Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
MULTI-MODE X-RAY STUDY OF SODIUM AND MAGNESIUM
SULFATE ATTACK ON PORTLAND CEMENT PASTE
N.N. Naik1, A.C. Jupe1, S.R. Stock2, A.P. Wilkinson3, P.L. Lee4, and K.E. Kurtis1,*
1
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta,
Georgia
2
Institute for Bioengineering and Nanoscience in Advanced Medicine, Northwestern University,
Chicago, Illinois
3
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia
4
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois
ABSTRACT
Sulfate attack is potentially one of the most damaging forms of degradation affecting portland
cement-based materials. However, despite over six decades of study, considerable uncertainty
remains in optimally selecting and proportioning materials for sulfate resistance, and current
understanding of the actual mechanisms of degradation in sulfate environments remains
incomplete. In this research, x-ray microtomography (μCT) and energy dispersive x-ray
diffraction (EDXRD) are used synergistically to produce a time-resolved correlation between
progressive physical and chemical damage to sulfate-exposed cement pastes. Companion
expansion and strength data has also been obtained for comparison with the x-ray data. For a
subset of Type I cement paste samples exposed to 33,800 ppm sulfate in either sodium sulfate
(Na2SO4) or magnesium sulfate (MgSO4) solutions, synthesis of results obtained through both
traditional testing and x-ray characterization suggests that expansion and cracking are more
prevalent forms of damage under sodium sulfate attack, while loss in compressive strength was
more prevalent under magnesium sulfate exposure. It is proposed that the expansion and
cracking observed in the Na2SO4-exposed samples resulted from the formation of ettringite
and/or gypsum in the near-surface region and that the loss of material in the MgSO4-exposed
samples resulted, indirectly, from the formation of brucite [Mg(OH)2], which is known to lead to
decalcification of the strength-giving C-S-H.
INTRODUCTION
With worldwide consumption of portland cements at 1.6 billion metric tons per year, the
durability of portland cement concrete has a tremendous impact on the economy and the
environment [1]. Mehta and Monteiro [2] indicate that in industrialized countries over 40% of
the total resources of the building industry are applied to repair and maintenance of existing
structures and less than 60% to new installations. Improved durability of infrastructure materials
is also ecologically beneficial because of the energy consumption associated with cement and
concrete production and because of the CO2 emissions generated during cement manufacture. A
durable concrete has far reaching beneficial influences on sustainable development in the
construction industry [3].
*
Corresponding author: Dr. K.E. Kurtis, 790 Atlantic Dr., Atlanta, GA 30332-0355; phone: 404-385-0825; fax:
404-894-0211; email: [email protected]
63
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Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
The durability of portland cement concrete can be compromised by several chemical degradation
processes such as alkali-aggregate reactions, carbonation, and reaction with neutral or acidic
groundwater, among others. Sulfate attack is potentially one of the most damaging degradation
mechanisms [4]. Case studies [5-9] demonstrate that sulfate attack can occur in a wide variety of
environments. A concrete structure may come in contact with sulfate ions dissolved in ground
water (e.g., concrete foundations and buried containment vessels) or water in canals and
spillways (e.g., concrete canal linings and dams), seawater, or acid rain. Sulfate ions penetrating
into concrete from external environments, react with some components of the hardened cement
paste and can lead to distress which over time may even render a structure unserviceable.
Therefore, concrete structures exposed to sulfate-containing water in service must be designed
for sulfate resistance.
However, despite over six decades of study of sulfate attack, considerable uncertainty remains in
optimally selecting and proportioning materials for sulfate resistance in concrete, and current
understanding of the actual mechanisms of degradation in sulfate environments is incomplete
[10-12]. Generally, two forms of sulfate attack are believed to exist: (1) reaction with aluminabearing cement hydration products, and/or unhydrated tricalcium aluminate (C3A) to produce
ettringite, and (2) reaction with calcium hydroxide to produce gypsum. Sulfate attack manifests
itself in the form of cracking, spalling and expansions or as loss of mass, adhesion and strength
[2]. It is widely believed that damage by expansion and cracking occurs primarily due to
ettringite formation, and loss of adhesion and strength occurs primarily as a result of gypsum
formation. However, it is precisely this – the linkage between the chemical reactions and the
physical and mechanical consequences to the material as a whole – which remains poorly
understood and which hinders our ability to produce concrete that will with certainty exhibit
long-term sulfate resistance.
