Vol.2, No.8, 809-816 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Tracking chloride and metal diffusion in proofed and
unproofed concrete matrices using ablative laser
technology (ICP-MS)
Avin Pillay1*, Mirella Elkadi1*, Fadi Feghali2, Sai Cheong Fok3, Ghada Bassioni4, Sasi Stephen1
1Department of Chemistry, The Petroleum Institute, Abu Dhabi, UAE; *Corresponding Author: apillay@pi.ac.ae
2PO Box, 2431, Abu Dhabi, UAE; *Corresponding Author: melkadi@pi.ac.ae
3Department of Mechanical Engineering, The Petroleum Institute, Abu Dhabi, UAE
4Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE. On leave from the Chemistry Department, Faculty
of Engineering, Ain Shams University, Cairo, Egypt
Received 8 May 2010; revised 17 June 2010; accepted 22 June 2010.
Depth profiling studies (laser ICP-MS) of ions
(Cl-, Na+, Mg2+) in concrete-based material can
be used to provide useful information on the
migration paths of these ionic species. In par-
ticular, deterioration of concrete through infil-
tration of chloride could lead to costly corrosion
problems with serious impact on the environ-
ment. Many modeling studies on concrete ma-
trices depend on the tortuosity of these trans-
port paths. Our work showed that dispersion
paths of ionic species in concrete are intermit-
tent and sporadic, suggesting that applications
of simplifying assumptions in treatment of such
data could lead to appreciable perturbations in
related mathematical models. This paper ex-
amines the capability of using a high resolution
ICP-MS laser ablation technique to track Cl mi-
gration in concrete samples in the presence of
other ions such as Na+ and Mg2+. Cationic mi-
gration in such materials is underexplored and
data in this particular area could contribute to
modeling studies. Concrete bricks (with and
without surface coatings) were specially pre-
pared in cubic configurations and allowed to
saturate in a ponding medium (sea water). The
study subsequently examined the distribution
of Cl, Na+ and Mg2+ with depth in protected (ep-
oxy coated) and unprotected cored concrete sliv-
ers (5 mm diameter; 2 mm thick) using an 80 µm-
diameter laser beam coupled to an ICP-MS in-
strument. The laser (213 nm) was programmed
to ablate a total depth of 50 µm at each point at
5-µm intervals. The results in unprotected sam-
ples indicated that chloride intensity showed a
general decline with depth, suggesting that mo-
bility of the chloride is a function of its interac-
tion with the concrete matrix. In some cases
‘hotspots’ were observed at certain points indi-
cating that transport of the intruding ion was
limited. No significant mobility was observed in
coated samples. The depth-profiling results for
Na+ and Mg2+ were somewhat unexpected. Strong
similarities in their spectra purported that the
matrix was indifferent to charge and size of the
ion. Our experimental data further showed that
the matrix itself offers natural protection to the
reinforced steel rebars by limiting chloride and
metal diffusion at certain locations. Clearly, if
the composition of these protective environ-
ments within the concrete could be simulated
on a larger scale and introduced into the matrix
it would offer scope for extended research in
this area. Our work would be of definite interest
to materials and environmental research; and
mechanistic studies on aggregates.
Keywords: Concrete; Cl, Na+; Mg2+; Laser
Ablation; Depth-Profiling; ICP-MS
In vitro tracking of chloride diffusion (by ICP-MS depth
profiling) in the presence of multi-ionic species (Na+,
Mg2+) can provide information on two fronts: 1) the tor-
tuosity of the migration pattern through the bulk material;
and 2) the extent of diffusion in proofed concrete. It has
been widely reported that concrete matrices incur serious
environmental damage through invasive attacks of Cl in
the bulk material [1-5]. Concrete structures in contact
with seawater or salt water are, therefore, expected to
incur some form of environmental degradation. Oil and
A. E. Pillay et al. / Natural Science 2 (2010) 809-816
Copyright © 2010 SciRes. OPEN ACCESS
gas companies that have colossal concrete structures
offshore tend to incur financial losses through salt-deg-
radation of this nature. Salt sprays and splashes can
also affect roadside concrete structures and buildings in
the vicinity of the sea. The mechanism of corrosion due
to Cl invasion in concrete is still a subject of active
study. It has been documented that when a concrete
structure is exposed to seawater, chloride ions from this
medium will gradually infiltrate the matrix, largely
through the pores and fissures in the hydrated cement
mixture [6]. Some migration studies have alleged that
chloride ions linked to MgCl2 have different migratory
properties than those associated with NaCl [7], and
further work in this area would be useful. In the case of
reinforced concrete, penetration of the chloride ions to
the steel rebars, and subsequent accumulation to be-
yond a certain level, initiates corrosion in the steel,
especially in the presence of moisture and oxygen at
the steel-concrete interface.
