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Corrosion of Concrete by Water-Induced Metal-Proton Exchange Nicolas Giraudo, Peter Georg Weidler, Fabrice Laye, Matthias Schwotzer, Joerg Lahann, Christof Wöll, and Peter Thissen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07347 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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The Journal of Physical Chemistry

Corrosion of Concrete by Water-Induced Metal-Proton Exchange

Nicolas Giraudo1, Peter G. Weidler1, Fabrice Laye1, Matthias Schwotzer1, Joerg Lahann1,2, Christof Wöll1 and Peter Thissen1*

1

Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-von-

Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2

Department of Chemical Engineering, Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan, United States *corresponding author Abstract We have investigated basic mechanisms of concrete corrosion by studying Wollastonite in aqueous environments. This well-defined crystalline mineral is well suited as a model system for Calcium-Silicate phases, the main constituent of this important building material. A detailed peak-shape analysis of x-ray diffraction (XRD) signals recorded for Wollastonite powders exposed to water allowed to monitor dramatic changes in particle shape as a result of the so-called Metal-Proton Exchange Reaction (MPER). Since these experiments were carried out for well-defined particles with known orientation, state-of-the art calculations using density functional theory (DFT) could be employed to more precisely study this behavior, which previously has not attracted much attention. The free energies of the different crystalline surfaces of Wollastonite are strongly affected when brought in contact with water, thus providing a strong driving force for changes in particle shape. A more detailed analysis of the corresponding Wulff constructions reveals, however, that a quantitative description of these phenomena also requires a detailed analysis of the kinetics. Implications for concrete corrosion will be discussed, and strategies for its prevention will be outlined. Keywords: Calcium-Silicate; Wollastonite; Wulff Construction; Metal-Proton Exchange Reaction; Cement; Corrosion;

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Introduction In all developed countries Portland-cement concrete is the most widely used construction material.1-3 However, concrete degradation is a major problem for industrialized countries. According to a study published in 2014, in 2002 a total of 8.3 billion dollars had to be spent only for the repair of highway bridges, with the total amount of expenses required for the removal of corrosion damage to all concretebased materials being at least an order of magnitude higher.4 There are many reasons for the failure of concrete-based constructions: corrosion of supporting (metal) bars, mechanical stress caused by vehicles, or rapid heating-cooling cycles resulting from day/night temperature changes. One of the largest problems, however, is the interaction of Calcium-Silicate (CS) and Calcium-Silicate-Hydrate (CSH) phases (forming the structural basis of concrete) with water. Among the various corrosion reactions of CS and CSH phases, the so-called Metal-Proton Exchange Reaction (MPER) schematically represented in Fig. 1 is the most important.5 The MPER leads to a leaching of metal ions, in particular Ca, out of the concrete, a process which leads to massive structural changes of the corresponding CS and CSH phases. Although this reaction has been studied in many previous works, many details and implications are not sufficiently well understood. Here, we will use a model system, the mineral Wollastonite, to unravel important aspects of the MPER and, more importantly, to demonstrate a crucial consequence leading to shape-changes of the micrometer-sized particles of this mineral in aqueous environments. Wollastonite is a naturally occurring mineral with triclinic structure.5-8 While the bulk structure is well characterized, only little information is available about the surface chemistry of this material and, particularly, about the interaction with water. Previously, Schott et al. reported an experimental investigation of the dissolution reactions occurring at the surfaces of this mineral under acidic conditions.9 The reported experimental reaction rates were in good agreement with theoretical results obtained from Density Functional Theory (DFT) studies.5 When extending the detailed theoretical analysis to specific surfaces of Wollastonite, we found substantial differences as a result of strong anisotropic behavior. This observation has important consequences, since changing the relative stability of different surface types, e. g. when brought from contact with air into contact with water, will change the shape of the particles constituting concrete. While a number of studies have demonstrated the importance of such shape changes of mineral particles upon immersion in water, this approach has, so far, not been applied to model systems for concrete.5, 10 A straightforward way to predict the equilibrium shape of particles from the free energies of the different surface orientations is provided by the Wulff construction.11 Application of this method yields the thermodynamic most stable shape for a particle in equilibrium with the environment. 12-13 While most Wulff constructions are reported for particles in equilibrium with the vapor phase, recently also equilibria with other environments have been considered. Such calculations represent a major challenge, since in order to obtain the corresponding surface free energies, e.g. via DFT, a water layer with sufficient thickness has to be included in the calculations. As a result, DFT-based Wulff constructions for materials in aqueous environments have only been reported in a few cases, e.g for MgO.14-15 Here, we will first describe the outcome of XRD-experiments, which allow to determine the shape of micrometer-sized powder particles before and after immersion into water. Subsequently, Wulff constructions for two materials, α-SiO2 and Wollastonite, will be presented. While for α-SiO2 the waterinduced changes are small, as expected, substantial changes are seen for Wollastonite. Surprisingly,


