Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Passivation of Hydrated Cement Nicolas Giraudo, Jonas Wohlgemuth, Samuel Bergdolt, Marita Heinle, and Peter Thissen* Institut für Funktionelle Grenzflächen, Karlsruher Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *
ABSTRACT: Interface phenomena are the starting point for every corrosion reaction. In this work, the corrosion of calcium−silicate−hydrate (C−S−H) phases by water is investigated by implementing silicon wafers as substrates. The carbonation of these phases was avoided by synthesizing the samples in a controlled atmosphere. The passivation of these mineral surfaces is a significant step for materials sustainability. The coating of the surfaces with four different molecules is presented here: carbonation, sodium silicate layers, tetraethyl orthosilicates (TEOS), and octadecylphosphonic acid (ODPA) monolayers. The performance of every technique against corrosion by water is evaluated by infrared spectroscopy, inductively coupled plasma optical emission spectroscopy, water contact angle, and pH measurements. Since the concentration of protons in water is the relevant parameter in this corrosion process, three different values were analyzed at the experiments. After comparing the passivation techniques, the results obtained by coating with ODPA are the most promising ones, and it is, therefore, applied to a more realistic model: hydrated cement. The passivation by ODPA is then analyzed to unravel the mechanism of hydrophobization, finding its dependence with the Ca/Si ratio of the surfaces, supported by first-principles calculations. The passivation by applying this kind of molecule is very promising because of its efficiency against water corrosion and due to the easiness of preparation of the surface before its application. KEYWORDS: Calcium−silicate−hydrate phases, Fourier transform infrared spectroscopy, Density functional theory, Passivation, Corrosion, Octadecylphosphonic acid
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INTRODUCTION The development of new materials to be implemented in the construction industry is very promising and will be applicable in the near future.1,2 Even more interesting are the advances in the development of recycling technologies for concrete.3,4 However, concrete production does not stop nor decrease, and its consumption grows steadily. Therefore, the most urgent solution is to avoid the deterioration of this kind of material. The concept of the corrosion of concrete has been analyzed in the past regarding many aspects. The corrosion of metal supporting bars is well-known, and many efforts are being made to reinforce the structures in order to avoid such deterioration.5−7 The discussion of whether the solution for the degradation of metals has to be settled before the degradation of concrete is still open; however, understanding regarding the mechanism of both types of degradation is rather unbalanced. The deterioration mechanism denoted as alkali−aggregate reaction (AAR) or, more specifically for siliceous aggregates, alkali−silica reaction (ASR) is also well-known to produce cracks in these materials due to their increments of volume.8 The corrosion of hydrated cement itself by water has been discussed, defining the deterioration of the calcium−silicate− hydrate and calcium−silicate phases by the metal−proton exchange reaction as the main factor responsible under ambient conditions.9 Nevertheless, the principal concern remains open: how to avoid the corrosion produced by water. © XXXX American Chemical Society
The mechanism of the metal proton exchange reaction (MPER) was shown before as an interface phenomenon, i.e., at the exchange of calcium by protons of the water, the solid phase of the calcium−silicate−hydrate (C−S−H), and calcium− silicate (C−S) phases, which are in contact with the liquid phase of water.10 Thence, surface approaches to slow down this mechanism should be able to interact with both phases, solid and liquid, and act as a barrier for the MPER. The solid phase in this case study consists of a surface of concrete, mainly hydrated cement. This surface is normally carbonate terminated due to the presence of CO2 in the atmosphere.11 It was shown in an earlier publication that the passivation implementing several molecules competes with the carbonation of the surface. In that work, it was demonstrated that the molecules frequently used to perform the passivation of C−S−H surfaces are not able to form covalent bonds with the carbonates.12 The activation was investigated by four different techniques in that work, but most of them with further changes in the silicate structure. The challenge to be faced now is finding molecules that are able to replace the carbonates (passivation by activation) or to activate the surface without Received: August 31, 2017 Revised: November 21, 2017 Published: November 27, 2017 A
DOI: 10.1021/acssuschemeng.7b03045 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. IR spectrum of C−S−H phases synthesized on a silicon wafer, taking the piranha-cleaned silicon wafer as a background.
molecules is achieved, which confirms their suitability for further application.
