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DISCUSSION OF THE EVOLUTION OF THE CHLORIDE
MIGRATION COEFFICIENT OF PORTLAND CEMENT CONCRETE
TESTED BY THE RAPID CHLORIDE MIGRATION (RCM) TEST AT
LONG-TERM CURING PERIODS UP TO 5 YEARS
Zhuqing Yu (1), Guang Ye (1, 2), Martin Hunger (3), Reinier van Noort (3)
(1) Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands
(2) Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Ghent, Belgium
(3) Heidelberg Cement Benelux, ENCI B.V. Rotterdam, the Netherlands
Abstract
Chloride-induced corrosion of reinforced concrete is one of the main deterioration mechanisms leading to shortening of the service life of concrete structures. Therefore, assessment of the resistance of concrete to chloride ingress plays an important role in predicting the service life of such structures. The Rapid Chloride Migration (RCM) test is one of the accelerated test methods commonly used in Europe. It is commonly assumed that for a properly cured Portland cement concrete without degradation, the chloride migration coefficient should decrease with increasing curing age due to on-going hydration and microstructure densification. However, recent experiments show that the rapid chloride migration coefficient of Portland cement concrete decreases at early curing age, but increases at later curing ages up to 5 years.
In this paper, the evolution of the rapid chloride migration coefficient of Portland cement concrete during long-term curing periods up to 5 years is discussed. The test conditions for the RCM test and the test results obtained from RCM tests are described. Finally, possible causes for the increase in DRCM that occurred during late curing of the samples are investigated in order to explain this increase.
452 1. INTRODUCTION
Chloride-induced corrosion of reinforced concrete is one of the main deterioration mechanisms leading to shortening of the service life of concrete structures. Therefore, assessment of the resistance of concrete to chloride ingress plays an important role in predicting the service life of concrete structures. Several standard test procedures have been developed to evaluate this resistance . Among them, the Rapid Chloride Migration (RCM) test is one of the accelerated test methods which is commonly used in European countries. RCM is a non-steady state migration test using an externally applied electrical field for accelerating chloride penetration. The parameter, non-steady state migration coefficient (DRCM) obtained
from the RCM test describes the rate of chloride transport under a condition of reduced chloride binding [1].
In general, for a properly cured Portland cement concrete without degradation, the chloride migration coefficient should decrease with increasing curing age, due to on-going hydration and microstructure densification. However, recent experimental results show that the rapid chloride migration coefficient of Portland cement concrete decreases at early curing age, but increases in the later curing age up to 2 years. Maage and Neithalath [2-3] found that the DRCM of Portland cement concrete with a water/cement (w/c) ratio of 0.7 and 0.4 changed
little with an increase in curing age from 56 to 90 days. This is probably because the cement in Portland cement concrete is nearly fully hydrated and matured after a certain number of days of moist curing [5]. Furthermore, van Dalen presented that at the curing age of 112 days, the DRCM was higher than that at 56 days, and the DRCM decreased after 112 days [4]. Similar
data presented by other researchers [6-9] show that the DRCM of Portland cement concrete
with w/c ratio of 0.4 to 0.5 decreased at early curing ages of up to 365 days, but increased at the later curing age. Figure 1 shows the variation of the DRCM of Portland cement concrete
with curing age in cluing the testing parameters, such as cement type, w/c ratio, and curing conditions. It should be noted that the Portland cement concrete specimens were all cured under water (18 ± 2 °C) or with lime bath (20 ± 2 °C, around 100% humidity) without extreme deterioration such as freeze-thaw attack, carbonation, etc. It is commonly assumed that for such a properly cured Portland cement concrete without degradation, the DRCM should
decrease with increasing curing age.
In this paper, the evolution of the rapid chloride migration coefficient of Portland cement concrete during long-term curing periods up to 5 years is discussed. Independent tests were carried out at two laboratories, namely the Microlab at Delft University of Technology and the Research and Development center of ENCI (a cement company). The test conditions for the RCM tests and the obtained test results are described. In order to explain the observed increase in the chloride migration coefficient of Portland cement concrete, the microstructural development of cement paste (porosity) is investigated by Mercury Intrusion Porosimetry (MIP) and using BET. Mineral compositions and hydration products in cement paste and Portland cement concrete at different curing ages are monitored using X-ray diffraction analysis (XRD), Thermogravimetric analysis (TGA) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Finally, the possible causes for the observed increase of the rapid chloride migration coefficient are discussed with regard to the leaching of calcium hydroxide (CH), the formation of high-density Calcium Silicate Hydrate (CSH) and Delayed Ettringite formation (DEF).
