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The Influence of Admixed Micelles on Corrosion Performance of Reinforced Mortar J. Hu1, D.A.Koleva1, K.van Breugel1, J. M. C. Mol2, J. H. W. de Wit2 1,2Delft University of Technology, The Netherlands 1Faculty of Civil Engineering & Geosciences, Dep. Materials & Environment; Stevinweg 1, 2628 CN, Delft; 2Faculty 3mE, Dep. Surfaces & Interfaces, Mekelweg 2, 2628 CD Delft.

Summary This study reports on the corrosion behavior of reinforced mortar in the presence of very low concentration (0.006 wt % per mortar weight) polymeric nano-aggregates (PEO113-b-PS70 micelles). After curing in fog room conditions (98% RH and 20°C) for 28 days, the specimens were immersed in tap water (control groups) and 5% NaCl solution (corroding groups). Open circuit potential (OCP) mapping, Electrochemical impedance spectroscopy (EIS) and Potentio-dynamic polarization (PDP) were employed for evaluating the corrosion performance of the embedded steel at different time intervals. The results from electrochemical measurement indicate that for the control groups, the micelles can increase both the electrical resistivity of the mortar and the corrosion resistance of the steel reinforcement. For the corroding groups, the micelles are able to delay corrosion initiation and improve the corrosion resistance of the reinforcement. However, after corrosion was initiated, the micelles showed no obvious influence on the corrosion behavior of the reinforcing steel. Morphology observations reveal that the presence of the micelles results in a more homogenous and compact layer on the steel surface.

1. Introduction Reinforcing steel corrosion is the main cause of the deterioration of reinforced concrete structures [1, 2]. In normal condition, the steel reinforcement is corrosion resistant because the concrete is a high alkaline material with a pH between 12.6 and 13.5 and a stable passive layer is formed on the steel surface [3, 4]. However, the corrosion of the steel reinforcement can be initiated when the aggressive aggregates penetrate into the concrete. The main corrosion-inducing agents are carbon dioxide and chloride [5]. The CO2 can react with the alkaline hydroxides in the concrete pore solution and result in concrete carbonation. The carbonation can cause uniform corrosion due to the acidification of the pore solution [6]. The chloride can lead to localized depassivation of the protective layer on the steel surface and cause pitting corrosion of the reinforcement [7]. Polymers are known to be used in cement-based materials for different purposes [8]. Due to ultrafine size, when nano particles are incorporated in cement paste, mortar or concrete, materials with different characteristics from conventional materials can be obtained [9, 10]. There are many papers describing the influence of the incorporated nano particles on the material properties, such as mechanical properties, permeability and microstructure of the cement-based materials [11-13]. However,

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there is no investigation on the corrosion protection of reinforced concrete by nano particles. This study is part of a two-year research project on nano-materials with tailored properties for self-healing of corrosion damages in reinforced concrete. The motivation for applying nano particles in this project is to establish an innovative technology for corrosion control of steel reinforcement. On one hand, the global performance and microstructure of the cement matrix will be improved by the incorporated tailored nano particles; on the other hand, after the corrosion is initiated, the reinforcing steel can be protected by using self-healing mechanisms. In our previous work, the influence of PEO113–b–PS218 micelles on the material properties and microstructure of the plain mortar has already been investigated [14, 15]. The results indicate that the micelles are able to significantly decrease the porosity and permeability of the mortar. In this present work, another similar type of micelles, PEO113-b-PS70 micelles, was admixed in the reinforced mortar, and the electrical resistivity of the mortar and the electrochemical behavior of the steel reinforcement were investigated.

2. Experimental Material specification: The materials used in the present study were reinforced mortar cylinders, cast from Ordinary Portland cement OPC CEM I 42.5R (cast according to EN 196-1). The water to cement ratio (w/c) was 0.5 and cement to sand ratio (c/s) was 1:3. The mortar cylinders comprised 2 main groups: “Ref” group represents specimens cast with tap water; “Nano” group represents specimens cast with polymeric micelles solution (0.5 g/l PEO113-b-PS70 micelles previously dissolved in demi-water). The core-shell micelles were synthesized by atom transfer radical polymerization (ATRP), employing the macroinitiator technique [16]. The mortar cylinders were cured in fog room (20ºC, 98% RH) for 28 days and further lab conditioned (2/3rd of height immersed in water or 5% NaCl solution respectively; the experimental set-up is similar to previously reported such for studying the electrochemical behavior of steel reinforcement in cement-based materials [17]). Sample designation: Four main groups were investigated in this study. For “Ref” group (cast with tap water), “RefW” represents the specimens immersed in tap water and “RefN” represents the specimens immersed in 5% NaCl solution. Accordingly, for “Nano” group, “NanoW” represents the specimens immersed in tap water and “NanoN” represents the specimens immersed in 5% NaCl solution. Methods: The electrochemical measurements involved in this study included electrochemical impedance spectroscopy (EIS) and potentio-dynamic polarization (PDP). The measurements were performed at open circuit potential (OCP) for all cells and in immersed condition (mentioned above). The PDP measurement was performed in the range of −0.2 to +1.2 V vs OCP. The EIS measurements were carried out in the frequency range of 50 kHz to 10 mHz by superimposing an ac voltage of 20 mV. The used equipment was EcoChemie Autolab-Potentiostat PGSTAT30, combined with FRA2 module, using GPES and FRA interface. Relevant to the morphological aspect, at the end of the test, the mortar cylinders were broken and the surface of the steel bar was checked by scanning electronic microscopy 2

