RELATIVE IMPORTANCE OF CORROSION RATE AND EXPOSURE CONDITION ON THE PRACTICAL USE OF NEW ENVIRONMENTALLY FRIENDLY BINDERS

Abstract

Lowering the clinker content of concrete using SCMs can contribute significantly to reduce the energy consumption and the CO2 emissions of concrete. Uncertainty about long-term durability, especially carbonation induced corrosion, is the main factor limiting the practical use: containing less CaO they have less capacity to neutralize CO2 and thus higher carbonation rates, which may lead to premature corrosion of steel reinforcement. Results in literature concerning corrosion of steel in carbonated concrete are rare and refer mostly to ordinary Portland cement. Generally, a trend to higher corrosion rates at higher relative humidity was found. To estimate the service life of concrete structures made with new blended cements, corrosion rate data are urgently needed, because the so called “corrosion propagation stage” might significantly contribute to the total service life. Corrosion rate has to be measured for different blended cements, w/c ratios and exposure conditions. To collect data of corrosion rates in a reasonable time, a new experimental set up has been designed. The new test setup consists of small (8 x 8 cm) and thin (6 mm) mortar samples instrumented with reference electrode, 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample allows full carbonation within 3 weeks (4% CO2). Parameters that can be measured are electrical resistivity, corrosion potential, corrosion rate and oxygen diffusion. These results should allow to investigate the mechanism, particularly the kinetics, of carbonation induced corrosion. The first results show that new blended cements could be more susceptible to corrosion in certain exposure conditions. Depending on the environment the steel dissolution rate can vary by a factor of 200, from < 0.1 µm/year at 50% RH, to 20 µm/year in wet conditions. To define the application limits of new binders, the interaction with variable exposure conditions has to be carefully evaluated.

Full text

1st International Conference on Construction Materials for Sustainable Future,
19-21 April 2017, Zadar, Croatia
1
RELATIVE IMPORTANCE OF CORROSION RATE AND
EXPOSURE CONDITION ON THE PRACTICAL USE OF
NEW ENVIRONMENTALLY FRIENDLY BINDERS
Matteo Stefanoni1, Ueli Angst1 and Bernhard Elsener1,2
1 ETH Zurich Faculty of Civil Engineering, Institute for Building Materials
Stefano-Franscini-Platz 3, CH-8093 Zürich, Switzerland
2 University of Cagliari, Department of Chemical and Geological Science, I-09100 Cagliari
e-mail: matteost@ethz.ch, uangst@ethz.ch, elsener@ethz.ch
SUMMARY: Lowering the clinker content of concrete using SCMs can contribute significantly to reduce the
energy consumption and the CO2 emissions of concrete. Uncertainty about long-term durability, especially
carbonation induced corrosion, is the main factor limiting the practical use: containing less CaO they have less
capacity to neutralize CO2 and thus higher carbonation rates, which may lead to premature corrosion of steel
reinforcement. Results in literature concerning corrosion of steel in carbonated concrete are rare and refer
mostly to ordinary Portland cement. Generally, a trend to higher corrosion rates at higher relative humidity was
found. To estimate the service life of concrete structures made with new blended cements, corrosion rate data
are urgently needed, because the so called “corrosion propagation stage” might significantly contribute to the
total service life. Corrosion rate has to be measured for different blended cements, w/c ratios and exposure
conditions. To collect data of corrosion rates in a reasonable time, a new experimental set up has been
designed. The new test setup consists of small (8 x 8 cm) and thin (6 mm) mortar samples instrumented with
reference electrode, 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample
allows full carbonation within 3 weeks (4% CO2). Parameters that can be measured are electrical resistivity,
corrosion potential, corrosion rate and oxygen diffusion. These results should allow to investigate the
mechanism, particularly the kinetics, of carbonation induced corrosion. The first results show that new blended
cements could be more susceptible to corrosion in certain exposure conditions. Depending on the
environment the steel dissolution rate can vary by a factor of 200, from < 0.1 µm/year at 50% RH, to 20
µm/year in wet conditions. To define the application limits of new binders, the interaction with variable
exposure conditions has to be carefully evaluated.
KEY WORDS: Corrosion, Carbonation, Durability, SCMs, Exposure conditions.
1 INTRODUCTION
In the past years environmental issues in the building industry have become an increasingly hot topic.
According to Trends in global CO2 emissions: 2015 Report [1] cement production accounts for roughly 8% of
global CO2 emissions. On the raw materials side lowering the clinker content of cements using supplementary
cementitious materials (SCM) can contribute significantly to reduce the energy consumption and the CO2
emissions of building materials, thus they become more environmental friendly. Uncertainty about durability,
especially carbonation-induced corrosion, is the main factor limiting the practical use of these blended
cements. As a matter of fact, the carbonation rate is faster for blended cements [2] due to their inherent
chemical properties, in particular the reduced pH buffering capacity (lower calcium hydroxide content):

