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Carbonation Competing Functionalization on Calcium-SilicateHydrates: Investigation of Four Promising Surface-Activation Techniques Nicolas Giraudo, and Peter Thissen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00870 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016
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ACS Sustainable Chemistry & Engineering
Carbonation Competing Functionalization on Calcium-Silicate-Hydrates: Investigation of Four Promising Surface-Activation Techniques
Nicolas Giraudo and Peter Thissen*
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Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
*corresponding author: Dr. Peter Thissen Head of the group: Model development of mineral interfaces Karlsruhe Institute of Technology (KIT) Institute of Functional Interfaces (IFG) Hermann-von-Helmholtz-Platz 1, Building 330 76344 Eggenstein-Leopoldshafen, Germany Phone: +49 721 608-28223 E-mail:
[email protected] Homepage: http://www.ifg.kit.edu/english/379.php
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Abstract In this study ultrathin Calcium-Silicate-Hydrate (C-S-H) phases on silicon wafers were prepared, which are partially terminated by calcium carbonates. First, a Density Functional Theory (DFT) analysis was performed, to define the nature of the carbonates that are stable in the structure, concluding that two different kinds of them will be present on the surface. Then, by means of four different experimental handling techniques, the C-S-H phases were activated by disposing the carbonate termination: (1) UV-light (365 nm) radiation as a function of time, (2) direct heating between room temperature (RT) and 840°C, (3) wet chemical treatment by an aqueous solution with a defined pH value as a function of time and (4) Ar/O2-plasma treatment. Fourier Transform Infrared (FTIR) spectroscopy was implemented to confirm that every method was successfully reducing the carbonate termination of the ultrathin C-S-H phases. Interestingly, the effects of the diverse treatments on the C-S-H phases are very different. UV-light radiation eliminates partially carbonates from the C-S-H phases; but in contrast to the other treatments the rate of this activation is very low. Temperatures up to 700°C are necessary to remove the carbonates by direct heating. Remarkably, at these high temperatures the remaining Calcium-Silicate (C-S) phases start to change their crystal structure, which was proved by means of X-ray Diffraction (XRD). During wet chemical treatment, in addition to the carbonates removal, C-S-H phases were also affected, due to the low pH value (≤ 4) of the implemented solution. Finally, the most rapid activation at RT was provided by Ar/O2-plasma treatment, without drastic impacts on the C-S-H phases. Keywords: Calcium-Silicate-Hydrate (C-S-H); Calcium Carbonate; Calcium-Silicate Carbonate; MetalProton Exchange Reaction; Infrared Spectroscopy;
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ACS Sustainable Chemistry & Engineering
Introduction Corrosion of hydrated cement has to be understood and avoided. In this manuscript the idea of activation before functionalization is suggested. Activation means, in this case, the removal of carbonates which inhibit the covalent bonding of passivation molecules on the surface. Consequently, carbonates should be removed from the surface before any functionalization. Thereby, the reactions of passivation will be more efficient. Consequently, the aim of the techniques explained in this work is to remove the calcium carbonates/silicate-carbonates which are acting as a barrier for the functionalization. Carbonates are present in the industry and in the quotidian life with several applications. Metal carbonates like calcium carbonate are used to produce clinker, that later will be implemented for cement; sodium carbonates are used to produce glasses, soaps, detergents and paper; lithium carbonates are very popular in the pharmaceutical industry; cobalt carbonates in the catalysts industry, between tens of examples.1-2 However, other kind of applications using carbonates production is nowadays of rising importance: its utilization to the sequestration of CO2.3-4 Since CO2 is the principal gas responsible for the Greenhouse Effect, one of the greatest but partially successful efforts of the research community during the last century was to control the release of this gas, regulating thereby the global warming.5 During the production of cement, a lot of CO2 will be released, and therefore alternatives to reduce it, like for example storage of the gas through sequestration via carbonation, are in constant development.3, 6-10 Carbonates can have negative effects on life, inhibiting for example heavy metals to be transferred from soil to plants.11 The presence of carbonates in many minerals as impurities is well known, and its presence in C-S-H phases has been well characterized.12-14 If these compounds were considered to be desirable on the surface, like intended for CO2 storage, in that case C-S-H phases are really promising, for example using supercritical CO2 conditions.15-16 However, the discussion if the hydrated cement can be applied for CO2 storage remains controversial, since the impact of such process on the mechanical properties of the concrete is still unknown. The consumption of cement is steadily growing. It is expected to grow 7.5 % in 2015 and 7.9 % in 2016.17-18 In contrast to that, the advances in detecting and preventing the corrosion of hydrated cement are scarce. C-S-H phases are the most important compounds of hydrated cement and responsible for its mechanical properties.19 Chemical corrosion of hydrated cement is occasioned mainly by water through the Metal-Proton Exchange Reaction (MPER), i.e. a Ca is removed from the structure and replaced by protons from water, building calcium hydroxide as a product.20-21 Longo et. al. described the MPER of such C-S-H phases in presence of CO2, to understand the dependence of the building of carbonates in presence of water.22 One of the most transcendental conclusions of their work is that the reaction of carbonation in C-S-H phases where MPER has succeeded, forms calcium carbonates very quickly, due to the availability of the Ca on the surface. At surfaces where MPER did not take place, CO2 will be also adsorbed, but bonding more weakly. The end products of both reactions are predicted and their structures are calculated and discussed later in this work.23-24 One approach to bridge the problems caused by MPER and carbonation is the passivation of C-S-H surfaces. Such passivation can be, for instance, the functionalization of C-S-H surfaces with waterrepelling films made of silanes. Molecules like tetraethoxysilane (TEOS) can bind with hydrated cement, between others.25 However, it is for C-S-H surfaces not clear how they would interact with 3 ACS Paragon Plus Environment
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silanes, given that the nature of such bindings is unknown. Besides, C-S-H phases are normally carbonate terminated and they are even more difficult to passivate, since their reaction with silanes appears less probably.26 As observed by Okhrimenko et. al., on the adsorption of alcohols on the calcium carbonate, the Ca−CO3 pair tends to delocalize charge by ordering the −OH end of small organic molecules so that O associates with Ca and H associates with CO3, making a covalent bond quite improbable.27 In contrast, very different would be the effect observed in crystal like silicates, for example in SiO4 containing minerals, on which surfaces many molecules could be implemented for the functionalization.25 Supplementary calculations were performed in this work, relaxing the concerning molecules for the C-S-H-phases system on calcite. The results support the conclusion that carbonates are not able to bind strongly with functionalizing molecules aiming passivation and, consequently, should be first removed. Four promising approaches to activate C-S-H surfaces by removing the carbonates are now presented: UV-light radiation, direct heating, wet chemical treatment and Ar/O2 plasma treatment. These investigations are mainly done by means of in-situ infrared spectroscopy. For all approaches, the effectiveness of removal of carbonates as well as the impact on the C-S-H phases is examined. Interestingly, every approach has very specific results concerning those two mentioned parameters. The UV-light radiation was able to remove part of the calcium-silicate carbonates but not the calcium carbonates. The direct heating first removes the calcium-silicate carbonates in the range from room temperature (RT) to 300°C and the calcium carbonates from 400°C to 800°C. At even higher temperatures (T > 800°C) the released CaO starts to incorporate into the existing C-S-H phases. The wet chemical treatment in aqueous solutions requires a pH value below the acidic strength of carbonic acid (pH
