds to complete hydration. The signal decreases linearly with the degree of hydration.


    Conceptual Model
To summarize the di fferent processes taking place in a gypsum, a schematic representation of the experiment is shown in Fig. 2. Three curves are shown: temperature (T), hydrogen content (H), and the approximated vapour pressure (Pv).


Figure 2. The combined hydrogen (H, solid line) and temperature (T, dashed line) profi le. The moisture peak is indicated in grey. The hydration degree,
 and the moisture content are shown on the right vertical axis; the temperature is indicated on the left axis.  The small arrows indicate the two
dehydration fronts. An approximate pressure peak is shown to illustrate that the vapour transport is directed in two directions.


The moisture peak is highlighted in grey. Especially the fi rst dehydration front is a source of vapour, which is resulting in a local increase in vapour pressure. The exact location of the pressure peak is somewhere in between the dehydration front and the surface, due to the increase of temperature towards the surface.The maximum in vapour pressure, causes the vapour to di ffuse in two directions (indicated by the double arrow). The vapour which is diff using towards the drying surface will flow into a region of higher temperatures, and can exit the porous material at the drying surface. However, the vapour which is di ffusing towards the back of the sample will flow into a cooler region. At lower temperatures the vapour will condensate, which will result in the built up of the a moisture peak. The consequence of the increase of free moisture just behind the dehydration front, is that the front which initially starts as a pure dehydration front, changes into a front as a result of a mix of boiling and dehydration.

Experimental setup
For the experiments in this study, a NMR scanner built used. This instrument was especially designed for quantitative measurements of moisture in porous materials with short transverse relaxation (T2) times (unlike standard Magnetic Resonance Imaging (MRI), which is generally used in a qualitative way). The machine makes use of the magnet of a whole body MRI scanner (Gyroscan, Philips) which operates at a main field of 1.5 Tesla. The setup is placed within the scanner and a schematic diagram is given in Fig 3. The sample is heated with a halogen lamp. The reflector of the lamp was gold plated to ensure maximum reflection of the infra-red radiation. The sample is placed inside the birdcage coil and is thermally insulated using mineral wool in order to create a 1D experiment.


Figure 3: Schematic diagram of the NMR setup. A 1.5 T MRI scanner provides the main magnetic field B0. Two
Helmholtz coils provide a gradient G of 86.5 mT m-1 in the x direction. A bird-cage coil is used for both applying
RF pulses B1 and receiving the NMR signal. An array of four 100 W halogen lamps is used to heat the sample,


Results
In situ, building materials will have a moisture content in equilibrium with an environmental relative humidity. Therefore, in this experiment the gypsum was equilibrated at 50 % RH. The gypsum has a very low moisture content of 6.10-4m3 m-3. This relative humidity was chosen close to the average humidity in a building. Compared to the amount of chemically bound water in gypsum, the free water content is only 0.4 % of the total water content. The moisture and temperature profi les are shown every 4.7 minutes in Fig. 4.


Figure 8. Hydrogen (a) and temperature (b) pro les during heating of gypsum in equilibrium with 50 % RH. The initial moisture content is  6.10-4m3 m-3. The pro files are shown every 4.7minutes. Initially the sample is fully hydrated ( =1). Two separate dehydration fronts can beobserved (horizontal arrows). The temperatures on these fronts are marked in the temperaturepro les. A large amount of water is released in the rst dehydration reaction, whichresults in an increase in free moisture content


 The magnitude of the signal at the start of this experiment is about 7 % of the initial signal in the saturated experiment.  The initial moisture content in this experiment is so low that we will split the y axis in two parts: hydration degree, and free moisture content.  Based on the relaxation times and the densities of we can estimate the signal contribution of free moisture in Fig. 4a to be in the order of 2 %, which is smaller than the noise on the moisture profi les. The gypsum sample is initially fully hydrated ( =1). Any increase of the signal above the maximum hydration degree of one must therefore originate from free moisture. Furthermore, we will use the term hydrogen pro files since it is more suited in this experiment. Three observations can be made from the hydrogen pro les. First, a front is moving through the sample (horizontal arrow at a hydrogen content roughly between 0.45 and 1). The temperature at the front is about 100 oC. Dehydration is taking place since the hydration degree is lower than one. Based on the temperatures, this front can be attributed to the first dehydration reaction. The first dehydration reaction releases 75% of the chemically bound water. As a result the vapour pressure in the sample will increase.
The second observation is related to the increase in partial vapour pressure due to dehydration. Behind the dehydration front, a significant increase in hydrogen content above =1 is observed. Therefore, the increase in signal cannot be coming from chemically bound water, and must be coming from free moisture.The maximum signal intensity at the peak of the hydrogen pro le can be compared with the saturated experiment. The moisture content is approximately 0.04 m3 m-3. This built up of the moisture peak is due to the condensation. Note that the temperature at which the moisture is condensating is about 70 oC.
Thirdly, a second front can be observed moving through the sample behind the first front (indicated by the bottom arrow, hydrogen content between 0.45 and 0). The temperatures of the dehydration front are marked in the temperature pro les.  The temperatures range from 200 oC at a position of 0 mm to 230 oC further into the material, which is signi ficantly higher than the first front. The temperatures measured on this front are comparable to the temperatures related to the second dehydration of gypsum. This second front can therefore be identi fied as the second dehydration reaction.



G.H.A. van der Heijden, L.Pel, H.P. Huinink and K.Kopinga., One-dimensional scanning of moisture in heated porous building materials with NMR, J. of Magn. Reson 208 235-242 (2011)

G.H.A. van der Heijden, L. Pel, H.P. Huinink, K.Kopinga, Moisture transport and dehydration in heated gypsum, an NMR study, Chemical Engineering Science 66 4241–4250 (2011)

G.H.A van der Heijden, NMR imaging of moisture inside heated porous building materials, Eindhoven University of Technology (2011). (Download 8 Mb)