One significant impediment to the study of sulfate attack and the many reactions that occur in
concrete is the lack of appropriate characterization tools. Cement-based materials are hydrated
systems, which may be significantly altered by the removal of water, as required by many highresolution imaging methods. In addition, cracking commonly occurs during drying and may be
furthered during epoxy impregnation, which are standard methods for characterization of
cement-based materials. When investigating damage by an expansive reaction, such as sulfate
attack, formation of cracks during sample preparation for characterization can further complicate
the interpretation of damage mechanisms. In addition, drying or further sample preparation may
not only damage the samples but will also halt reactions, making in situ observations impossible.
Characterization of durability reactions such as sulfate attack is best done, therefore, with
techniques that do not require such sample preparation. With sulfate attack, in particular,
coupling of chemical analysis and crystallographic data with imaging is particularly beneficial in
linking chemical and physical/mechanical manifestations of damage.
Here, x-ray microtomography (μCT) and energy dispersive x-ray diffraction (EDXRD) are used
synergistically to correlate physical and chemical changes through non-destructive interrogation
of the same samples after various periods of sulfate exposure. Sample expansion and strength
data complement the x-ray data, adding to the picture of the failure mechanism. By relating
physical changes in microstructure in response to sulfate ingress (using data obtained through
μCT, measurements of expansion, and mechanical testing) to chemical changes in microstructure
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Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
(using data obtained by EDXRD), linkages can be developed between chemical and
microstructural responses to sulfate attack and changes in bulk mechanical properties. This
research approach provides the fundamental understanding essential for specifying concrete for
enhanced sulfate resistance.
While research results have been presented more extensively elsewhere [13-17], the objective
here is to illustrate how two x-ray methods – μCT and EDXRD – may be combined with more
routine materials testing to provide new understanding of material performance. Here, ongoing
damage under sodium sulfate attack and magnesium sulfate attack is described for a set of Type I
cement paste samples exposed to severe sulfate attack (33,800 ppm sulfate ion concentration),
characterized as Class 3 exposure by American Concrete Institute Committee 201 on Durability
[18].
EXPERIMENTAL PROGRAM
In an investigation of the fundamental relationships between materials and mixture
characteristics and sulfate resistance in a range of sulfate environments (i.e., varying solution
concentration and associated counter ion), the influence of sulfate exposure on strength, length
change (expansion), structure, and chemical composition were monitored over time. Results
from these more extensive studies can be found in [13-17, 19].
With regard to the results to be presented here, samples were cast from ASTM Type I cement* at
w/c of 0.485 and were subsequently exposed at room temperature to sodium sulfate (Na2SO4)
and magnesium sulfate (MgSO4) solutions at 3.38% (33,800 ppm) sulfate ion concentration. The
ratio of solution volume to sample surface area was kept constant at 2.4cm3/cm2 for all sample
types, and solutions were changed weekly.
Cylindrical cement paste samples approximately 40 mm long and 12 mm in diameter were cast
in plastic vials from mixtures of Type I cement and de-ionized water for characterization by µCT
and energy dispersive x-ray diffraction (EDXRD); these methods are in more detail elsewhere
[14,16,17]. Scanning electron microscopy with x-ray imaging, confocal microscopy, and stereo
microscopy have also been performed on these specimens, as described in [19].
Based upon the sulfate resistance test method described by Mehta [20] and previously
implemented by Kurtis et al. [21], 12.7 mm (0.5 in) cement paste cubes were cast from the same
cement. Compressive strength measurements were performed on the cement paste cubes using a
9980 kg-capacity (22,000 lb), screw driven universal testing machine with a load rate of 272
kg/min (600 lb/min). Compressive strength measurements were made on six replicate samples
for each condition.
In addition to strength measurements, measurements of length change were made on mortar bar
samples cast in 25 mm x 25 mm x 285 mm (1 in x 1in x 11 in) brass molds from the same
cement, with w/c of 0.485 and sand-to-cement ratio of 2.75 by mass. Length change
measurements were conducted weekly, generally on six replicates but on no fewer than three
replicate samples (as some samples failed during exposure).
*
Cement composition has been previously reported elsewhere [13,17].