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(SEM), using environmental SEM (ESEM Philips XL30) to investigate the product layers or corrosion products, formed on the steel surface.

3. Results and discussion 3.1 Micelles Identification The micelles were previously identified by TEM [16]; hereby performed was Atomic Force Microscopy (AFM) and Dynamic Light Scattering (DLS) measurement for determining the size of the micelles, which are shown in Fig.1. As seen in AFM image (Fig.1 (a)), the micelles showed an approximately spherical morphology with an average diameter of 50 nm or lower. The DLS measurement (Fig.1 (b)) also showed an apparent hydrodynamic radius of 50 nm.

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Fig.1 (a) AFM microscopy and Dynamic light scattering (DLS) of the micelles 3.2 Open circuit potential (OCP) mapping Generally speaking, open circuit potential (OCP) mapping determines the time to corrosion initiation. In chloride containing medium, corrosion initiation is due to the passive layer breakdown on the steel surface i.e. localized corrosion. For a reinforced mortar system, the potential threshold of passivity is generally accepted as -200 ±70 mV [18-20]. The steel is in passive state if the OCP is equal or more anodic then this value.

Fig.2 OCP readings of steel reinforcements in different studied medium

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Fig.2 shows the OCP readings of the steel reinforcements in different immersion medium. The micelles didn’t cause obvious influence on the OCP of reinforcement for the control groups (immersed in tap water). The OCPs of the reinforcing steel were always very anodic for both specimens RefW and NanoW, meaning that a stable passive layer was formed on the steel surface. For the corroding groups (immersed in 5% NaCl solution), at the early immersion days, the OCPs were also very anodic in both conditions. The reason is that at the early immersion days, the chlorides had not penetrated through the cement matrix and reached the steel surface yet; the steel was in passive state. When the immersion continued, the micelles were able to delay the corrosion initiation of the steel which was evidenced by the more anodic potential for micelles-containing specimen (“NanoN”) at 31 and 35 immersion days. However, after the corrosion was initiated, the micelles didn’t show influence on the OCP of the steel reinforcement. 3.3 Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) is a widely used method in the reinforced concrete system [21, 22] and it is able to give information on interfacial phenomena and electrical and electrochemical properties of the reinforced concrete including both the bulk cement matrix and the steel reinforcement [23, 24]. Generally, the EIS response in the high frequency domain gives the bulk matrix properties i.e. mortar electrical resistivity, related to the porosity, pore network connectivity and matrix permeability respectively. The EIS response in the low frequency domain is related to the electrochemical behavior of the steel reinforcement [25]. Fig.3 shows the EIS response in Nyquist and Bode format for the steel reinforcement at different immersion days. It can be observed that for the specimens immersed in tap water, the magnitude of |Z| for micelles-containing specimen NanoW was higher both in the high frequency domain and in the low frequency domain, meaning that the electrical resistivity of the mortar and the corrosion resistance of the steel reinforcement were all increased in the presence of the micelles. As mentioned before, the mortar electrical resistivity is related to the porosity and pore network connectivity of the cement matrix. In previously reported and related to this work investigations, the influence of the PEO113–b–PS218 micelles on the global performance and microstructure of the mortar has already been investigated. The results indicate that the micelles can significantly reduce the porosity (8.23% for the Nano and 14.98% for the Ref samples at 1d curing age) and water permeability (the coefficient of water permeability K for the Nano samples was about 6 orders of magnitude lower than the control (Ref) ones at 7d curing age) of the mortar [14, 15]. Therefore, the results indicate that the higher mortar electrical resistivity of the micelles-containing specimen is resulted from its lower porosity and pore network connectivity. Moreover, the micelles incorporated at the interface between the steel reinforcement and cement paste can improve the barrier effect of the product layer on the steel surface which will result in an improved corrosion resistance of the reinforcement. For the specimens immersed in NaCl solution, there was no obvious difference on the electrical resistivity of the mortar. As previously reported [14], compared to water permeability, the results showed that the NaCl permeability decreased for control specimen due to Cl- binding mechanisms and known Cl- effects on the microstructure [26], whereas increased for nano specimen due to the 4

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shrinkage of micelles caused by chloride [27]. This is corresponding to the EIS data derived for specimens immersed in NaCl solution. However, the steel reinforcement of micelles-containing sample still had a higher corrosion resistance before corrosion initiation.