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Figure 1: The development of the DRCM with curing age [4, 6-9]
2. MATERIALS AND METHODS
2.1 Materials and properties
Portland cement (CEM I 42, 5 N), aggregate, and tap water were used in preparing concrete mixtures. Graded Dutch river sands with a maximum grain size of 4 mm and gravel with a 16 mm maximum grain size were used as fine and coarse aggregates, respectively. Demineralized water was applied to cast cement paste. The chemical compositions of the Portland cements used in the two laboratories are shown in Table 1.
Table 1: Chemical compositions of the Portland cements used Chemical
composition (% by mass)
SiO2 Al2O3 CaO free CaO Fe2O3P2O5 K2O MgO SO3 Na2O Calcite
CEM I 42,5 N
(TUD) 20.36 4.96 64.4 0.6 3.17 0.18 0.64 2.09 2.57 0.14 1.45 CEM I 52,5 N
(ENCI) 19.80 4.91 64.0 0.97 3.33 - - 0.97 2.19 0.32 3.01
Note: TUD: Delft University of Technology; ENCI: Dutch cement producer and supplier;
Table 2: The w/c ratios and specimens preparation
Type w/c Specimen preparation
TUD Concrete 0.4, 0.5, 0.6 Cast in cylinder molds
Paste 0.4 Cast in plastic bottles
ENCI Concrete 0.45 Cast in cylinders
Table 2 lists the w/c ratios and specimen preparation. The Portland cement paste to be used for MIP, XRD, TGA, SEM/EDS, and BET measurements was cast in a plastic bottle. The
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Portland cement concrete mixtures used in the RCM tests were cast in cylinder molds (diameter of 100 mm and height of 300 mm) or cube molds (length × width × height = 20 × 20 × 20 cm). After 24 h, the concrete specimens and hardened cement paste samples were demolded and cured according to the curing conditions presented in Table 3. After a certain curing age, three slices with the diameter of 100 mm and a thickness of 50 mm were cut from the concrete samples to be used in the RCM test.
Table 3: Curing conditions and curing age
Type Curing conditions Curing age (days) TUD Concrete
Fog room with lime bath
(humidity: 100%; T: 20°C ±
1°C) 28, 90, 180, 365, 730
Paste Sealed 7, 28, 90, 180, 365, 730
ENCI Concrete Under water (T: 20°C) 28, 56, 91, 182, 365, 730, 2101
2.2 Test methods
The Rapid Chloride Migration (RCM) test was conducted according to the NT Build 492 standard [10]. Following the standard, a corrected voltage and appropriate test duration were chosen. After the migration test, all specimens were split and sprayed with 0.1 M silver nitrate (AgNO3) solution to determine the penetration depth of chloride. The average penetration
depth, Xd of chloride was calculated, and from this the non-steady-state migration coefficient
(DRCM) was calculated [10].
Mercury Intrusion Porosimetry (MIP) measurements were performed to determine the total porosity of cement paste [11]. During the sample preparation for MIP testing, the specimens were demolded from the plastic bottle and crushed into small particles of about 1 mm. The small species were frozen by immersion into liquid nitrogen for 5 min in order to stop hydration. Afterwards, the specimens were then dried in a freeze-dryer until a constant weight loss (0.05%) was reached. The dried specimens were also used for BET, TGA, SEM, AND XRD tests.
The internal specific surface area of cement paste was determined by Nitrogen adsorption. When a dried porous material is put into a gas environment, the internal surface of pores will adsorb a certain quantity of gas molecules. The nitrogen gas pressure and the adsorbed nitrogen quantity are recorded during the test. The pore surface area is calculated according to the Brunauer-Emmett-Teller (BET) method [12].