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Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
All samples (i.e., portland cement-based cylinders, cement paste cubes and mortar bars) were
demolded after 1 d of accelerated curing, using ASTM C1012 [22] conditions, or normal curing
and were subsequently cured for an additional 2 d in limewater at room temperature. The threeday curing period was determined as described in ASTM C 1012, where the curing period is
determined by the time necessary for the compressive strength of 50.8 mm (2 in) mortar cubes to
reach 20.0 MPa. After curing, the samples of each type were placed in sulfate solutions, while
some samples remained in saturated limewater baths (referred to as unexposed samples) or were
stored in sealed containers at room temperature to serve as controls.
RESULTS
Results obtained from combined analysis of data resulting from the different methods employed
(e.g., length change, compressive strength, μCT and EDXRD) will be presented to examine the
influence of the associated counter ion (Na+ vs. Mg2+) during sulfate attack. Here the discussion
is limited to specimens of Type I cement paste and to sulfate ion concentrations of 33,800 ppm.
First, results from physical and mechanical testing will be presented. Data from the x-ray
methods will be discussed subsequently.
Physical Consequence of Sulfate Attack: Length Change
Mortar bar expansion for samples exposed to sodium and magnesium sulfate solutions are shown
in Figure 1; these results were obtained as described in ASTM C 1012 [22]. The average length
change was measured for 19 weeks for sodium sulfate and 39 weeks for magnesium sulfate.
Longer exposures produced expansions beyond the capacity of the length comparator.
The data in Figure 1 shows that until 8 weeks of exposure the average expansion measured under
sodium sulfate (0.23 % at 8 weeks) was similar to that under magnesium sulfate attack (0.22 %
at 8 weeks). At times of exposure greater than 8 weeks, in general, the average expansion was
greater under sodium sulfate attack. For example, at 10 weeks average expansions were 0.41 %
and 0.19 % for sodium and magnesium sulfate exposures respectively.
(a)
(b)
Figure 1. Measurements of mortar bar expansion with continued exposure to Na2SO4 (a, left)
and MgSO4 (b, right) solutions with sulfate concentration of 33,800 ppm.
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67
Mechanical Consequence of Sulfate Attack: Compressive Strength Loss
Average compressive strengths for cement paste cubes are shown in Figure 2 for exposures up to
24 weeks. For both cation types, strength initially increased presumably due to continued
cement hydration, as observed by [21] and [23]. Average compressive strength, however, was
lower at all times for those samples under magnesium sulfate exposure. Thus, it appears that
expansion is more prevalent under sodium sulfate exposure (Figure 1), while reduction in
compressive strength is more prevalent under magnesium sulfate attack. The peak in the
maximum compressive strength was greater and occurred at a later time under sodium sulfate
attack as compared to magnesium sulfate attack. The maximum average compressive strengths
were 8760 psi (at 4 weeks) and 6700 psi (at 2 weeks), respectively. It is suggested that the
greater initial strength gain under sodium sulfate attack led to lower susceptibility to loss of
compressive strength at later ages, as compared to magnesium sulfate attack. A closer
examination of the underlying mechanisms is presented through the μCT and EDXRD results.
Figure 2. Average compressive strength for Type I cement paste samples exposed to Na2SO4
and MgSO4 solutions with sulfate concentration of 33,800 ppm.
Microtomography
Microtomography of both sample groups showed damage in the form of edge cracks,
longitudinal surface cracks, body cracks and loss of material at specimen edges, i.e., rounding.
Table 1 shows the exposure time for the onset of these different forms of damage in the two
sample groups.
Table 1. Number of weeks of sulfate exposure until the first observation of different forms of
damage by μCT.
Manifestations of Damage Observed at Time (weeks)
Sample
Edge
Longitudinal
Description Edge Cracks
Rounding
Surface
Body Cracks
Cracks
Type I, Na
Type I, Mg
6
―
―
6
10
―
10
12
Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
As shown in Table 1, damage was first observed at the same time (6 weeks) under both sodium
and magnesium sulfate attack, but the manifestations of damage differed. Under sodium sulfate
attack, the initial damage appeared as cracking at the sample edges, while under magnesium
sulfate attack it occurred as rounding at the corners of the cylinders. The sodium sulfate-exposed
samples completely disintegrated by 33 weeks, whereas the samples exposed to magnesium
sulfate solution remained generally intact, although severely damaged, through the 78-week
exposure period.