Knowledge of the dispersion of Cl and other species
in a concrete matrix could contribute to mechanistic
studies of ionic diffusion in such materials. Chloride
mobility is particularly important and several factors
could influence its dispersion pattern [8-11], such as: 1)
the extent of transportation through the bulk material (by
aqueous media); 2) salinity of the aqueous medium in
contact with the concrete; 3) the level of interaction of
the Cl with chemicals and ingrained impurities in the
concrete itself; 4) the porosity of the material; 5) tem-
perature; 6) humidity; and 7) material homogeneity. A
point to note about the last factor is that uneven mixing
of the concrete could lead to considerable material in-
homogeneity, which in turn could lead to aberrations in
Cl migration patterns.
We have developed an ultrasensitive technique for as-
sessing the diffusion of Cl (and cations such as Na+ and
Mg2+) in bulk concrete samples, and pinpointing areas in
the matrix to study their spatial and depth dispersion.
The technique uses laser ablation linked to a high per-
formance ICP-MS instrument. Our work demonstrates
that systematic depth profiling in concrete samples can
provide clues to the tortuosity of the transport path,
which could be useful in modeling studies. The aim of
this paper, therefore, is to explore the potential of our
method for rapidly tracking migration of Cl and other
ionic species in suitable bulk samples.
2.1. Sample Preparation/Sample Handling
The cement samples used in this study were regular
Portland cement material (Type 1), manufactured by the
Ras Al Khaimah cement plant in the United Arab Emir-
ates (UAE)1. The concrete mix was designed using fine
and coarse aggregates from the UAE. The maximum
aggregate size was 10 mm. The concrete mix preparation
was based on the British Method Design (BRE 106).
Two lots of concrete mixes were prepared to meet min-
imum target strengths of 20 N/mm2 and 40 N/mm2 (la-
beled Grade 20 & Grade 40 respectively). The details of
mix proportions are given in Table 1. Slump tests were
conducted to ensure the practical workability of the con-
crete mixtures.
Bricks were cast from the two lots of concrete mixture.
After casting, the bricks were cured under wet hessian
blankets and polyethylene film. Compressive strength
tests were performed on three concrete cubes (150 × 150
× 150 mm) from each lot of concrete mix after seven
days to establish if the samples had attained the appro-
priate strengths. Table 2 shows the compressive strength
results, which confirmed the quality of the cast con-
The cured bricks in each lot were further separated
into two batches. Solvent free 100% epoxy resin (water-
proof, chloride and carbonation resistant coating for
protection of concrete) was applied to the bricks in one
batch. The coating generally has excellent resistance to
chemicals and UV exposure (i.e., suitable for the petro-
chemical industry) with an expected life span of about
ten years. No coating treatment was applied to the bricks
in the other batch. The two batches of coated and un-
coated bricks in each lot were immersed in a pond of sea
water. After several months, all the bricks were removed
from the pond and samples were cored (discs of 5 mm
diameter by 2 mm thick) from the bulk material using a
standard coring tool [Makita TB131, Taiwan] (Figure 1).
Laser experiments were conducted on coated and un-
coated samples (bottom left and right of Figure 1, re-
2.2. ICP-MS Laser Ablation Technology
Samples were investigated with a Perkin Elmer SCIEX
DRC-e ICP-MS (Connecticut, USA) fitted with a New
Wave UP-213 laser ablation system. Laser ablation
technology uses a micro-beam (from an Nd: YAG solid
state laser) to ablate samples in a special sample cham-
ber. The fine ablated material is transported by a carrier
gas (Ar) to a hot plasma where it is atomized and con-
verted to ions (characteristic of the elements of the sam-
ple), which are subsequently carried to a mass spec-
trometer for detection (Figure 2).
The extremely high temperature of the plasma (about
10,000 K) separates the sample into individual atoms.
The plasma subsequently ionizes these atoms (M M+
+ e-) so that they can be detected by the mass spec-
1The Petroleum Institute, Abu Dhabi, UAE
A. E. Pillay et al. / Natural Science 2 (2010) 809-816
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Table 1. Concrete mix proportions.