strong modification of the materials (activation before passivation). The synthesis of calcium silicates on silicon wafers was described before as a versatile tool for atomic understanding.13,14 In this paper, these ultrathin layers are the key substrates to investigating the passivation techniques with different molecules. As was mentioned before, the carbonate’s termination impedes the reaction of many molecules with the surface. Therefore, a carbonate-free synthesis is implemented here. The results of the passivation techniques are evaluated by means of Fourier transform infrared spectroscopy (FTIR). The selection of molecules used to perform the passivation is based on already performed studies on the hydrophobization of ceramic and concrete surfaces.15−17 Four different techniques of passivation are applied here: carbonation, sodium silicate layers, tetraethyl orthosilicates (TEOS), and octadecylphosphonic acid (ODPA) monolayers. FTIR allows a description to be made about the kind of bindings of the molecules with the protected surface. Afterward, the coatings produced by these molecules are chemically tested when wet to determine their performance against corrosion. This performance is also evaluated by means of FTIR. Furthermore, the resulting solutions of the chemical corrosion were investigated by means of inductively coupled plasma optical emission spectroscopy (ICP-OES) to confirm the exchange of calcium from the samples. An in situ investigation of the changes in the degradation solution was performed in every sample through resistance measurements. Some examples are mentioned here and shown in the Supporting Information. The coating technique that showed the best results against chemical corrosion was the one made with ODPA. In addition to that, the hydrophobization achieved because of the long organic chains of this molecule have shown remarkable results in the protection against MPER. This passivation technique was also implemented in a more realistic model: a hydrated cement surface. Since this surface is terminated by carbonates, it is representative of realistic situations, and therefore, it is also tested under harsh conditions. It was shown in earlier publications that the MPER depends on the availability of calcium at the surface; thus, surfaces have to be analyzed while taking this parameter into account.10 With this aim, first-principles calculations were performed to investigate the mechanism of ODPA reacting on the same orientation of different Ca/Si minerals: (001) slabs of tobermorite (0.83), wollastonite (1), and jennite (1.5). From the combination of experimental and theoretical results, a complete description of the mechanism of passivation by these
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MATERIALS AND METHODS
Sample Preparation. A 3 cm × 1 cm Si(111) crystal was chemically cleaned with a 30 min exposure at 80 °C to a 1:3 solution of aqueous H2O2:H2SO4 (piranha solution). Afterward, the crystal was rinsed thoroughly with deionized water with a resistivity of 0.055 μS/ cm. Then, the wafer was immersed in a solution of 5 mM Ca(OH)2 at 60 °C in 100 mL reactors during 6 h. Aqueous solutions of Ca(OH)2 as bought from Sigma-Aldrich were prepared in an N2(g) atmosphere to avoid any effects due to CO2 contamination. After the synthesis, the samples were rinsed with water and dried with argon inside the N2(g) atmosphere. Passivation Techniques. Octadecylphosphonic Acid. Solutions of 97% ODPA as bought from Sigma-Aldrich were prepared in methanol at a concentration of 1 mM. Samples were put into a closed container, with 10 mL of such solutions at 60 °C for 12 h. Tetraethylorthosilanes. A 0.1 mL drop each of 98% TEOS as bought from Sigma-Aldrich was placed at the bottoms of 15 mL cylindrical containers containing the sample. The process lasted 12 h at 90 °C. Sodium Silicates. Water solutions of Na2SiO3 (sodium metasilicate as bought from Sigma-Aldrich) with a concentration of 10 mM were prepared in 10 mL containers. Samples were immersed in these solutions at 60 °C for 6 h. Carbonates. To terminate the samples with carbonates, as was explained in earlier publications, the simple exposure of the samples in the atmosphere addresses the desired results.11,12 Samples were let in contact with the atmosphere during 12 h; the relative humidity was measured to be 20%. Fourier Transform Infrared Spectroscopy. FTIR measurements were done in a N2(g) purged glovebox with a Bruker Vertex 70, and recorded with a nominal 4 cm−1 resolution. Spectra were collected from 400 to 4000 cm−1 in transmission mode with an angle of incidence of 64° with respect to the silicon surface normal. A room temperature pyroelectric detector (DTGS) was employed for data collection. In all experiments, 1024 scans were taken. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Calcium concentrations of the solutions after contact with the samples were measured by inductively coupled plasma optical emission spectrometry (ICP-OES; OPTIMA 8300DV, PerkinElmer). The sample flow was set to 1 mL/min. The HF-generator was operated at 1400 W. Gas flows were 15 L/min for the plasma, 0.5 L/ min for the thrust gas, and 0.55 L/min for the vaporizer gas. pH Measurements. A benchtop pH meter (Orion Star A211, Thermo Fisher Scientific, Waltham, MA, USA) was implemented to determine the pH values of the solutions. Water Contact Angle. The water contact angle (WCA) was measured at ambient temperature with an optical contact angle meter (DSA100, Kruess, Hamburg, Germany) using the sessile drop measuring method. To determine the static contact angles of 10 μL water droplets, the drop shapes were evaluated using the Young− B
DOI: 10.1021/acssuschemeng.7b03045 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. IR spectrum of carbonates formed on the surface of C−S−H phases synthesized on a silicon wafer, taking the sample before exposure to the atmosphere as a background.
