EMPA20100601
International Conference on Material Science and 64th RILEM Annual Week in Aachen - MATSCI
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NEUTRON TOMOGRAPHY MEASUREMENTS OF WATER RELEASE
FROM SUPERABSORBENT POLYMERS IN CEMENT PASTE
P. Trtik1, B. Muench1, W.J. Weiss2, G. Herth3, A. Kaestner4, E. Lehmann4, P. Lura1
1
Empa, Swiss Federal Laboratories for Materials Testing and Research, Dübendorf, Switzerland
2
Purdue University, School of Civil Engineering, West Lafayette, IN, US
3
BASF Construction Chemicals GmbH, Trostberg, Germany
4
Paul Scherrer Institut, Neutron Imaging and Activation Group, Villigen PSI, Switzerland
ABSTRACT: Internal curing of high-performance concrete (i.e., concrete with a low water to
cement ratio (w/c)) can be effectively achieved with superabsorbent polymers (SAP). The
SAP are added dry during mixing, absorb part of the mixing water and form water-filled
inclusions in the concrete. After setting, the SAP release water as needed during the first few
days of hydration, thereby limiting self-desiccation and autogenous shrinkage. For internal
curing to be efficient, the water must be released from the SAP at the appropriate time and the
SAP must be well dispersed, since water can only travel a limited distance in a hardening
cement paste with low w/c. In this study, the emptying of SAP in a low w/c cement paste was
monitored with neutron tomography. The measurements show that the SAP start emptying
around setting time and release most of the water in the first day of hydration.
1
INTRODUCTION
In the internal curing (IC) process, water is delivered to hardening concrete from water
reservoirs located within the concrete [Lur07]. In practical applications, two primary
approaches have been explored for IC [Jen06]: one approach is based on the use of watersaturated lightweight aggregates (LWA) [Ham92], while the other is based on the use of
superabsorbent polymers (SAP) [Jen01, Jen02]. Both types of IC reservoirs release the water
during the first few days of hydration, thereby limiting self-desiccation and early-age
shrinkage [Jen06, Lur07]. For IC to be efficient, it is essential that the water is promptly
released from the reservoirs and that the reservoirs are well dispersed, because water can only
travel for a limited distance in a hardening cement paste [Ben99, Jen06, Lur07]. Knowing
how much water is released, when it is released, and how far it can travel in a hydrating
cement paste is paramount for understanding and optimizing the IC process.
Lura et al. [Lur06a] and Henkesiefken et al. [Hen09] measured the 1D water transport from a
single, large LWA to a hydrating cement paste by means of X-ray radiography. They found
that, while water could move freely at very early ages, a few hours after setting it could travel
only a few mm. Bentz et al. [Ben06] used synchrotron X-ray tomography to study water
movement in three dimensions in a cement paste containing LWA in the first days of
hydration. They showed that with X-ray tomography it was possible to observe the emptying
of the LWA, while the resolution and the contrast were considered not sufficient to clearly
determine the distance to which the water travelled. Maruyama et al. [Mar09] performed a
neutron radiography study of a single saturated LWA embedded in a hardening cement paste,
where both the emptying of the aggregate and the transport of water to the paste could be
detected. In view of these results, neutron tomography, due to inherent high sensitivity for
water, was considered as a suitable experimental technique for studying the process of IC in
cementitious materials.
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TRTIK ET AL.: Neutron tomography investigation of water release from SAP in cement paste
In this project, absorption of SAP and transport of water from SAP in a sample of hardening
cement paste was studied by means of neutron tomography. This 3D visualization and
quantification technique possesses a high sensitivity for water and sufficient spatial and
temporal resolution for investigation of the internal curing process in the first day of
hydration.