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- Lower amount of calcium hydroxide formed during the hydration reaction [3], as consequence of a
reduced amount of CaO with respect to Ordinary Portland Cement (OPC 65% CaO), variably down to
40% [4-6], depending on the type and amount of substituents;
- Consumption of calcium hydroxide [3,7,8] in presence of SiO2-rich components by the pozzolanic
reactions.
The carbonation rate, viz. the penetration of the carbonation front in the concrete matrix, is faster in case of
blended cements as documented in comprehensive reviews of data collected since 1968 for ground
granulated blast-furnace slag (GGBS) [9] and for fly ash concrete [10].
Carbonation also influences differently the microstructure properties depending on the type of binder.
Carbonation leads to a reduction in total porosity [11-15] which is ascribed to the positive difference of molar
volume between the calcium carbonate formed and the initial hydration products. However at lower clinker
contents a shift of the capillary porosity to coarser distribution was reported [15-19], possibly due to the
disappearance of the clusters of CH crystals replaced by a packing of calcium carbonate crystals leaving new
voids. These changes in the material pore size distribution can be of major importance in defining the
adsorption and diffusion properties of water and oxygen in the concrete matrix, influencing the corrosion rate
as a consequence.
Considering the schematic representation of the service life (Fig. 1), increasing addition of SCMs leads to a
shorter induction period for the onset of corrosion (depassivation) and it becomes obvious that the corrosion
rate of steel in carbonated concrete becomes a critical factor for reaching the expected service life of a
structure. However, results in literature on the corrosion rate of steel in carbonated concrete are rare and refer
mostly to ordinary Portland cement.
Figure 1: Schematic representation of the service life of a reinforced concrete structure (Tuutti diagram) showing the
importance of the propagation-stage of corrosion in carbonated concrete.
For service life prediction of concrete structures with new, blended cements, corrosion rate data are
urgently needed because the so-called “corrosion propagation stage” might be a significant part of the total
service life. Being the corrosion process a system property, influenced both by the material and the
environment, the combination of different materials (composition and mix design) together with different
exposure conditions (constant RHs and wet-dry cycles) have to be studied.

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The final objective is to evaluate the application limit, case by case, and develop guidelines for the use of
new blended cements. In this paper an approach for carbonated mortars is presented, aiming to achieve fast
testing of corrosion properties, thanks to an innovative sample setup. The relative importance of type of
binder, w/c ratio and exposure condition, with respect to corrosion propagation rate, is evaluated.
2 MATERIALS AND METHODS
2.1 Sample design
To be able to collect data of corrosion rate of steel in carbonated mortar in a reasonably short time, a new
experimental set up has been designed. The new test setup consists of small (8 x 8 cm) and thin (6 mm)
cement mortar sample instrumented with a reference electrode, 5 steel wire electrodes and a stainless steel
grid counter electrode (Fig. 2). The thin sample allows rapid full carbonation (max 3 weeks in 4% CO2) and
rapid equilibration of environmental humidity (checked by the sample weight). Parameters that can be
measured are electrical resistivity of the mortar, corrosion potential and corrosion rate (LPR measurements) of
the steel wires, oxygen diffusion and consumption rate. From these data the mechanism of steel corrosion in
carbonated concrete made of different blended cements can be evaluated.
Figure 2: Sample after casting and hardening (left) and schematic representation of dimensions (right).
2.2 Materials
For the realization of the mortar samples Holcim Optimo 4 cement (CEM II/B-M (T-LL) 42,5) and Holcim
Normo 5R (CEM I 52,5 R) were used. The mix design was chosen to allow the best fluidity while maintaining a
high stability of the cementitious suspension. The w/b ratios tested were 0.4, 0.5 and 0.6; the sand/binder ratio
was 2 and the sand had a maximum particle diameter of 1mm. A poly-carboxylate ether superplasticizer with
de-foaming agent was added to the mix in order to increase the fluidity and be able to fill the mould, the
amount was chosen in order to achieve a visually similar fluidity of the mortars.