SEM/EDS was used to capture images, and to determine the chemical composition of the specimens. XRD measurements were carried out on a Philips X’ pert diffract meter system with Cu KĮ radiation. Scans were run from 5 to 70° (2ș), with a step size of 0.02° (2ș) and a dwell time of 2 s per step. The content of calcium hydroxide (CH) of the sample was determined by TGA. Analyses were conducted using a TG-449-F3-Jupiter instrument at a heating rate of 10°C/min from 40 °C to 1100 °C under flowing argon. The following steps were taken during sample preparation for XRD and TGA. First, the concrete samples were crushed and the coarse aggregate was removed. Next, the smaller aggregate and sand were
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eliminated as much as possible in order to reduce the characteristic peak of silica. Finally, the remaining mortar was powdered by grinding.
3. RESULTS
3.1 The influence of curing age on DRCM of Portland cement concrete
The chloride migration coefficient of concrete is typically measured at ages of 28 or 91 days. It is assumed that after this period, ongoing hydration in Portland cement concrete with a w/c ratio larger than 0.4 will continue to result in an increasingly dense microstructure. However, measurements performed on concrete specimens older than one year are rarely reported.
Figure 2 shows the development of the DRCM of Portland cement concrete with w/c varying
between 0.4 and 0.6 over 2 and 5 years, respectively. As can be seen the w/c ratio has a clear impact on the DRCM which rises from initially (after 28 days) 13h10-12 m2/s to 28 h10-12
m2/s with increasing w/c. Despite having a higher water/cement ratio, the DRCM of the sample
prepared at ENCI is similar to the value measured for concrete with w/c of 0.4 by TUD. This is likely a consequence of the higher fineness (Blaine) of the CEM I 52, 5 N compared to the CEM I 42, 5 N. This is in line with the expectation. However, the development of DRCM in
time is not. After an initial ongoing decrease in DRCM, an increase is noted which even moves
the DRCM back to initially measured values or even beyond. Also here the w/c seems to have
an effect on the rate of the increase. The period where the development turns seems to be similar in all four examples, namely around half a year.
Figure 2: The influence of curing age on DRCM for different concrete binders
It is clear that the test results from our experiments are very similar to the results reported from literature as discussed in the introduction.
3.2 Pore structure of cement paste
Figure 3 shows the total porosity obtained from MIP tests, and the BET surface area obtained from Nitrogen absorption of cement paste with w/c of 0.4. As expected, increasing curing age results in a lower total porosity. This is consistent with test results proposed by other researchers [13-15]. The specific surface area increases at early curing age and
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decreases after the curing age of 91 days. The total porosity and BET surface area of other samples with different w/c ratios show similar trends.
Figure 3: Total porosity and BET surface area of cement paste 3.3 SEM image observation and EDS analysis
In the scanning electron microscope, Ettringite-like (AFt) needle-shaped crystals are observed in the Portland cement concrete sample at a curing age of 2 years (Figure 4, left). An EDS spectrum confirms Ettringite formation (Figure 4, right). Some micro-cracks with a width of 3-5 μm are observed in the same concrete sample as shown in Figure 5, left.
Figure 4: SEM image and EDS spectrum 3.4 XRD test results
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E: Ettringite; Q: SiO2; CH: Ca (OH) 2; CO: CaCO3; Mc: Monocarboaluminate
Figure 5: XRD spectrums of Portland cement concrete
The X-ray diffraction spectrums of concretes with a w/c-ratio of 0.4 at curing ages of 215 days and 2 years and concrete with a w/c-ratio of 0.6 at curing age of 2 years are shown in Figure 5. In the samples that are 2 years old, the main compounds are Ettringite (AFt), Quartz (SiO2), Portlandite (CH), Calcite (CaCO3), and Monocarboaluminate (Mc). Ettringite and Mc
were not observed in the concrete with a w/c-ratio of 0.4 at 215 days. 3.5 TGA test result
The TGA test results are shown in Figure 6. The mass loss at a temperature of 450 °C is associated with the dehydration of calcium hydroxide (CH). The content of CH determined by graphical technique [16] from TGA curves is listed in table 4. The concrete with a w/c-ratio of 0.6 has similar content of CH at the curing age of 2 and 3 years. This indicates that the hydration of cement in this concrete has almost entirely stopped after 2 years of curing.