Three-dimensional renderings from μCT of a sample exposed to sodium sulfate attack at 11 and
17 weeks are shown in Figure 3. Figure 4 shows a companion sample subjected to magnesium
sulfate attack for 11, 17 and 78 weeks. Exposure to sodium sulfate solution produced cracking at
the edges and surface of the specimen (11 weeks), which progressed to cracking within the body
and eventual spalling (17 weeks). Exposure to magnesium sulfate solution produced rounding of
cylinder’s ends (11 weeks), continued rounding with some body cracking (17 weeks), and
continued degradation as loss of material and cracking (78 weeks).
Figure 3. Three-dimensional renderings from μCT of Type I cement sample (w/c = 0.485)
exposed to Na2SO4 solution with sulfate concentration of 33,800 ppm. At (a) 11 weeks, edge
and surface cracks are observed, at (b) 17 weeks, spalling is apparent at the sample edges and (c)
at 17 weeks of exposure, extensive cracking in the sample interior is observed.
Figure 4. Three-dimensional renderings from μCT of Type I cement sample (w/c = 0.485)
exposed to MgSO4 solution with sulfate concentration of 33,800 ppm. At (a) 11 weeks, rounding
of the specimen corners is observed, at (b) 17 weeks, some cracking in the interior of the sample
is observed, and (c) at 78 weeks of exposure, continued spalling and more extensive body
cracking is apparent.
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Thus, the samples showed greater susceptibility to cracking and spalling under sodium sulfate
attack, which agreed well with the corresponding mortar bar expansions (Figure 1). Under
exposure to magnesium sulfate solution, the samples were more prone to loss of material, which
is suggested to have contributed to the more rapid loss of compressive strength (Figure 2)
through loss of effective sample cross-section resisting the external compressive load.
EDXRD
To examine the changes in the chemical composition that accompany the changes observed in
length and strength and the physical damage observed by μCT, companion samples were
characterized at key intervals by EDXRD. Through-depth profiles for different crystalline
products of hydration and sulfate attack were analyzed using plots of normalized intensity for
each phase of interest with respect to sample depth. Samples were interrogated through a depth
of ~ 4mm in the radial direction.
Figure 5 shows through-depth profiles obtained by EDXRD on cement paste samples not
exposed to sulfate solution (i.e., control samples), while Figure 6 shows results after 10 weeks of
exposure to the solutions of sodium and magnesium sulfate. Sample depths are accurate to ±
0.05 mm because of uncertainties in determining the position of the sample surface and in
counting statistics. In the absence of sulfate exposure, the crystalline products of cement
hydration ettringite (Ca6Al2(SO4)3(OH)12·26H2O), monosulfate hydrate (both the 12 and 14water forms), and calcium hydroxide (Ca(OH)2) are observed throughout the sample depth
examined, although the relative amounts of ettringite apparently decreases beyond a sample
depth of ~0.5mm. In addition, the presence of tetracalcium aluminoferrite (C4AF) suggests that
some unhydrated cementitious phases remain.
Figure 5. Through-depth chemical profile, obtained from EDXRD, of a Type I cement paste
sample (w/c = 0.485) not exposed to sulfate environment.
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(a)
(b)
Figure 6. Through-depth chemical profile, obtained by EDXRD, of a Type I cement sample (w/c
= 0.485) exposed to 33,800 ppm sulfate (a, left) in Na2SO4 solution for 10 weeks and (b, right)
in MgSO4 solution for 10 weeks.
After 10 weeks of sodium sulfate exposure, ettringite, gypsum, calcium hydroxide, and
monosulfate hydrate, are detected as well as some residual C4AF (Figure 6a). As compared to the
unexposed sample, changes in composition with depth are observed. These include the depletion
of Ca(OH)2 in the ~ 0.6mm nearest the surface, presumably due to leaching and/or reaction with
ingressing sulfates (where the products would be gypsum and/or ettringite). Gypsum, which was
not detected in the unexposed sample, is present after 10 weeks of exposure, particularly in the
0.6mm or so nearest the surface, suggesting the reaction of Ca(OH)2 with the ingressing solution.