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2.3 Carbonation procedure
The samples were carbonated in a carbonation chamber at room temperature, 65% relative humidity and
4% CO2 concentration in the controlled atmosphere. The time required for complete carbonation was:
 2 weeks for CEM II mortars;
 3 weeks for CEM I mortars.
Complete carbonation was ensured by the phenolphthalein test.
2.4 Exposure conditions
The corrosion behavior of the carbonated samples was studied in different exposure conditions. The first
tests were performed in controlled and constant environments (50% and 95% relative humidity and 20 °C).
Another series of experiments studied the response to wet and dry cycles, samples have been provided with a
silicon sealed wall for the ponding solution (Fig. 3). The cycles have been carried out by placing 3 mm of
water on the samples and let it adsorb, the parameters have been monitored over time from the water
adsorption to the drying of the sample.
2.5 Electrochemical tests
All the electrochemical experiments were performed using a potentiostat Metrohm Autolab PGSTAT30.
The embedded Ag/AgCl sensor was always used as reference electrode and its reference potential was
checked by means of an external Ag/AgCl reference electrode. One steel wire was used as working electrode
and the stainless steel grid was used as counter electrode depending on the test performed. The
measurements were repeated over time for each exposure condition.
Corrosion rate: the instantaneous corrosion current density was determined by polarization resistance
measurements. The polarization resistance Rp of the single steel wires was measured with the stainless steel
grid as counter electrode at ± 10 mV around the open circuit potential with a scan rate of 0.1 mV/s. The IR-
drop in the mortar was taken into account indirectly. Impedance measurements were performed right before
each polarization resistance test, and the ohmic resistance obtained was subtracted from the result each time
at the end of the tests.
Figure 3: Samples used for wet and dry cycles.

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Linear polarization measurements allowed the determination of an electrical resistance Rp’ that is the sum
of the polarization resistance Rp of the steel wires and the ohmic resistance RΩ between working electrode
and counter electrode. Electrochemical impedance spectroscopy tests were performed in order to measure
RΩ. The values of RΩ were subtracted from the total resistance Rp’ to get the real polarization resistance
values.
The corrosion rate was then calculated using the Stern Geary relation (1):
𝑖𝑐𝑜𝑟𝑟 =𝐵
𝑅𝑝 (1)
Where Rp is the polarization resistance and B is a parameter depending on the electrochemical properties
of the considered system; for iron in actively corroding state a value of 26 mV is commonly used.
3 RESULTS
At 50% RH the corrosion rate both of CEM I and CEM II at all w/c ratios was lower than 0.01 µA/cm2 (lower
than 0.1 µm/year) (Fig. 4). Initially the corrosion rate slightly increased over time, together with the corrosion
potential shifting to slightly more negative values for both cement types.
At 95% RH corrosion rates were higher for samples made with blended cement and at higher w/c ratio, but
overall found to be lower than 0.1 µA/cm2 (ca. 1.2 µm/year) (Fig. 4). Also in this case, in the first period of
exposure, the corrosion rate increased with time and the corrosion potential decreased, for both cement types.
Samples exposed to wetting showed a very rapid decrease of the open circuit potential and of the
polarization resistance (Fig. 5), the maximum value of the corrosion rate after wetting was 1.7 µA/cm2 (ca. 20
µm/year) (Fig. 4). The process of drying out took much more time, at the end the corrosion potential and
polarization resistance reached values similar to 50% RH (Fig. 5). The maximum corrosion rate in the wet
state was not influenced by the w/c ratio. On the contrary, the type of binder did play a role: for CEM II higher
corrosion rates were measured in the wet phase and it was also noticed that CEM II showed a faster drying.
Figure 4: Corrosion rates measured for every exposure condition, w/c ratio and type of binder (for wet-dry cycles the
maximum value for each cycle was taken).