2
MATERIALS AND METHODS
A Portland cement paste (w/c 0.25) was used in this study. The cement was a CEM I 42.5
Portland cement with density 3130 kg/m3 and a Blaine fineness of 2800 cm2/g. A commercial
polycarboxylate-based superplasticizer (solid content 30% by mass) was added at a dosage of
0.72% by mass of cement. Deionized water was first boiled for about 15 seconds and was
afterwards cooled down to ambient temperature to minimize the potential for forming air
bubbles in the paste. The cement paste was mixed in a vacuum mixer (Twister evolution
1822-0000) for two minutes at 450 revolutions per minute. The mixer had a maximum
volume of 250 ml and the volume of cement paste was about 50 ml. The cement paste was
characterized with isothermal calorimetry (TAM air calorimeter) at 28°C on two replicate
samples; details on samples and measurements can be found in [Lur10].
Due to the available spatial resolution of neutron tomography, large and geometrically welldefined, almost spherical SAP were produced specifically for this experiment by inversesuspension polymerization. Two particles were selected, with a diameter of about 1 mm and
weighing 0.5±0.1 mg in the dry state. The SAP used in this study are large compared to the
range of SAP particles normally used in concrete, typically 50 to 250 µm in the dry state and
100-800 µm in the swollen state [Jen02, Lur06b]. One SAP particle was inserted dry into the
cement paste, while a second particle was immersed for several minutes in a synthetic pore
solution ((mmol/l): [K+]=350, [Na+]=28, [OH-]=378 [Lot06]) before inserting it into the
cement paste; after immersion, the SAP weighed 4.8 mg. Inserting single, large particles into
the cement paste allowed positioning them close to the axis of the sample where they are
surrounded by a sufficient amount of paste.
The field of view of the neutron tomography setup was 27 mm. However, the maximum
thickness of a sample is limited by the amount of hydrogen it contains, since hydrogen has a
large neutron cross-section. The neutron cross-section of an element is an indication of the
probability of an interaction, either scattering or capture, between a neutron and a target
nucleus, expressed in barns (1 barn=10-28m2). The large neutron scattering cross-section of
hydrogen (about 30 barn for thermal neutrons (25meV) and 108 barn for cold neutrons
(1.25meV), depending on the applied neutron spectrum [Jan10]) makes neutron tomography
particularly sensitive to changes in water content of a material.
The mould for the cement paste was made of a glass-fiber reinforced Teflon cylinder with
internal diameter 6 mm, height 13 mm and thickness 1 mm. The cylinder was fitted with an oring and an aluminium cap that could be tightly screwed to the holder. Teflon has a relatively
small neutron cross-section and it does not react chemically with cement paste.
The freshly-mixed cement paste was inserted into the Teflon mould with a small spatula; a
layer of a couple of mm of cement paste was first consolidated on the bottom of the sample.
The SAP saturated with pore solution was placed onto the first layer of cement paste and
gently pressed into it. The SAP was then covered with more paste (about 2 mm thickness) and
the sample was consolidated by tapping the mould. At this point the dry SAP was pressed into
the paste, covered with more paste, and then the whole sample was consolidated again by
tapping. The whole cement paste sample including the two SAP was about 9 mm high.
International Conference on Material Science and 64th RILEM Annual Week in Aachen - MATSCI
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Finally, the o-ring was fitted onto the top of the mould and the aluminium lid was screwed
into place.
The sample was then transferred onto a cylindrical sample holder and tomographed using cold
neutrons at the ICON beamline, Paul Scherrer Institut, Villigen, Switzerland. The reader is
referred to [Kuh05] for details about the beamline experimental arrangement and to [Leh07]
for details about the tomographic experimental setup.
During the entire measurement time, the sample was kept inside the beamline hutch at
28±1°C. However, the sample holder was moved out of the beam between some of the
experiments, to allow measurements on different samples.