Figure 3. IR spectrum of Na2SiO3 on the surface of C−S−H phases synthesized on a silicon wafer, taking the C−S−H phases as a background.
cm−1 and δ(O−C−O) at 875 cm−1). The spectrum shows other vibration modes after the synthesis, corresponding to the Si−O (ν(Si−O) with a maximum at 976 cm−1 and δ(O−Si−O) at 660 and 460 cm−1), which is some evidence of the formation of the C−S−H phases. Additionally, the thickness was investigated by scanning electron microscopy and was measured around 400 nm; the surface of the layer shows a high roughness and a needlelike appearance.14 These phases are the starting point for all techniques that are applied in this work to passivate the surfaces. In the past, several works confirmed that carbonates are not able to slow down the corrosion of C−S−H phases significantly; however, the experimental confirmation of this statement is shown here.11,12 Figure 2 shows the spectrum that results from the exposure of C−S−H phases synthesized on a silicon wafer to atmospheric conditions for 12 h. As can be observed in this spectrum in Figure 2A, the formation of carbonates produces negative peaks corresponding to the stretching vibration modes of SiO−H at 3740 cm−1 and CaO− H at 3690 cm−1 and the appearance of negative peaks corresponding to the vibration modes of silicates in Figure 2B, ν(Si−O) at 1048 and 936 cm−1, which are compensated by the appearance of a peak at higher wavenumbers, ν(Si−O) at 1146 cm−1. This shift to higher wavenumbers indicates that, by the formation of calcium carbonates, the removal of calcium from the C−S−H phases is produced. The formation of this kind of carbonates is due to the presence of water (20% relative humidity), as was explained by Longo et al. This statement is confirmed by the presence of Si−O vibration modes corresponding to less coordination by calcium.11,13 Besides, in Figure 2B, the vibration modes corresponding to the carbonates are observed: νas(C−O) with a maximum at 1480 cm−1 and δ(O−C−O) at 870 cm−1. Comparing the intensity
Laplace equation (software DSA3, Kruess, Hamburg, Germany). The WCA values were averaged from five independent measurements. Computational Methods. 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).18 The electron−ion interaction was treated within the projector-augmented-wave (PAW) method.19 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 × 2 × 1 meshes of Monkhorst−Pack k-points.20 The electron−electron exchange and correlation energy was approached making use of the PW91 functional contained in the generalized gradient approximation (GGA).21−25
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RESULTS AND DISCUSSION Passivation Techniques. Figure 1 shows a well-known spectrum of the C−S−H phases synthesized on silicon wafers.12,13,26 However, it is important to depict the characteristic vibration modes corresponding to these phases, since they are relevant later to understand the reactions of passivation. The spectrum shown in Figure 1 is divided into two parts; this procedure is repeated for all spectra in this work to depict every vibration mode better. In Figure 1A, the vibration modes corresponding to molecular water and dissociated water are observed. Two features related to the presence of OH can be depicted: a broad band between 3100 and 3500 cm−1 corresponding to free water adsorbed and sharp peaks at 3690 and 3740 cm−1 that confirm the presence of dissociated water with the stretching vibration modes of Ca−OH and Si− OH, respectively.27−29 As can be seen in Figure 1B, the synthesis shows not only vibration modes associated with the presence of silicates but also carbonates that are identified by their vibration modes (in νas(C−O) with a maximum at 1487 C
DOI: 10.1021/acssuschemeng.7b03045 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. IR spectrum of TEOS on the surface of C−S−H phases synthesized on a silicon wafer, taking the C−S−H phases as a background.
Figure 5. IR spectrum of ODPA on the surface of C−S−H phases synthesized on a silicon wafer, taking the C−S−H phases as a background.