Mix Proportion Concrete Targeted
Strength (N/mm2) Cement kg/m3 Water Cement RatioDensity kg/m3Aggregate Cement Ratio % Fine aggregate
20 (Grade 20) 320 0.54 2409 5.98:1 53.0
40 (Grade 40) 400 0.41 2440 4.68:1 52.0
Table 2. Compressive strength of concrete mixes.
Compressive strength after 7 days (N/mm2)
Concrete Targeted
Strength N/mm2 Cube 1 Cube 2 Cube 3
Average Measured Compressive
Strength N/mm2
20 (Grade 20) 37 38 37.5 37.5
40 (Grade 40) 48 50 53 50
Figure 1. The coring instrument (top); and typical samples of
proofed and unproofed cored discs (bottom left and right, re-
trometer. The technique is highly sensitive and can attain
a limit of detection of 10-6 mg/kg (parts per trillion) for
most elements. The concrete cored discs were placed
into a special sample holder with dimensions 5 cm × 5 cm.
No serious pre-treatment was necessary prior to irradia-
tion. Samples were subjected to 213-nm laser irradiation
at different points on the sample (16-point gridFigure
2). The level of the beam energy was 30%, with a flu-
ence of approximately 3 J/cm2 and beam diameter of 50
µm. The laser was programmed to ablate a depth of 5
µm at each point and repeatedly scanned the surface;
recording measurements after each ablation to a total
depth of 50 µm.
Figure 2. The ICP-MS instrument (top); 16-point grid showing
the arrangement of points on a sample irradiated by the laser
2.3. Instrumental Performance
Characteristic intensities originating from the elements
of interest were measured; and valid considerations were
given to potential interferences and matrix effects. Prior
to each run, the performance of the instrument was vali-
dated [12,13]. The study was largely semi-quantitative in
the absence of standardization, and for purposes of
comparison, all measurements were conducted under
identical experimental conditions. Appropriate spectra
were produced to observe variations in characteristic
elemental profiles spatially and with penetration depth.
A point to note is that the ponding medium produced Cl,
A. E. Pillay et al. / Natural Science 2 (2010) 809-816
Copyright © 2010 SciRes. OPEN ACCESS
Na+ and Mg2+ ions which are in elemental form in the
solid matrix. Validating the analytical performance of the
laser technique was conducted on an available certified
NIST standard (National Institute of Standards and
Technology, Maryland, USA; Certificate 613, glass
bead). To ascertain that the instrument was optimally
functional we examined its performance for different
isotopes by taking replicate measurements (n = 3) for
equivalent counting times at random points on the stan-
dard. Relative standard deviations of less than 5% were
attained in general (Table 3) indicating that the opera-
tional performance of the instrument was acceptable. It
is necessary to underscore that in the absence of match-
ing-matrix standards our method was based on evaluat-
ing relative intensities (counts/sec) for purposes of com-
3.1. Chloride Diffusion in Unprotected
Concrete Matrices-Depth Profiling
Laser ablation technology is capable of depth and sur-
face analysis, displays the elemental (ionic) intensities
and produces an elemental profile [14]. Ablative tech-
nology is particularly useful for the measurement of ul-
tra-trace intensities of metals in solids. Chloride detec-
tion is more energetically demanding due to the higher
ionization potential of chlorine; therefore, the technique
is limited to determination of minor concentrations of
this element. Interferences from the matrix are generally
common in ICP-MS, but with the availability of sophis-
ticated software and collision reaction cells they are be-
coming easy to detect and handle.
Depth profiling is a special process to investigate ele-
mental distribution beneath the surface. Very few con-
temporary instrumental methods have the capability to
study metal intensity with depth [12]. X-ray methods are
useful, but lack the ability to control depth penetration.
Nuclear particle irradiation is equally useful, but such
techniques require nuclear accelerators, and tend to be
limited to only a few microns below the surface. The
competence, therefore, of the laser approach to delve to
discreet depths below the surface of a sample is attrac-
tive for homogeneity studies in bulk materials. A typical
spectrum depicting the profile of chloride with depth
appears in Figure 3(a). The general trend portrays oscil-
lating intensities suggesting that migration of chloride
through the bulk material leads to interaction with the
matrix. Similar trends appeared in both sample grades.