The temporal resolution of neutron tomography experiments is predominantly limited by the
available flux of neutrons at each particular neutron imaging beamline. Since the dynamic
process was to be visualised in three dimensions, a trade-off between the spatial and the
temporal resolutions was necessary. This was achieved by acquiring a significantly lower
number of projections than required for fulfilling the sampling theorem. In each tomography
experiment, only 46 raw projections were acquired in equiangular positions over 360 degrees;
the 1st and the 46th projection were taken from the same angular position. Four-times binning
of the CCD chip was employed for the projection acquisition, leading to pixel size of
approximately 52.7 µm. The exposure time for each projection was 10 s. The total time
required for acquisition of all raw projections was about 10 minutes for each tomography
measurement. The first tomography was completed about 35 minutes after mixing of the
cement paste, while the last measurement was completed at about 20.5 hours after mixing. A
total of 23 tomographies were performed.
After the application of dark- and flat-field corrections of the raw projections, the data were
reconstructed using the algebraic technique based on penalized likelihood image
reconstruction [Ahn06]. A standard reconstruction by filtered back-projection was used as the
initial approximation in this iterative algorithm.
3
RESULTS
Figure 3 shows the cumulative heat release measured on two replicate samples of cement
paste with w/c 0.25. The measurements lasted for 72 hours and were performed at 28°C,
corresponding to the average temperature at the beamline during the neutron tomography
experiments. At 20.5 hours, the time of the last neutron tomography, the cement pastes had
developed about 80% of the heat at 72 hours.
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TRTIK ET AL.: Neutron tomography investigation of water release from SAP in cement paste
Cumulative heat (J/g)
250
200
150
100
50
0
0
12
24
36
48
60
72
Time (hours)
Fig. 1.
Cumulative heat of a w/c 0.25 Portland cement paste measured at 28°C.
Corresponding vertical ortho-slices from the reconstructed 3D datasets of the sample at
approximately 4, 8, 12.5 and 20.5 h after mixing are shown in Figure 2, from left to right.
Materials with high neutron cross-section appear bright. The SAP are visible as almost white
circles in the first image, surrounded by cement paste that appears light gray. The dark gray
rim around the cement paste is the Teflon mould, while air on the top of the paste appears
black. With the progress of hydration, black areas appear within the original location of the
SAP, indicating the formation of vapour-filled space. The bottom SAP appears to empty by
detaching from part of the wall, while in the top one, an empty area initially forms within the
SAP. In the last orthoslice, the upper SAP appears almost empty, while an area with high
contrast is still present in the lower one.
Fig. 2.
Corresponding vertical orthoslices from the reconstructed 3D datasets at approximately 4,
8, 12.5 and 20.5 h (from left to right) after mixing.
International Conference on Material Science and 64th RILEM Annual Week in Aachen - MATSCI
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a)
b)
c)
d)
Fig. 3.
3D representation of the superabsorbent polymer particles within the cement paste
sample. The outer cylindrical shell is the Teflon mould, the inner cylinder is the cement
paste and the two spherical particles are the superabsorbent polymers. The tomographies
were taken at approximately 4, 8, 12.5 and 20.5 h (from a to d in the figure) after mixing.
Figure 3 shows a three-dimensional representation of the whole sample subjected to neutron
tomography, including the Teflon mould (outer cylindrical shell, transparent), the cylindrical
cement paste sample (inner cylinder, transparent), and the SAP particles (the two dark,
originally almost spherical particles within the inner cylinder). The tomographies were taken
at approximately 4, 8, 12.5 and 20.5 h (from left to right) after mixing. The SAP particles are
initially filled with water that has high neutron contrast (dark in Figure 3). The water is
progressively lost during hydration, leaving behind empty pores that are shown as a lighter
shade of gray in Figure 3.
At about 3 hours after casting, the SAP on the bottom of the paste cylinder, which was
immersed into a synthetic pore solution for several minutes before inserting it into the cement
paste, has a volume of 8.5 mm3. Assuming for simplicity a density of 1 for both the polymer
and the absorbed pore solution, this implies that the bottom SAP absorbed further 3.7 mg
while in the cement paste. The total absorption of the bottom SAP corresponded to about 16 g
pore solution per g SAP. The bottom SAP has an almost spherical shape, with a diameter of
about 2.5 mm in the swollen state. Approximately 4 hours after casting, the bottom SAP
began to lose water, partially detaching from the wall of the cavity from the left and shrinking
gradually, ending as a remnant in the right side of the cavity (Figure 3).
