Progress in Nanoscale Studies of Hydrogen
Reactions in Construction Materials
J.S. Schweitzer, R.A. Livingston, J. Cheung, C. Rolfs, H.-W. Becker, S. Kubsky,
T. Spillane, J. Zickefoose, M. Castellote, N. Bengtsson, I. Galan,
P.G. de Viedma, S. Brendle, W. Bumrongjaroen, and I. Muller
1
Abstract. Nuclear resonance reaction analysis (NRRA) has been applied to measure the nanoscale distribution of hydrogen with depth in the hydration of cementitious phases. This has provided a better understanding of the mechanisms and
kinetics of cement hydration during the induction period that is critical to improved concrete technology. NRRA was also applied to measure the hydrogen
depth profiles in other materials used in concrete construction such as fly ash and
steel. By varying the incident beam energy one measures a profile with a depth
resolution of a few nanometers. Time-resolved measurements are achieved by
stopping the chemical reactions at specific times. Effects of temperature, sulfate
concentration, accelerators and retarders, and superplasticizers have been investigated. Hydration of fly ashes has been studied with synthetic glass specimens
whose chemical compositions are modeled on those of actual fly ashes. A combinatorial chemistry approach was used where glasses of different compositions are
hydrated in various solutions for a fixed time. The resulting hydrogen depth profiles show significant differences in hydrated phases, rates of depth penetration
and amount of surface etching. Hydrogen embrittlement of steel was studied on
slow strain rate specimens under different corrosion potentials.
J.S. Schweitzer, T. Spillane, and J. Zickefoose
University of Connecticut, Storrs, CT, USA
R.A. Livingston
University of Maryland, College Park, MD, USA
J. Cheung
W.R. Grace, Cambridge, MA, USA
C. Rolfs and H.-W. Becker
Ruhr-Universität Bochum, Bochum, Germany
S. Kubsky
Synchrotron SOLEIL, Saint-Aubin, Gif-sur-Yvette CEDEX, France
M. Castellote, N. Bengtsson, I. Galan, and P.G. de Viedma
Institute of Construction Science "Eduardo Torroja" (CSIC), Madrid, Spain
S. Brendle
Delft University of Technology, Delft, The Netherlands
W. Bumrongjaroen and I. Muller
Catholic University, Washington, DC, USA
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1 Introduction
At the previous International Symposium on Nanotechnology in Construction
(NICOM2) in 2005 in Bilbao, the use of nuclear resonant reaction analysis
(NRRA) to investigate the induction period in the hydration of cementitious
phases was described [1]. This method gives in-situ measurements of hydrogen
concentration with depth at a depth resolution of a few nanometers [2]. It uses the
1
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ER = 6.400 MeV resonance in the H( N,αγ) C reaction [3]. A nitrogen ion beam
with precisely regulated energies and good energy resolution is produced by a 4
MeV Dynamitron tandem accelerator at Ruhr-Universität Bochum, Germany
which provides an H-detection sensitivity of about 10 ppm and an H-depth resolution of a few nm at the surface [4]. This has enabled the detailed investigation of
the effect of temperature and other factors on the induction period [4-6].
Since NICOM2 the application of NRRA has gone beyond the silicate phases
in Portland to look at the hydration of the calcium aluminate phase. It has also
been applied to study the pozzolanic reactions of fly ash glasses and the process of
hydrogen embrittlement of steel. These studies are reported here.
2 Experimental Approach
The experimental procedure for studying hydration has been described in detail
elsewhere [2]. A major difference between this method and others for studying
cement, like calorimetry, is the material being studied is not in powdered form, but
is rather a solid pellet, that presents a smooth surface to the ion beam. Cementitious phases such as tricalcium silicate (C3S) and tricalcium aluminate (C3A) are
molded into cylindrical pellets of 12.7 mm diameter and fused. A typical experiment involves 8-12 samples. They are hydrated in a common solution bath of
specific composition and temperature; individual samples are removed at specific
times. Samples are stored and handled under inert atmosphere both before and after the chemical reaction. Reacted samples are kept in vacuum until analysis.
Each sample is a single point in the material’s hydration time history. To obtain
an H depth profile, the beam energy is increased stepwise from just below the
resonance energy of 6.400 MeV. As this is an isolated resonance in the H cross
15
section the reaction only occurs when the N ion energy is at the resonance energy. If its energy is greater, no reactions occur until the beam loses enough energy by scattering to get down to the resonance energy. At each energy step, the
15
N ion will reach the resonance energy at a particular sample depth, and the
hydrogen concentration at that depth is measured. For each energy step, a
gamma-ray spectrum is acquired, typically 10,000 cts per minute. The beam energy is increased in 10 keV steps to 7 MeV to resolve thin surface layers, and then
in coarser steps (100-500 keV) as the profile typically changes more slowly in this
region. The maximum beam energy is limited to 12 MeV to avoid interference
from the next higher energy resonance.
A plot of the H signal as a function of incident beam energy allows a visualization of the H depth profile, as shown in Fig. 1 for a C3S sample during the induction period. It shows the typical Gaussian peak associated with a surface layer, on
the left edge of the figure, followed by a diffusion-type region at greater depths.