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however, the predicted changes in the shape of the Wollastonite-particles are not consistent with our experimental XRD results. We will show that by expanding the Wulff construction by explicitly considering kinetic control we can derive a consistent picture of experimental and theoretical findings. Materials and Methods Sample preparation Wollastonite powder particles with dimensions of less than 20 µm were produced by milling of a mmsized natural crystal and subsequent sieving. After characterization with XRD, the powder was immersed in an aqueous HCl solution (pH 4.3). The solution which was continuously purged with argon in order to avoid unwanted reactions other than MPER, e.g. carbonation.16 After 5 minutes of purging the recipient was closed. After a reaction time of 24 hours the suspension was filtered. The powder remaining on the filter paper was dried to constant weight and immediately characterized again with XRD. Aliquots of the solution were analyzed by inductively-coupled plasma optical emission spectroscopy (ICP-OES) to determine the amount of calcium which had been leached out from the powder. X-ray Diffraction (XRD) Out-of-plane XRD measurements were carried out with a Bruker D8 Advance in Bragg-Brentano geometry (θ - θ setup) by employing a PSD Lynxeye (192 Si-strips) detector equipped with a 9-fold sample holder. Cu-radiation was implemented (K-alpha). A scan speed of 0.2 seconds/step and a step size of 0.01° were used. Diffracted intensities were recorded in an interval between 10° and 70°. For all measurements the same holder was used. Scanning Electron Microscope (SEM) A Philips XL30 ESEM-FEG operated at 20 kV was used for imaging the samples. Theoretical determination of surface free energies at the solid/vacuum and solid/water interface All ab-initio electronic structure calculations reported here were carried out using Density Functional Theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).17 The electron-ion interaction was treated within the projector-augmented wave (PAW) method.18 The plane waves into which the valence electron wave functions are expanded have a kinetic energy cutoff of 360 eV. This energy limit yielded converged structures for all calculations. The Brillouin zone sampling was completed with 2 x 2 x 1 meshes of Monkhorst−Pack k-points.19 The electron-electron exchange and correlation energy was approached making use of the PW91 functional contained in the generalized gradient approximation (GGA), which in previous work has been found to reliably describe the structure and energetics of hydrogen bonded water molecules.20-24 Every slab used for the calculation of free surface energy was extended and relaxed from a single crystal, which was optimized with the third order Birch-Murnaghan equation of state. In order to assess the stability of the numerous model structures and different surface orientations the thermodynamic grandcanonical potential was investigated, using previously published methods found in Sanna et al. (14). For the corresponding calculations, it is important to note that not only the number of water molecules but also the numbers of surface Ca and O atoms as well as H and electrons will be different for the different structures.25



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Additional anions like chloride have not been considered in these calculations. Results and Discussion The Lorentzian fitting of the XRD peaks recorded for the Wollastonite particles before and after immersion to the aqueous solution are shown in Fig. 2.26 While the positions remain unchanged, the width of the peaks showed a significant variation. These changes in width result from a change of the size of the coherent scattering domains (CSD) along the corresponding crystallographic directions also provided in Fig. 2. A broadening of a particular XRD-peak reveals a decrease of the CSD-size along the corresponding direction.27 Scherrer’s equation can be used to quantitatively determine the average CSDsize (∆) from the FWHM (full width half maximum) value:

2 = ⇔ %∆ =

∗ 100 ;

(1);

where, β is the FWHM, after subtracting the instrumental line broadening. K is the constant of the proportionality (Scherrer constant). is the wave length. L is a volume average of the CSD thickness in the direction normal to the reflecting planes. The changes on the FWHM observed in the Fig. 2, resulting from a fitting with a Lorentzian function are: from 0.052 ± 0.001 to 0.073 ± 0.003 in (200), from 0.061 ± 0.001 to 0.090 ± 0.003 in the (002) and from 0.100 ± 0.003 to 0.089 ± 0.003 in (120) before and after 24 hours MPER, respectively. In the microstructure of Wollastonite, both, the (100) and the (001) direction run parallel to the so-called b-axis. Strong changes of the CSD sizes appear as a result of the immersion of Wollastonite in water for 24h. Interestingly, both (200) and (002) domains change in a similar way, becoming smaller (broader peaks) upon immersion in water. In contrast to that, changes of along the (120) direction are negligible. Measurements recorded after 48h of immersion showed that Wollastonite particles became amorphous, in good agreement with recent literature.9 Corresponding experiments for α-SiO2 particles immersed in the same aqueous solutions revealed no changes in XRD peak shapes. Fig. 3 shows selected SEM pictures recorded for particles after different exposures to water. The initial structure of the particles after the grinding process is rather elongated. Similar nail-shapes have been reported in earlier studies.9, 28 After 24 hours of immersion in the solution, MPER has clearly changed the shape of the particles, the now reveal a needle-like form. The CSD-data reveal that the extension along the 010-directions has decreased substantially. After 48 hours of immersion most particles are amorphous, in full agreement with the XRD – results. We now come to the theoretical description of particle shapes in thermodynamic equilibrium with the environment. The first step in a Wulff construction is to determine the total free surface energy γ of the most important surface orientations.14 γ is defined as =

!" #$∗

%&

;

(2);

where Esl denotes the free energy of the slab; n the number of unit cells used to build the slab, Eb is the free energy of the unit cell of the bulk, and A is the area of the slab. In case of α-SiO2, the calculated surface energies amount to 2.5 J/m2 for α-SiO2(001) and to 1.6 J/m2 for α-SiO2(100) and α-SiO2(010). These values are in good agreements with previous results reported in the literature.29-31 After adsorption of water, the values change to 8.8 J/m2 for α-SiO2(001) and to 4.0 J/m2 for α-SiO2(100) and αSiO2(010).


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In Fig. 4 the Wulff constructions of clean α-SiO2 (A) and α-SiO2 after adsorption of water (B) are depicted. In case of α-SiO2 the water molecules just adsorb on the surfaces, no strong chemical interactions are present. Consequently, the water-induced changes in shape are negligible (see Fig. 3). For the calculations of Wollastonite slabs it is crucial that both sides of the slabs (top and bottom) exhibit the same surface termination. Among the different orientations studied in case of Wollastonite, special care has to be taken for the (010) orientation. All orientations running not parallel to the b-axis will create unit cells where the chains of Si-O have to be broken. In order to saturate the resulting dangling bonds, a H2O molecule was added to the unit cell, '( − * − '( + ,% * '( − * − , + '( − * − ,;

(3);

and the total free energy of the slab was adjusted accordingly: 12345 = ./0/ − 678 9 ∗ μ,% * ; ./0/

(4);