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Figure 5: Variation of the open circuit potential and the corrosion rate during wet-dry cycles
(measurements on two steel wires per sample).
4 DISCUSSION
The influence of the cement type and w/c ratio on the corrosion rate could be influenced by the pore size
distribution of the carbonated mortars (see introduction section). Being corrosion an electrochemical process
that needs an electrolyte layer, on the steel surface, to take place, the condensation behaviour of water could
be a possible limiting factor in non-saturated conditions. At 50% RH CEM I samples show a higher corrosion
rate than samples made with blended cement (Fig. 4) in agreement with data presented in the literature [20].
One possible explanation could be found in the finer pore structure of CEM I carbonated mortars that would
allow some water condensation also at such low relative humidity. At 95% RH the dissolution rate is higher in
CEM II mortars (Fig. 4). Also this fact might possibly be attributed to the pore structure, if it is coarser in
carbonated blended cement: a higher amount of large pores would increase the amount of free water present
at high relative humidity and increase the corrosion rate as a consequence.
The highest corrosion rates were found during wet and dry cycles (exposure class XC4). In the wet periods
maximum values as high as 1.7 µA/cm2 were measured (Fig. 4). The corrosion rate in the wet phase is up to
20 times higher than at constant 95% RH. Such high corrosion rates of about 20 µm/year can be critical and
markedly limit the service life of a structure. At the end of the drying phase, which seems to be faster in the
case of CEM II mortar, perhaps due to a more open pore structure, the dissolution rate turns back to negligible
values.
The better durability performance claimed for blended cements (associated to a finer pore structure and
lower permeability [11, 21-23]) is perhaps relevant only in a non-carbonated state. After carbonation blended
cements have been found to develop a coarser pore size distribution that might be influencing the corrosion
behaviour of the embedded steel, leading to higher dissolution rates in a humid/wet environment (Tab. 1). In
wet conditions (XC4) steel in carbonated mortars made of CEM II showed a corrosion rate higher by a factor
up to 2 compared to mortar made of CEM I. Note that the reasoning regarding the limiting mechanism is only
based on from literature data, as no porosity data of the tested samples are available yet. Further research is
therefore needed, and currently ongoing.

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Keeping in mind that depassivation of steel in blended cements occurs at shorter times (Fig. 1), the total
amount of corrosion in the propagation state becomes important. As shown in this work, the dissolution rates
vary between < 0.1 µm/y and 20 µm/y (factor 200) and the maximum values are up to 2 times higher in
carbonated mortar made of CEM II. Thus for a safe, long-term durable application of blended cements not
only the general exposure class (XC3 or XC4) has to be considered, but a careful evaluation of the site
specific climatic conditions (number of wet/dry cycles, average relative humidity etc.) is necessary. From this
study it can also be concluded that corrosion in carbonated concrete at a given exposure condition is
influenced to a greater extent by the binder type than by the w/c ratio.
Table 1: Averaged corrosion rate values for each binder in each exposure condition (average over minimum 6
measurements). Ratio of dissolution rate of the steel embedded in the two binders for each exposure.
Corrosion Rate
(µA/cm2)
CEM I
CEM II
CEM II / CEM I
50% RH
0.0058 ± 0.0011
0.0023 ± 0.0005
0.39
95% RH
0.0410 ± 0.0025
0.0617 ± 0.0067
1.50
WET
0.6436 ± 0.1416
1.1842 ± 0.3116
1.84
5 CONCLUSIONS
The corrosion rate of steel in carbonated mortar made of CEM I and blended cement (CEM II) has been
studied for w/c ratios from 0.4 to 0.6 and different exposure conditions. Comparing CEM I and blended cement
(CEM II) at a given exposure condition, the corrosion rate in carbonated mortar made of blended cement is
about twice as high compared to mortar made of CEM I. However, the corrosion rate varies between ca. 0.1
µm/y at 50% RH, ca. 1 µm/y at 95% RH and 20 µm/y in wet conditions, thus the influence of exposure
conditions is crucial. In terms of service life, both the time to depassivation and the total amount of corrosion
of the steel (in µm) in the propagation period have to be considered.
From the results it is obvious that major concern for the application of blended cements in atmospheric
exposure conditions is related to wet/dry cycles (exposure class XC4). The use of blended cements in a
specific structure in exposure class XC4 can be recommended only after a careful evaluation of the site-
specific climatic conditions (number of wet/dry cycles, average relative humidity etc.) showing that the total
steel corrosion does not lead to spalling or cracking during the required service life.
ACKNOWLEDGMENTS
Research supported by the Swiss National Foundation for Research (SNF) project no. 154062 entitled
“Formulation, use and durability of concrete with low clinker cements” is gratefully acknowledged.

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