Also, at 10 weeks, relatively more ettringite is detected near the surface, as compared to the core
of the sample, suggesting that ettringite is also forming as a result of the Na2SO4 exposure. The
peak intensity for ettringite also occurs at a greater depth than in the control, again suggesting
that this phase is forming through reaction with ingressing sulfates, rather than by cement
hydration.
Loss of Ca(OH)2 in the near-surface region may contribute to a lowered pore solution pH and, as
a result, the instability of some products of cement hydration, which ultimately will lead to
decreased strength and adhesion. However, the presence of ettringite in the near-surface region
suggests that the pH remains suitably high for most products of cement hydration. This is
supported by data in Figure 2, which shows compressive strength in excess of 6000 psi in
companion samples at 10 weeks of sodium sulfate exposure.
Edge and surface cracking were observed by μCT (Figure 3a) at approximately the same time as
the EDXRD data shown in Fig. 5 and 6 were collected; these cracks extended (not shown here,
see [13]) to depths of ~0.1-0.15mm into the sample, well beyond the depth where Ca(OH)2 was
depleted and even beyond the depth of gypsum formation and presumed new formation of
ettringite. (It should be noted, however, that cracking was concentrated near the sample ends
where sulfate ingress may expected to be greater, whereas, to avoid edge effects, EDXRD data
was obtained further from the sample ends. The enhanced sulfate ingress at the sample ends
presumably only accelerates the rate at which damage appears at the ends of the sample and does
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Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
not affect the mechanisms of damage.) It is proposed, then, that the cracks result largely from
the formation of new products (i.e., ettringite and/or gypsum), rather than the combined effects
of new product formation and strength loss.
Figure 6b shows that after 10 weeks of magnesium sulfate attack, the same phases observed
under sodium sulfate exposure are also identified, but that an additional thin brucite (Mg(OH)2)
layer is observed in the near-surface region. Similar to the sodium sulfate-exposed sample, there
is Ca(OH)2 depletion, as compared to the control, in the first 0.5mm or so closest to the surface.
The trends in gypsum and ettringite formation with depth are similar to those observed in the
sodium sulfate sample, although the gypsum resides closer to the surface in the magnesium
sulfate-exposed sample. The most noticeable difference between the samples exposed to the two
sulfate solutions is the presence of a double layer, consisting of a nearer-surface layer of brucite
followed by a gypsum layer, at the surface of samples exposed to magnesium sulfate solutions.
The formation of brucite has been associated with decalcification of the strength-giving calcium
silicate hydrate (C-S-H) phase in portland cement-based materials. Decomposition of C-S-H,
then, would lead to loss of adhesion, loss of material, and decreased strength. Loss of material
at the sample edges was observed by 11 weeks of magnesium sulfate attack by μCT (Figure 4a).
Similar effects were also observed by visual inspection in the cement paste cubes used for
compressive strength measurements. The resulting reduction in the effective area resisting the
compressive load likely lowered the measured compressive strength of the MgSO4-exposed
samples (Figure 2).
SUMMARY OF OBSERVATIONS
The influence of the associated cation (Na+ vs. Mg2+) on the mechanisms and manifestations of
sulfate attack were examined synergistically by traditional measurements of length change and
strength gain/loss and by the novel, combined application of x-ray microtomography and energy
dispersive x-ray diffraction. To demonstrate the value of this approach, data from a small subset
of Type I cement pastes samples prepared at a single water-to-cement ratio was examined.
Changes in length and compressive strength and observations of cracking made through μCT
suggested that expansion and cracking were the most prevalent forms of damage under sodium
sulfate attack, while - after the initial strength gain - the decrease in compressive strength was
more severe under magnesium sulfate exposure. Through results obtained by EDXRD, it was
suggested that the expansion and cracking observed in the Na2SO4-exposed samples resulted
from the formation of ettringite and/or gypsum in the near-surface region and that the loss of
material in the MgSO4-exposed samples resulted, indirectly, from the formation of brucite,
which is known to lead to decalcification of the strength-giving C-S-H.
ACKNOWLEDGEMENTS
This research was supported by National Science Foundation (NSF) CMS-0084824. The
microtomography equipment was acquired under NSF OIA-9977551. The EDXRD work was
performed at the Advanced Photon Source which is supported by the Department of Energy
under contract W-31-109-ENG-38. Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do not necessarily reflect the views of the
sponsors.
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