Progress in Nanoscale Studies of Hydrogen Reactions in Construction Materials
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The NRRA coordinates of beam energy and counts per charge have been converted to depth and H concentrations on the upper and right axes, respectively.
Profiles for three times are shown to illustrate the diffusion region growth with the
Gaussian peak unchanged. The induction period ends with the surface layer
breakdown. This is easily recognized by the absence of the Gaussian peak and a
change in the shape of the diffusion region curve that allows the time for the induction period to be determined to a relative precision better than 5%.
o
Fig. 1 Evolution of hydrogen depth profile for triclinic C3S hydrated at 30 C. The inset expands the left portion of the figure for clarity
3 Cement Measurements
NRRA hydration studies have been reported on several of the silicate cement
phases including the effects of retarders, absorbers, and temperature. From the
temperature dependence it has been possible to determine the activation energy.
The studies have been extended to investigate the hydration properties of calcium
aluminate (C3A). As is well known, C3A appears to have rapid early hydration reactions. Therefore, to obtain better sensitivity for seeing changes in the hydration
o
properties, we have performed studies at temperatures of typically 5-10 C. Factors
that have been studied for C3A include the effects of gypsum, retarders, and superplasticizers. Figure 2 shows a comparison of the hydration profiles after 40
o
minutes of hydration at a temperature of 10 C. This figure shows the results for
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three different conditions. All samples are hydrated in a fully saturated calcium
hydroxide water solution. In one case, gypsum was also added to the solution. In
the third case, both gypsum and a superplasticizer were added.
o
Fig. 2 C3A hydration after 40 min. at 10 C for three different solutions
All three profiles are very different from the hydration profile for tricalcium
silicate (Fig. 1). When no gypsum is present the hydration profile has saturated
down to about ¾ of a micron with a diffusion tail to greater depths. This indicates
rapid early reaction and corresponds to flash set of Portland cement. When gypsum is added to the hydrating solution, hydration has only occurred to about a ¼
of a micron. As shown in the inset in Fig. 2, a very sharp rise and flat plateau
appears at the leading edge. We believe this is due to a crystalline surface layer,
presumably ettringite, that has impeded the hydration rate. Examination of the
profiles in pellets taken at other times in this series show essentially no difference.
The implication is that the formation of the ettringite layer stops the hydration reaction in less than five minutes. When a superplasticizer is also added to the gypsum and calcium hydroxide in solution, the leading edge has a different shape that
resembles a Gaussian curve that would be typical of a disordered noncrystalline
layer. Nevertheless, the rest of hydration curve falls off even more rapidly than
the one with the ettringite layer, suggesting that the superplasticizer interferes with
the formation of the surface layer or alters its character, but similarly stops the hydration reaction at a very early time.
Progress in Nanoscale Studies of Hydrogen Reactions in Construction Materials
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4 Fly Ash Measurements
A simple way to study fly ash hydration is with glasses whose chemical compositions are identical to those of a particular fly ash. The glass samples are made with
smooth flat surfaces for NRRA study, so we can study the hydration changes as a
function of small changes in the fly ash chemical composition.
NRRA provides direct observation of the depth profile of hydrogen diffusing
2+
into the fly ash in exchange for the alkalis and Ca as shown in Fig. 3. These H
depth profiles are for three calcium aluminate silicate glasses that have chemical
compositions based on data from actual fly ash glass samples [7]. The major difference among these specimens is the Ca/(Na+K) ratio. The specimens were hydrated for 72 hrs in simulated concrete pore solution (0.4 M KOH + 0.0215 M
Ca(OH)2, pH=13.5). The profile for the low Ca specimen appears to be much shallower than the others, which suggests that it reacts more slowly. Another possibility however, as shown by analysis of the dissolved ions in the solution, is that the
rate of etching of this glass is so rapid that a full depth gradient cannot develop, as
initial hydrated portions are etched leaving a fresh surface. The profiles for the
other two glasses show hydrogen profiles that reach a plateau that extends over the
1-micron range of the measurement. This indicates the presence of a saturated
phase. Given the higher Ca content of these fly ashes, this is likely to be a C-S-H
gel formed through the alkali-activated, or self-cementing reaction.
Repeating these measurements at different hydration times makes possible the
determination of parameters such as kinetic rate constants and diffusion coefficients. However, these profiles generally cannot be fitted to a simple Fick’s Law
model based on the erfc function. Instead, more complicated mathematical relations would be required, possibly involving concentration dependent diffusion coefficients [8] . This diffusion process within the unreacted core is not the same as
the one that is associated with the topochemical reaction that involves diffusion
through the surrounding C-S-H gel layer. Both diffusion processes affect the overall rate of growth of the gel. However, the reaction at the core/gel interface is
affected by the fly ash glass composition, whereas the diffusion to the core’s reaction surface is determined by the properties of the gel. By studying the hydration
Fig. 3 NRRA hydrogen depth
profiles of three synthetic fly
ash glass specimens
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reactions of the fly ash in isolation and in a cement paste, it is possible to determine the contributions of the individual diffusion processes.