Such interaction ultimately results in undesirable meta-
morphosis of the sample [15] causing material damage
[16]. Clearly, the path of migration through the concrete
would depend on its compositional make-up. The spec-
trum in Figure 3(a) shows that at certain depths the
chloride intensity changes sharply. This could be due to
minor ‘blockages’ that limit permeability and create a
diversion in the migration path itself. These blockages
could be made up of conglomerates of gravel/sand/ce-
ment in certain proportions and knowledge of such con-
stitutions would be useful to limit the infiltration of
chloride through the bulk material.
A good example of an obstructed path is shown in
Figure 3(b), which represents a spectrum delineating an
abrupt end to the diffusion of chloride after a depth of
about 25 μm. Evidently, some obstruction in the migra-
tion path at a depth of 25 μm prevented the chloride
from travelling any further. The data in Figure 3(b) sug-
gests that either permeability is limited or chloride mi-
gration stops. The mechanistic details are difficult to
devise and it is not clear at this stage exactly how per-
meability and chloride migration are linked. The spec-
trum in Figure 3(c) is interesting for the simple reason
that it shows dispersion of chloride up to 22 μm and then
a blocked path between 22-32 μm followed by continued
migration beyond a depth of 32 μm. This continued mi-
gration could be attributed to either another stream of
chloride ions that overlapped with the track of the laser
beam; or the same stream that went via another route and
again aligned itself with the main stream. This spectrum
in particular delineates the tortuosity of the transport
path, which if understated could lead to deviations in
related modeling studies.
3.2. Spatial Variation of Chloride
Iterative surface scans provided useful information on
the distribution of chloride on the surfaces of the con-
crete samples. Figure 4 presents a bar-graph showing
the typical variation of chloride intensity at different
ablation points on the surface of a selected sample. For
convenience, points on the plot represent maximum to
minimum intensities to exemplify the dramatic fluctua-
tion of chloride intensity.
The reason for such sharp variations on the surface is
not clear and could possibly be attributed to the inho-
mogeneous nature of the matrix itself. The pronounced
difference in intensity by as much as a factor of about 20
would indicate that there are sites on the surface that are
Table 3. Measurements (counts/sec) of reproducibility in a
NIST 613 standard.
Measurement 59Co 85Rb 51V 138Ba 140Ce 238U
1 567 1700 2900 7069 1333 1167
2 567 1767 2900 7103 1367 1200
3 633 1733 3101 7069 1367 1267
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Figure 3. Depth-profiling spectra of chloride: (a) an example
of an unbroken path (Grade 20); (b) an example of a blocked
path (Grade 20); and (c) an example of a path that is blocked
between 22-32 μm and continued chloride migration thereafter
(Grade 40).
Figure 4. Typical spatial distribution of chloride on sample
prone to accumulation of chloridewhereas at other
points chloride can easily infiltrate the sample (through
the medium of the ponding solvent). It would be of con-
siderable interest to identify the locations on the surface
matrix that tend to inhibit chloride diffusion. From the
accumulated data it would seem that the matrix itself
offers natural protection to the reinforced concrete by
limiting chloride diffusion at certain spots. A knowledge
of the composition of those inhibiting sites would be
useful and if simulated could be converted into a chemi-
cal additive or inhibitor to serve as a protective layer.
3.3. Hotspots/Degradation Problems
The appearance of sporadic abnormally tall peaks in a
spectrum demonstrates the presence of ‘hotspots’, (Fig-
ure 5). ‘Hotspots’ are sites in the interior of the sample
where chloride ‘agglomerates’. Once again, such accu-
mulation of chloride ions could be due to several factors.
Sudden blockages in the diffusion path; ‘bottlenecks;’
pores and pockets where the ponding solvent decelerates
in its progressive dispersion could collectively be re-
sponsible for the ‘hotspot’ phenomenon. The mechanism
linked to corrosion of reinforced steel is difficult to de-
fine. However, reports in the literature suggest that chlo-
ride plays a key role in the degradation process [6].
Chloride tends to weaken and destroy the thin coatings
of iron oxides on the rebars. The steel is subsequently
exposed to oxygen and moisture and begins to corrode.
It is known that a minimum level of chloride is neces-
sary to initiate corrosion [6,17]. Our data indicate that
‘hotspots’ are convenient internal sites in the matrix
where chloride ions have the opportunity to accumulate
and contribute to internal degradation. Such interactions
combined with increased stress in the material lead to
delamination in the matrix, which subsequently results in
staining of the concrete. Typical staining resulting from
these factors appears in one of our samples shown in
Figure 6.
3.4. Proofed Concrete
Most protective layers in concrete tend to shield it from
internal degradation. Effective proofing on concrete
should be impervious to infiltration of chloride ions.