The upper SAP, which was inserted dry into the cement paste and expanded by absorbing the
pore solution from the paste, has a volume of 6.5 mm3 at 3 hours. This means a diameter of
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TRTIK ET AL.: Neutron tomography investigation of water release from SAP in cement paste
about 2.3 mm in the swollen state (though the upper SAP has a more irregular shape) and
absorption of 12 g pore solution per g SAP. This different absorption of the two SAP,
however, may be a consequence of the inaccurate determination of the dry weight of the SAP,
which was determined on a balance with accuracy 0.1 mg. The upper SAP starts losing water
later than the bottom one, between 8 and 12.5 hours. The SAP detaches from the pore wall at
two different locations, finally ending up as a remnant attached to a wall of the cavity,
similarly to the bottom one.
In Figure 4, the total signal from the two SAP particles and their cumulative signal as a
function of hydration time are plotted. The signal is defined as the sum of the greyscale values
from the regions segmented as SAP and is expressed as a percentage of the maximum signal
of the SAP, which was reached at 3 hours. The SAP on the bottom, initially saturated with
synthetic pore solution, is stable in the first couple of hours. The majority of the absorption of
pore solution from the cement paste must have occurred the first 30 minutes, before the first
tomography was completed. The bottom SAP starts emptying at an age of approximately 3 h.
The rate of water loss accelerates between 6 and 8 hours, and then decreases slowly until the
end of the experiment at 20.5 hours. The upper SAP, added dry to the paste, shows however a
different picture. Initially, it appears to absorb some pore solution from the paste up to an age
of approximately 3 h. After a phase where the signal remains constant, the upper SAP starts
losing water at 9 h, a moment when about 40% of the water has already been lost from the
lower SAP. The loss of water from the upper SAP proceeds then faster and catches up with
the lower one after about 15 h. At 20.5 h, about 16% of the signal at 3 h is still present in both
the SAP. Assuming that the total signal of the SAP is coming from both the polymer and the
pore solution, at 20.5 h the lower SAP may contain about 13% of the initial pore solution and
the upper SAP about 8-9%.
An alternative to a plot of the change in signal from the SAP particles (Figure 4) would be a
plot of the volume change of the SAP particles, which can be directly derived from the
reconstructed 3D images shown in Figure 3. This second plot, not shown in this paper, is
almost identical to Figure 4. This confirms that the change in signal is equivalent to the
volume change of the SAP, and reinforces the interpretation of this phase as pore solution
which is transported from the SAP to the hydrating paste.
Cumulative signal (%)
100
Lower SAP (presaturated)
80
Upper SAP (initially dry)
Average of 2 SAP
60
40
20
0
0
4
8
12
16
20
24
Time (hours)
Fig. 4.
Signal from the two superabsorbent polymer particles and their average signal as a
function of time, normalized to the signal at 3 hours.
International Conference on Material Science and 64th RILEM Annual Week in Aachen - MATSCI
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Figure 5 shows the histograms of the first and last neutron tomographies. The four peaks in
the first histogram (Figure 5, left) correspond, from left to right, to air, Teflon, cement paste
and SAP particles filled with water. As expected, the first two peaks remain unaltered over
time, both in height, shape and position, from the first to the last tomography. However, the
small peak to the utmost right in the first histogram, corresponding to the region occupied by
the SAP particle, disappears in the last histogram when plotted in the full scale. However, as
is shown in Figure 5 right, a small peak of SAP remains in the last tomography histogram,
which corresponds to the regions occupied by the shrunk SAP particles. At the same time, the
third peak from the left, corresponding to the cement paste, becomes slightly shorter, broader
and shifts towards higher greyscale values, indicating an increase in neutron cross-section.