5 Hydrogen Embrittlement Measurements
The application of NRRA to investigate hydrogen embrittlement is illustrated by
Fig. 4 [9]. This presents the H profiles for three samples of a commercial cold
drawn pearlitic steel (0.77% C) that were exposed to different corrosion conditions
designed to accelerate hydrogen embrittlement. The samples were subjected to
the combined action of stress and aggressive environment through the Slow Strain
Rate Test (SSRT) in which a potential is applied so that H is generated within
cracks and pits in the specimen. The aggressive environment was a naturally
aerated 0.05 M aqueous solution of NaHCO3 (pH = 8.5) previously shown to be
capable of promoting SCC in cold drawn steels [10, 11]. An anodic potential of 300 mV (SCE) was applied to sample N1. This condition promotes pitting corrosion, a localized attack in which hydrogen is produced by acid hydrolysis of the
corrosion products in the interior of the pit[ 10, 11]. In contrast, sample N14 was
subjected to a cathodic potential of -1200 mV (SCE). At this potential, water is
not thermodynamically stable [12]. This produces molecular hydrogen at the metal
surface some of which diffuses into the metal and promotes embrittlement.
For the NRRA measurement, the sample was mounted so that the beam hit it
normal to the fracture surface. It can be observed in Fig. 4 that the resulting shape
of the H profile is different depending on the treatment. The -1200 mV (N14)
sample has a higher concentration near the fracture surface, but it diminishes to
values very close to that of the inert sample at about 0.3 µm. In between it exhibits
a convex shape, which indicates a departure from a simple Fick's Law diffusion
process. The N1 sample shows a more Fickian profile but exhibits a flat profile of
hydrogen deeper in the sample at depths greater than 0.5 µm. The total amount of
hydrogen in each sample can be determined by integrating the area under the
curve. The peak at the surface in each curve may be due to H adsorption, the first
step in the HE process. However, surface contamination such as grease from handling or exposure to atmospheric humidity may also be present. Therefore, to
eliminate this peak the integration of the hydrogen concentration was restricted to
the range between 0.020 µm and 0.48 µm, the maximum depth of the NRRA tech2
nique. The resulting area densities are 0.13, 0.67 and 0.46 µmol/cm for the N4
(inert), N1 (-300 mV) and N14 (-1200 mV) samples, respectively. Therefore, the
sample tested under pitting corrosion conditions took up a greater amount of hydrogen than the one tested at the more negative potential.
These results can also be used to estimate key parameters in the hydrogen embrittlement process. By the nature of the SSRT procedure, the H concentration in
the sample is the critical value for fracture. The average of the two samples N1
2
and N14 is 0.56 µmol/cm . Taken over a depth of 0.46 µm, this is an average con3
3
centration of 12 mmol/cm or 7.3 x 1021 atoms/cm . Steel has a number density of
22
3
8.53 x 10 atoms/cm . Hence the critical H value is 8.6 atomic percent.
Concerning the diffusion constant, as noted above it is not possible to fit the
profiles with Fick’s Law models. But it is possible to make a rough estimate,
given the time of exposure to solution of ~ 48 hrs, and the effective depth of 0.46
Progress in Nanoscale Studies of Hydrogen Reactions in Construction Materials
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2
µm. The resulting effective diffusion constant is on the order of 10 cm /sec. For
-10
-11
2
comparison, the values in the literature are in the range of 10 -10 cm /sec [20,
21]. However, these were determined indirectly, whereas these NRRA values are
measured directly. A major advantage of NRRA is that it is nondestructive and requires no special treatment of the samples. Thus it is possible to measure the same
steel sample repeatedly over time to observe how the depth profile evolves.
Hence, this time-resolved approach can be used to sort out the different diffusion
mechanisms that control the hydrogen embrittlement process.
6 Conclusions
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We have used NRRA with the H( N,αγ) C reaction to study physical and chemical processes at a nanometer scale involved in the hydration of various components of Portland cement. This has given new insights into the mechanisms that
control the setting and curing of concrete and has helped to resolve some longstanding controversies. For the calcium silicate phases, there is generally the
development of a semi-permeable surface layer that controls the hydration rate of
reaction during the induction period. The induction period length has a classic Arrhenius-type dependence on temperature.
Tricalcium aluminate shows a very different spatial pattern of hydration than
the calcium silicates. There is no semi-permeable surface layer. Instead a crystalline layer develops rapidly when gypsum is present and apparently slows further
reaction with water.
Initial experiments with accelerators and retarders have shown that they significantly affect the development the hydration profile, either by changing the
permeability of the surface layer or the diffusion coefficients in the substrate. For
a commercially available retarder like sodium gluconate, the rate of hydration is
strongly dependent on the dosage. These studies have demonstrated the great
promise of nuclear resonance reaction analysis for better understanding the hydration properties of cements and that the hydration of Portland cement is a valid
topic for nanoscience research.
The NRRA technique has also been used to study hydration reactions of fly ash
and hydrogen concentrations resulting in hydrogen embrittlement of steels.
Acknowledgments. The authors are indebted to the support of the National Science Foundation under contract CMS-0600532 that made this research possible.
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