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Fig.3 EIS response in Nyquist and Bode format for steel reinforcement (a) 21d, (b) 49d 3.4 Potentiodynamic polarization (PDP) The data derived from the EIS measurements are well supported by the potentiodynamic polarization (PDP) measurements in the same medium. Figure.4 presents the potentio-dynamic polarization curves for the steel reinforcement at different immersion days. As seen from Fig.4 (a), at early immersion stage, the corroding specimens depict similar behavior as control groups with external polarization which means the corroding samples were also in passive state. The PDP curves are well in line with the OCP readings and EIS data derived at the same immersion stage. However, when the immersion continued, the passive layer was broken down due to the penetrated chloride and the corroding specimens were transferred into corroding state, as shown in Fig.4 (b). Fig.5 shows the Rp values of the steel reinforcements which are derived from potentio-dynamic polarization curves. For the control groups, the Rp values were very high, from 36.67-444.59 KΩ.cm2 which indicates that a corrosion resistant

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passive layer was formed on the steel surface. Moreover, the Rp value of micellescontaining specimen NanoW was higher, compared to the micelles-free specimen RefW. This is consistent with the higher magnitude of |Z| in the low frequency domain for micelle-containing specimen. For the corroding groups, the same trend can be found before the corrosion initiation. Afterwards, the micelles didn’t show obvious effect on the Rp values. -3

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(a) 21d (b) 49d Fig.4 Potentio-dynamic polarization of steel reinforcement at different time intervals

Fig.5 Rp values of steel reinforcement derived from PDP 3.5 Morphology observation of the steel surface Fig.6 (control specimens RefW and NanoW) and Fig.7 (corroding specimens RefN and NanoN) present the morphology observation of the steel surface at the end of the test respectively. As seen from Fig.6, for the control groups, there is a more homogeneous and compact layer formed on the steel surface for the micellescontaining specimen NanoW, compared to the micelles-free specimen RefW. This is corresponding to the higher electrical resistivity and corrosion resistance of the micelles-containing specimen derived from the electrochemical measurements. For the corrosion groups, the layer on the steel surface of micelle-containing specimen NanoN is still more homogenous and compact, compared to the micelles-free specimen RefN (Fig.7 (a) and (b)). Besides, the corrosion products can be also observed on the surface of both specimens. However, obviously, the corrosion 6

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products are much less for the micelles-containing specimen. The results also indicate that the presence of micelles leads to a corrosion delay for the micellescontaining specimen.

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Fig.6 Morphology observation of steel surface for control specimens RefW and NanoW

(a) Product layer on specimen RefN

(b) Product layer on specimen NanoW

(c) Corrosion products on RefN (d) Corrosion products on NanoN Fig.6 Morphology observation of steel surface for corroding specimens 7

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Based on the electrochemical measurement results and the morphology observation of the steel surface, it can be stated that the incorporated micelles can increase the corrosion resistance of steel reinforcement before the corrosion initiation. However, after the corrosion was initiated, the micelles showed no obvious influence on the corrosion behavior. This is expected because of the low concentration of the micelles (just 0.006 wt% per mortar weight) in mortar. The most plausible reason for the enhancement of the corrosion resistance is due to the influence of the micelles on the microstructure of the mortar and the product layer on the steel surface. According to previously reported results [14, 15], the porosity and pore network connectivity of the cement matrix were reduced due to the effect of the micelles on cement hydration. This resulted in a higher electrical resistivity of cement matrix for micellescontaining sample. The lower porosity and pore network connectivity also lead to an impeded penetration of chloride into cement matrix which then resulted in the delay of the corrosion initiation. The improved corrosion resistance of micelles-containing sample is mainly due to the fact that the micelles present at the interface between the cement matrix and the steel reinforcement can improve the barrier effect of the product layer on the steel surface and result in a more homogenous and compact layer.

4. Conclusions In order to investigate the influence of micelles on the electrical resistivity of concrete and the electrochemical behavior of the reinforcing steel, a low concentration (0.006 wt% per mortar) of PEO113-b-PS70 micelles was admixed into reinforced mortar cylinder and the corrosion behavior of the steel reinforcement were studied. The electrochemical measurements indicate that the corrosion of steel reinforcement is delayed by the incorporated micelles. For the specimens immersed in tap water, the micelles are able to improve both the electrical resistivity of the mortar and the corrosion resistance of the steel reinforcement. The improvement is related to the lower porosity and pore network connectivity of the cement matrix in the presence of micelles. For the specimens immersed in NaCl solution, higher corrosion resistance is also observed for micelles-containing samples before the corrosion initiation. The most plausible reason is that the micelles incorporated at the interface between steel reinforcement and cement matrix are able to improve the barrier effect of the product layer on the steel surface.

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