The increase in neutron cross-section indicates the absorption of water by the paste. The total
signal from the sample lost about 0.57% from the first to the last tomography. However, the
total signal changes from tomography to tomography, with a maximum increase of 0.27% and
maximum decrease of 0.91% during 20.5 h of experiment. Therefore the change in signal is
likely due to measurement uncertainty rather to evaporation of water from the sample.
Fig. 5.
4
Histograms of the first (31 minutes after mixing, left) and last (20 hours and 21 minutes
after mixing, middle and right) neutron tomographies. The abscissas represent the
grayscale values in the neutron tomography reconstructed images and the ordinate axis
corresponds to the count of voxels. The peaks correspond, from left to right, to air,
Teflon, cement paste and superabsorbent polymer particles filled with water. The peak of
the shrunk SAPs becomes invisible in the full scale histogram (middle) and is therefore
presented in a zoomed-in version (right).
DISCUSSION
As shown in Figures 2-4, volume change and water content of single SAP particles within a
hydrating cement paste could be followed and quantified by neutron tomography. To the
knowledge of the authors, this is the first true visualization of the IC process by SAP. In fact,
low-field NMR relaxation measurements of the amount of water in the SAP as a function of
hydration time have been recently published [Nes09]. However, with NMR only the state of
water in a bulk sample was determined, while in the present study individual SAP particles
within a cement paste were followed in three dimensions in the first day of hydration.
The obvious drawback of neutron tomography is that IC water release can be imaged with
sufficient resolution only from SAP particles that are larger than those usually used in
practical applications. However, it was shown [Lur06b] that varying the size of SAP between
about 50 and 200 µm in the dry state (corresponding to about 100-600 µm in the swollen
state) did not significantly impact the autogenous shrinkage reduction in a cement paste. In
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TRTIK ET AL.: Neutron tomography investigation of water release from SAP in cement paste
this study, particularly large SAP, 2-2.5 mm in the swollen state (Figures 2 and 3), were
utilized. However, the results show that also smaller SAP, within the size range relevant for
practical applications, can be imaged with neutron tomography. Moreover, some general
features of the initial water absorption of the SAP in the cement paste and the later release of
water to the hardening cement paste can be conveniently studied on systems with large SAP.
For example, the distance of water transport in a hydrating cement paste may be easier to
determine if the IC reservoirs are large, almost round and surrounded by a large thickness of
cement paste.
Figure 4 shows that absorption of pore solution by a dry SAP particle took about 3 hours to
complete, even if 90% took place in the first 30 minutes. This should be compared to
absorption of different SAP in synthetic pore solutions [Jen02], which is completed within
about half an hour. The current opinion among researchers is that SAP absorb most of the
pore solution already during concrete mixing, which is also supported by rheology
measurements. The slow absorption measured in the present study may be a consequence of
the large size of the SAP particle. Other differences with the use of SAP in practice are that
the dry particle was inserted by hand into a cement paste, instead of introducing it during
mixing, and the consolidation by tapping of the paste.
The SAP particle on the bottom of the sample (Figure 2 and 3), was initially immersed in a
synthetic pore solution, before inserting it into the cement paste. This particle absorbed a
further amount of water after being inserted into the cement paste. A possible explanation is
that the actual pore solution in the fresh cement paste was of lower ionic strength than the
synthetic pore solution absorbed by the SAP. It is interesting to notice that in this case the
absorption was completed in the first 30 minutes and the SAP particle remained constant in
volume up to about 4 hours. The total absorption of the two particles by weight of SAP is of
16 g/g for the preswollen one and 12 g/g for the dry one. These values are within the expected
range and their difference may be overestimated because of possible inaccuracies in
determining the dry mass of the particles.
It is noticed that the SAP did not lose pore solution during the dormant phase of cement
hydration. The lower and larger SAP started emptying around 4-5 hours, which is about the
start of the acceleration period (Figure 1) and may correspond to the time of initial set of the
paste. The upper SAP started emptying at about 8-9 h, after final set of the paste. It is not
clear why the lower SAP emptied first. It may be due to the larger quantity of absorbed pore
solution and also to its larger size. In fact, larger filled pores are expected to empty first in a
self-desiccating cement paste [Ben99, Jen01].