Figure 6: TGA data for cement paste and concrete
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Table 4: The content of CH in cement paste and concrete
Type ratio w/c Curing age (Years) The content of CH (%) Cement
paste 0.4 2 13.03
Concrete 0.6 2 11.07
Concrete 0.6 3 11.09
4. DISCUSSION
In general, the concrete structure becomes denser and less permeable with increasing curing age. The chloride resistance of Portland cement concrete should thus be improved due to on-going hydration and microstructure densification. However, Figure 1 and Figure 2 both show an increase of chloride migration coefficient at later curing age. Based on the determination of hydration products and the observation of microstructure in the section 3, the possible reasons for this increase will now be discussed.
4.1 Calcium hydroxide (CH) leaching
The leaching of calcium hydroxide may be a matter of concern for the durability of concrete [17]. Tomas [18] performed leaching tests and permeability tests on Portland cement concrete and proposed that percolating pure water through concrete specimens has the ability to dissolve hydration products, mainly CH, and also to carry the dissolved ions out of the specimen by diffusion or convection. He found that an increased permeability will occur after a longer period of leaching [18].
A leaching process is the cause of an ambition to reach a lower energy level in a system [18]. Chemical bonds are broken and solids are dissolved. In concrete, the pore solution keeps in thermodynamic equilibrium. Once some parameters in the thermodynamic relation, such as temperature or pressure are changed, or if any of the solutes are carried away, the equilibrium is disturbed [18]. CH is the most soluble hydration product in hardened cement paste, and it is a weak link in cement and concrete from a durability point of view [19]. Under closed system conditions, the dissolution reaction of CH quickly reaches equilibrium, after which the concentration of Ca++ ions remains constant. But under flowing water or in a large reservoir, CH is leached. Ongoing dissolution and leaching of CH will then result in an increase in porosity.
Thus, the leaching of CH is a possible cause of weakening of concrete and of decreases of the resistance of concrete to chloride ingress. However, the concrete specimens used in this research were cured in a lime bath or still water. Furthermore, TGA showed that the content of CH in the concrete samples did not change between 2 years and 3 years of curing (see Table 4). Therefore, it can be concluded that the leaching of CH is not the reason for the increase of DRCM-values of concrete at curing ages beyond 5 years.
4.2 Development of pore structure in Portland cement paste
The specific surface area of a material is the area of the interface of a certain mass of solid . The specific surface area measured by nitrogen increases with hydration time until it plateaus at a maximum value, which agrees with the finding of Hunt [20]. Thomas and other
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researchers [21-22] suggest that once the available pore space has been filled with hydration products, the specific surface area will have reached its maximum value, even though hydration continues. This is a possible reason why the specific surface area of cement paste decreases again after certain curing ages.
It should be noted that the surface area of cement paste links to the amount of low-density C-S-H. At early curing age, the surface area increases while the amount of low-density C-S-H increases with the process of cement hydration. Around 3 months, the amount of low-density C-S-H reaches a maximum represented by the largest surface area. At the same time, the hydration of cement is almost completed in the cement paste with a w/c ratio of 0.4 [23]. At later curing ages, the surface area of cement paste decreases, corresponding with a decrease of the amount of low-density C-S-H. A simple description of the hydration process of cement is shown in Figure 7. This suggests that denser C-S-H arises from a transformation of less dense C-S-H phases [24]. During this transformation, larger pores may be left from the area of low-density C-S-H, thus decreasing the surface area of the pore network, while the porosity changes little. These larger pores result in an increase of the permeability, leading to a decrease of the resistance of the concrete structure to chloride ingress. Further experimental work is required to confirm this mechanism.
1 day 14 days 28 days
91 days 365 days
Figure 7: A simple description about the hydration process of cement 4.3 Delayed Ettringite Formation (DEF)
It is well known that Ettringite is formed in hydrating Portland cement systems from the reaction of calcium aluminate (C3A) with calcium sulfate (CaSO4) within the first few hours
after mixing with water. Essentially, all sulfate in the cement is normally consumed to form Ettringite within 24 hours. Once all calcium sulfate is used up, the Ettringite becomes unstable and reacts with any remaining C3A to form Monosulfate aluminate hydrate (AFm)
[25]. Day [26] proposed that if the SO3/Al2O3 (S/A) ratio is larger than a certain value as
shown in Figure 8, C3A will still be available after all of the Ettringite has been consumed.