where E(010) is the free energy obtained from the calculations for the (010) surface, nH2O is the number of water molecules integrated into the model, µ(H2O) is the chemical potential of the water. In case of Wollastonite, the calculated surface energies amount to 0.04 J/m2 eV for the (001) surface, 0.08 J/m2 for the (100) surface, and 0.29 J/m2 for the (010) surface orientation. As a result of the MPER, the thermodynamically most stable state of Wollastonite surfaces changes when brought in contact with water, the new values are 3.85 J/m2 for the (001) surface, 3.12 J/m2 for the (100) surface, and 7.80 J/m2 for the (010) surface orientation. More important than the absolute changes are the relative changes: Ratio of the surface energies before MPER (001/100/010) is (1/1.8/6.8) and after MPER it is (1.2/1/2.5). In the Fig. 5 the Wulff constructions of clean Wollastonite (A) and Wollastonite after the reaction with water (B) are depicted. Dramatic changes in particle shape are the result of the so-called MPER. The calculation of the ratios between the orientations shows that the orientations perpendicular to the baxis direction are elongated, having ratios of 7:1 for the (010) relative to the (001) and 3:1 for the (010) relative to the (100). Surprisingly, these changes in the aspect ratios do not agree with our XRD results. For example, along the [010] direction, the calculations predict a decrease in CSD diameters, whereas the corresponding XRDdata show no change at all. We take this unexpected discrepancy as an indication that the changes in the shape of the micrometer-sized Wollastonite particles in aqueous environments cannot be understood on the basis of thermodynamic arguments alone. Obviously, the equilibrium shapes are not reached and the possibility of kinetic limitations has to be considered. As has been demonstrated recently, the kinetic constants k001, k010 and k100 of the MPER occurring along the [001], [010] and [100]-direction, respectively, can be computed using Transition State Theory (TST) by the Eyring model. One of the most important conclusions of that work was that k010/k001 is so large, that the MPER on (001) and (100)-oriented surfaces did not take place on the time-scale (12 h) of our experiments. Furthermore, k100 and k001 can be assumed to be equal.5 Note the substantial difference in chemical behavior between surfaces oriented perpendicular to the silicate chains and surfaces not cutting through chains. The ratio of the surface energies before MPER (001/100/010) is (1/1.8/6.8) and after kinetic MPER it is now (1/1.8/16.2). In the same Fig. 5 the Wulff construction of clean and the kinetic controlled model of



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Wollastonite after MPER (C) is depicted. Now we find a consistent picture of experimental and theoretical trends. The Metal-Proton Exchange Reaction on Wollastonite is responsible for Calcium removal from the structure, being replaced by protons of the water, it modifies the shape of the crystals leading to their amorphization. This reaction runs faster in orientations that are perpendicular to the silicate tetrahedral chain, and therefore will affect more strongly the ones in which these orientations are more present, namely with shorter chains. Conclusions Using XRD we find substantial changes in the shape of Wollastonite particles upon immersion into water. A thorough theoretical analysis revealed that these shape changes, caused by a corrosion process referred to as MPER, cannot be explained by thermodynamic arguments alone, instead kinetic considerations have to be applied. The observed phenomena can be rationalized using an observation reported in an earlier work, where the MPER was found to be substantially accelerated on surfaces oriented perpendicular to the b-axis of Wollastonite (010-direction), whereas for orientations parallel to the silicate tetrahedral chains the reaction is delayed.5 These kinetic differences are responsible for the characteristic changes on the shape of the mineral particles when exposed to water. Corrosion of concrete induced by MPER leads to a phase transformation of crystalline to glassy C-S-H phases, due to the before-mentioned changes in the structure. The length of the silicate tetrahedral chain plays a very important role in the mechanical properties such as stiffness and hardness of this kind of compounds, and also determines how the Metal-Proton Exchange Reaction will affect their structure. Acknowledgments The authors acknowledge Thomas Sollich for experimental assistance. Results presented in this paper have been gained within the DFG-funded project TH 1566/3-2. The authors acknowledge the Texas Advanced Computing Center (TACC) for providing computational resources.


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Figures and figure captions

Figure 1. Metal-Proton Exchange Reaction (MPER) on a (001) Wollastonite surface.

Figure 2. XRD pattern of Wollastonite (black) before MPER and (red) after 24 hours MPER.

Figure 3. SEM captions of selected representative particles of the Wollastonite powder at different



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times of immersion in water.

Figure 4. (A) Wulff construction of clean α-SiO2. (B) Wulff construction of α-SiO2 after water adsorption.

Figure 5. (A) Wulff construction of clean Wollastonite. (B) Wulff construction of Wollastonite after MPER. (C) Wulff construction of Wollastonite after MPER with detailed kinetic analysis.


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