When the SAP lose water, the cavity originally occupied by them empties, but it does not
collapse even if the emptying occurs before final set (Figure 2). Different modes of emptying
are observed. The bottom SAP detaches from the wall of the cavity from one side, shrinks to
the other side and finally the polymer with little absorbed water remains attached to part of
the wall. The upper SAP appears to detach from the wall of the cavity at two different, almost
opposite locations. The further release of pore solution leads to coalescence of these two airfilled locations within the SAP, leading to a SAP particle with an apparent through-hole. In
the final step, the SAP shrinks further and remains attached to a wall of the cavity. In all
cases, it is clear that transport of the water from the SAP after partial detachment can only
happen to the side of the cavity in contact with the particle. It is noticed that models for IC
[Ben05] assume that transport of water can occur uniformly in all directions radiating from
the water reservoir. This may not represent the real case according to the present results.
However, as the size of the SAP diminishes, this effect should become less important.
According to Jensen and Hansen [Jen01], the optimum amount of entrained water for a
cement paste with w/c<0.36 can be calculated according to Powers’ model and is equal to
International Conference on Material Science and 64th RILEM Annual Week in Aachen - MATSCI
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w/ce=0.18·w/c. This amount of water allows filling all the available space in a low w/c paste
with hydration products, without initiating self-desiccation. For the cement paste used in this
experiment, the actual w/c was slightly lower than 0.25, because part of the water absorbed by
the two SAP particles was initially part of the mixing water. Subtracting the water absorbed
from the paste into the SAP, one obtains a w/c of 0.23. According to [Jen01], for a w/c of
0.23, w/ce=0.18·0.23=0.041 is calculated. If this value is multiplied by the amount of cement
in the sample, one obtains the mass of water needed for optimum IC of the cement paste: 18.6
mg. This compares quite well with about 14 mg of pore solution absorbed initially in the two
SAP particles. At 20.5 hours of hydration, the SAP still contained about 1.5 mg of water. It is
expected that the SAP were completely emptied in the following hours, as hydration
proceeded in the cement paste (Figure 1).
The shift in the histogram of the cement paste peak towards higher counts between the first
and the last tomography (Figure 5) confirms that the cement paste is receiving the pore
solution from the SAP. A preliminary analysis of the water transport from the SAP into the
cement paste indicates that water was transported fairly uniformly over the entire volume of
the hydrating cement paste in the first 20 h. In other words, there is no clear evidence in the
neutron tomographies of a developing gradient around the SAP particles. A more detailed
analysis is currently being performed [Trt10].
5
CONCLUSIONS
In this study, large SAP particles (about 2 mm in the swollen state) in a cement paste with a
w/c of 0.25 were imaged by neutron tomography during the first day of hydration. The shape
and volume changes of the SAP could be followed and the amount of water lost precisely
quantified. The SAP particles remained in the swollen state during the dormant period of
cement hydration, then rapidly lost water during the acceleration period to the point where
they were nearly empty by an age of 20.5 h. The rate of water loss closely followed the
kinetics of hydration as measured with isothermal calorimetry. The SAP particles shrunk as
water was lost, partially detaching from the cavity where they resided, ending up as a remnant
attached to a side of the wall.
The data are currently the object of a detailed analysis to evaluate the distance of water
transport from the SAP in the first day of hydration.
ACKNOWLEDGMENTS
Thanks to Erwin Pieper’s team at the Empa workshop that managed to produce reinforced
Teflon moulds on short notice. Thanks also to Carmelo Di Bella and Luigi Brunetti for help
with sample preparation and the calorimetry experiments. The first author would like to thank
Dr Federica Marone (SLS, PSI) for the initial discussion and help regarding the use of
algebraic reconstruction algorithms.
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