Figure 7(a) represents a depth-profile spectrum of the
proofing itself. Clearly, no chloride or other metals are
present. The proofing itself works efficiently provided
no cracks or fissures exist in the coating. A point to bear
in mind is that minor fissures and cracks permit permea-
tion of the ponding solvent and access of chloride into
the sample. A meticulous scan of the coating was made
with the laser and Figure 7(b) represents an imperfec-
tion in the epoxy where the ponding medium infiltrated
the coating and traces of chloride were seen. In general the
A. E. Pillay et al. / Natural Science 2 (2010) 809-816
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Figure 5. Example of ‘hotspot’ in depth-profiling spectrum of
chloride (Grade 20).
Figure 6. Staining and corrosion damage due to invasive attack
of chloride.
Figure 7. (a) Depth-profiling spectrum of epoxy proofing (Grade
20); (b) depth-profiling spectrum of cracked epoxy proofing
showing chloride infiltration (Grade 40).
absence of chloride would proclaim the efficiency of the
proofing. However, as with all epoxies exposure to a
combination of solar radiation and sea water tends to
cause the surface to 'chalk' and degrade a few mi-
crons each year. Hairline cracks are formed in this
way through which the immersion solvent can gain free
3.5. Na+ and Mg2+ Diffusion
A report by Mussato et al. [7] suggests that the migra-
tory properties of chloride ions originating from MgCl2
differ from those linked to NaCl. Whether or not this
theory has been verified is not clear, but our work tends
to lend credence to the view that differences in such
transport properties could exist. Figures 8(a) and 8(b)
represent typical depth profiling spectra of Na+ and Mg2+
respectively in the unproofed sample. Similar trends
appeared in both sample grades. Control samples
showed no serious levels of ionic species.
Both spectra seem to mimic each other. This was an
unexpected development and purported that the matrix
‘adsorbs’ both cations concurrently irrespective of size
and charge of the ion. It could also suggest that there are
points or ‘bottlenecks’ within the matrix where physical
conditions promote supersaturation of the salts of these
metals thus leading to their agglomeration and the spec-
tral similarities observed in Figure 8. However, of sig-
nificance is that the level of sodium chloride is about
five times higher than magnesium chloride in sea water
and a comparison of the heights of the peaks in Figures
Figure 8. Typical depth-profiling spectra of a Grade 20 sample:
(a) sodium; and (b) magnesium.
A. E. Pillay et al. / Natural Science 2 (2010) 809-816
Copyright © 2010 SciRes. OPEN ACCESS
8(a) and 8(b) indicate that the intensities of Mg2+ and
Na+ do not vary appreciably, indirectly suggesting that
transport differences linked to chloride could exist and
should be more closely investigated. Both spectra also
delineate areas of high and diminished intensity. The
areas of low intensity indicate that the region in the inte-
rior of the sample is not conducive to ‘adsorption’ of the
metal cations, and could be attributed to meager interac-
tion with the matrix. As in the case of chloride, the areas
of high intensity suggest that the metals linger due to
some interaction with the matrix. Figures 9(a) and 9(b)
portray typical depth-profiling spectra of a proofed sam-
ple. Again the spectrum of Na+ mirrors that of Mg2+.
Both spectra (Figure 9) reveal peaks up to a depth of
10 μm suggesting that the ponding medium penetrated
the proofing and left traces of these metals on the surface.
This leads us to believe that such penetration could have
taken place through a hairline crack or fissure as we ob-
served in Figure 7(b), and that the unbroken proofing
itself was impervious to the seawater.
Our experimental data showed that the matrix itself of-
fers natural protection to the reinforced steel rebars by
limiting chloride and metal diffusion at certain locations.
Clearly, if the composition of these protective environ-
ments within the concrete could be simulated on a larger
scale and introduced into the matrix (in the form of addi-
tives or inhibitors) it would offer scope for extended
Figure 9. Spectra of Na+ and Mg2+ in the proofed sample
(Grade 40) showing infiltration up to about 10 μm.
research in this area. Unquestionably, adding ‘chemical
inhibitors’ to the concrete mixtures could be a useful
way to control the rate of corrosion (provided that they
complement the natural function of the matrix in shield-
ing the steel rebars). Exploring the possibility of differ-
ences in migratory properties of chloride ions associated
with MgCl2 and NaCl could also be the subject of future
The authors thank the petroleum institute for financial assistance.
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