Moisture transport, condensation and
of gypsum under fire conditions
Gypsum is widely used as a building material in dierent
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
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
(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.
To summarize the different 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)
profile. The moisture peak is indicated in grey. The
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 first
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 be
diffuse in two directions (indicated by the double arrow). The
vapour which is diffusing 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
diffusing 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.
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
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 profiles are
shown every 4.7 minutes in Fig. 4.
Figure 8. Hydrogen (a) and
temperature (b) proles during heating of gypsum in equilibrium
with 50 % RH. The initial moisture content is 6.10-4m3 m-3. The
profiles 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 temperatureproles. 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 profiles. 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 profiles since it is
more suited in this experiment. Three observations can be made from
the hydrogen proles. 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 prole 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 proles. The temperatures
range from 200 oC at a position of 0 mm to 230 oC
further into the material, which is significantly 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 identied as the second
- An extensive description can be found in:
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
G.H.A van der Heijden, NMR imaging of moisture inside heated
porous building materials, Eindhoven University of Technology
to main page