The peak centered at 970 cm−1 corresponding to the stretching vibration of Si−O indicates the formation of silicates that are not polymerized, also confirming conclusions drawn in older works.13 Also, the bending vibrations of O−Si−O at 660 and 460 cm−1 support this statement. Silanes were already used in the past by many other authors to make surfaces more hydrophobic. Among these surfaces, examples are ceramic and hydrated cement.30 However, as was mentioned before, these kinds of molecules are not able to react with surfaces that are terminated with carbonates. In this case, TEOS was applied to a carbonate-free surface, and the resulting spectrum is observed in Figure 4. Apart from the negative peak corresponding to the stretching vibration mode of Si−OH at 3740 cm−1, the characteristic vibration modes of this molecule, e.g., νas(C−H3) at 2976 cm−1, νas(C−H2) at 2922 cm−1, νs(C−H3) at 2896 cm−1, and νs(C−H2) at 2850 cm−1 in Figure 4A; ν(C−OSi) at 1580 cm−1, δ(H−C−H) in CH3 at 1445, 1389, and 1164 cm−1, δ(H−C−H) in CH2 at 1295 cm−1, ν(Si−O) at 1100, 1082, 1023, and 793 cm−1, and δ(O−Si−O) at 475 cm−1 in Figure 4B are clearly observed, confirming the presence of the molecule on the surface.31,32 However, as observed in the case of Na2SiO3, the interaction with the C−S−H phases is not intensive, which can be associated with a weak or nonexistent reaction between both phases. The missing interaction is observable in the intensity of the negative peaks corresponding to the C−S−H phases, represented by the stretching vibration modes of Si−O at 1023 and 938 cm−1. This weak interaction has a strong influence on their performance against corrosion. However, from the intensity of the Si−O vibration modes, one can assume that the layers of TEOS adsorbed on the surface are of a thickness similar to the ones of the C−S−H phases. Due to the weak interaction of the hydrophobic compounds and passivation layers with the Si−O bonds of C−S−H phases,
and frequency of the peaks observed in the spectrum with the ones of the synthesis in Figure 1 and with the ones of an older publication allows for the conclusion that the carbonates are present as multilayers on the surfaces.12 The occurrence of the metal proton exchange reaction (MPER) depends not only on the concentration of the protons in the solutions but also on the present metal in the structure. In the case of C−S−H phases, the metal is calcium, but if the metal is changed, the rate of the reaction could be either slowed down or accelerated. Therefore, a compound similar to the C− S−H phases is deposited on the surfaces. Figure 3 shows the spectrum resulting from the passivation technique that implies the deposition of Na2SiO3 on the C−S−H phase surface. The similarity of this spectrum to the one shown in Figure 1 confirms the statement made before about the similarity of the compounds. Since the C−S−H phases synthesized on silicon wafers are not presented as long chains, the missing polymerization of sodium silicates makes the spectra really alike.13 The effects on the vibration modes corresponding to the C−S−H phases are not significant, which is attributed here to the absence of a strong reaction between both phases and will also be confirmed during the corrosion processes. However, regarding the intensities of the Si−O vibration modes, it can be assumed that the layer built on the surface is even thicker than the ones of the C−S−H phases. The vibration modes observed in Figure 3A correspond to the increment of the presence of SiOH (ν(SiO−H) at 3740 cm−1) on the surface, stemming from the Na2SiO3 and the further adsorption of water caused by this compound (broad peak at 3300−3600 cm−1). Also, the presence of some carbonates can be detected in Figure 3B, since sodium carbonates are also easily formed on sodium silicate surfaces, and even if the synthesis was performed under protected conditions, some CO2 is present in either the water or the preparation powder. D
DOI: 10.1021/acssuschemeng.7b03045 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 6. Difference spectra of clean C−S−H phases after 2 h of contact with different solutions, taking the previous state as a background.
Figure 7. Difference spectra of C−S−H phases passivated with carbonates after 2 h of contact with different solutions, taking the previous state as a background.
phosphonates built on the surface are monodentate.37,38 Furthermore, this statement will also be confirmed by supporting first-principles calculations. The huge negative peak observed at 972 cm−1 corresponding to the ν(Si−O) shows the strong interaction of the phosphonates with the C− S−H phases. This shift in the vibration modes to higher wavenumbers (ν(Si−O) at 1060 and 1040 cm−1) indicates that calcium is the key to the interaction of these molecules with the surfaces as will be confirmed by the first-principles calculations. Efficiency of Passivation Techniques against Corrosion. Once the surfaces were treated with different wet chemical approaches to adsorb different molecules, the MPER is analyzed for each one in order to prove their efficiency against corrosion. As was mentioned in older publications, the rate of MPER depends on the pH value of the solution, and therefore a drastic effect can be observed if solutions with pH