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Figure 8: Critical values for SO3/Al2O3 ratio [26]
In this study, the SO3/Al2O3 mass ratio is around 0.518. The amount of C3A is around 7%
as calculated by the Bogue calculation based on Table 1. Therefore, all Ettringite is converted to AFm during early curing as shown in Figure 8. However, Ettringite is found in the concrete samples at curing ages of 2 years as shown in Figure 4 and Figure 5. Furthermore, DEF is an expansive reaction, Ettringite formed takes up more space than monosulfoaluminate [27].
In the present study, several reasons for the formation of Ettringite were explored. Some researchers reported [27] that when concrete is exposed to water for a long period (many years), the Ettringite can slowly dissolve and reform in less confined locations. Klieger and Hooton proposed [28] that if CaCO3 is present in the cement, it will slowly dissolve and
subsequently react with any kinds of calcium monosulfoaluminate hydrate or calcium aluminate hydrate to form the AFm carboaluminate phase and Ettringite (equation 1).
2 3 4 2 3 2 2 3 3 2 2 3 4 2 3 3 12 2 18 2 3 11 3 3 32
CaO Al O CaSO H O CaCO H O CaO Al O CaCO H O CaO Al O CaSO H O
o
(1)
As shown in Table 1, the contents of CaCO3 in the cement used by TUD and ENCI are
1.45% and 3.01%, respectively. The XRD patterns of CaCO3, Monocarboaluminate, and
Ettringite were also reported in Figure 5. Therefore, Ettringite likely did form in the samples studied by the reaction of CaCO3 and monosulfoaluminate occurs in this study. It is noticed
that Ettringite is more stable due to its lower solubility as shown in table 5 (which gives the solubility products of relevant salts, such as Calcite, AFm, AFt and Ettringite).
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Table 5: Solubility products of Calcite, AFm, AFt and Ettringite [29]
Salt Solubility products at 25 °C
Calcite CaCO3 8.7 ×10-9
Monosulfoaluminate
3CaO Al O 2 3
CaSO412H O2 1.7 ×10-28
Monocarboaluminate
3CaO Al O 2 3
CaCO311H O2 1.4 ×10-30
Ettringite
3CaO Al O 2 3
3CaSO432H O2 1.1 ×10-40
Therefore, it can be concluded that the presence of CaCO3 in cement can result in the
formation of Monocarboaluminate and Ettringite. As shown in Figure 4, some micro-cracks around 5 μm are filled with needle-shaped Ettringite. As discussed above DEF can result in a volume increase in the concrete structure. Therefore, it can be concluded that the formation of DEF in this study could lead to possible crack formation and cause a decrease of chloride ingress resistance.
Until now, there is controversy in the discussion whether expansion caused by Ettringite is the primary cause of structural damage or the consequence of a predamage of the microstructure [30]. The cement hydration and microstructure formation in Portland cement concrete are also affected by many factors such as, the variations of used materials (cement, tap water, aggregate), curing conditions, test methods etc.
462 5. CONCLUSIONS
This paper presents a discussion on the possible reasons why a proper cured Portland cement concrete has decreased resistance to chloride ingress at later curing age. Various possible mechanisms for this are explored.
The leaching of CH in Portland cement concrete is unlikely to result in a significant increase in concrete permeability under the applied curing conditions.
The transformation of low density CSH into high density CSH that occurs at later curing age is a possible cause of the increased DRCM of Portland cement concrete. However, to
confirm this, further research has to be carried out.
Although DEF did occur at later curing age, there is no hard evidence to conclude that DEF caused cracks and thus lowered the resistance of Portland cement concrete to chloride penetration.
As the possible decrease in resistance against chloride ingress at later curing age discussed here may be of major importance for the durability of concrete made with Ordinary Portland Cement, more research on the cause of this decrease is required.
ACKNOWLEDGEMENTS
We hereby express our gratitude to our technicians and colleagues at the Microlab, at Delft Technical University, and at the ENCI Concrete Laboratory in Rotterdam.
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