An Electrochemical Approach to Study Carbonation of Novel Lime Based Materials

Lead Research Organisation: University of Bath
Department Name: Architecture and Civil Engineering


This project seeks to advance the current understanding of carbonation in lime and cement materials. A novel approach is proposed where the reaction with carbon dioxide will be studied using micro pH electrodes. pH variations in a thin water film at the sample surface will allow the reaction mechanism to be determined. A detailed understanding of the surface morphology and composition will be provided by a comprehensive electron optical and surface analytical study. The reaction of these materials with carbon dioxide is or great interest as sequestration of carbon dioxide is a key initiative aimed at reducing climate change. Lime has the ability to adsorb significantly higher quantities of carbon dioxide during the setting process in comparison to alternative products such as cement and has important applications in the restoration and conservation of historic buildings in addition to renovation and new build projects. Cement acquires its strength from hydration of reactive silicate and aluminate clinker phases however in the long term carbonation of these phases can lead to a reduction in mechanical performance and the corrosion of steel reinforcements if present.Although the chemical process of carbonation is well known the mechanisms in lime and cement mortars are poorly understood. This research programme seeks to address this issue.In recent years the producers of low carbon footprint materials such as hemp and wastes from a range of industrial processes have expressed interest in the incorporation of their materials as fillers and additives in lime and cement products including mortars, renders, plasters and concrete. The addition of these environmentally friendly materials not only influence the micro and macro pore structure but soluble constituents may introduce ionic species into the pore water, the influence of which on carbonation is unknown. This proposal aims to study the carbonation process by monitoring ion concentrations at different locations within the liquid film on the surface of a calcium hydroxide substrate. Proton concentrations will be measured using specially manufactured microelectrodes consisting of nanostructured palladium hydride discs approximately 10 micrometers in diameter and electrodeposited on the end of a normal microdisc electrode. Held precisely with a micropositioner the pH microelectrode will be brought to within a few micrometres from the surface under investigation. In a similar way, commercially available ion selective electrodes will be used to determine the calcium ion concentration. In the second phase of the project the effect of ions leached from additives commonly used in conjunction with lime will be investigated. These can be divided into the following five groups, blast furnace slag (GGBS), bottle glass, wood ash, hemp and metakaolin.Additives of interest will include glass, ashes / slag and organic surfactants.

Planned Impact

The proposed project aims to impact academia, industry and society as a whole through research and development of advanced lime based materials for the construction industry. These will include nano lime for conservation and restoration applications, and composites incorporating industrial waste materials, such as ground granulated blast furnace slag, and low embodied energy fillers, such as natural plant fibres for new build. Recent research at the University of Bristol highlights the importance of the micro structural changes associated with the carbonation of calcium hydroxide and hydraulic phases on in-service performance. Carbonation is a physicochemical process which occurs in thin water films on the outside and within the pore structure of a material. This project will further the current understanding of carbonation through development of a method of measuring pH in thin films between liquid and gaseous phases representing in-service mortar conditions in a laboratory environment. The research proposed will offer an opportunity to push the development of pH microelectrodes beyond the conventional pH range. Most pH microelectrodes are designed for biological applications and are therefore optimised for a very narrow pH range. Here we propose to develop and test electrodes to operate in a very alkaline environment where even the traditional glass electrode suffers from the alkaline error . Moreover we propose to conduct long experiments, from hours to days. This will also push the development of the microelectrodes to achieve stability sufficient for the timescale considered. This is undoubtedly a challenging part of the work but it is clear that any success in this area will have a significant impact on the use of microelectrodes to monitor pH reliably in industrial systems. The impact will not be restricted to academia since pH is such an important chemical parameter. Advances made in the development of nanostructured pH microdisc electrodes of high reliability will very quickly benefit industrial and environmental processes. The use of nanostructured pH microelectrodes will be beneficial to the UK's leading role in scanning electrochemical microscopy (SECM). The development of reliable pH probes for the SECM is of interest to Uniscan Instruments Ltd who manufacture and commercialise SECM since it adds new functionalities to the instrument. Our proposed work will demonstrate that the SECM can be employed to probe pH variations at surfaces, even in very alkaline conditions. There is no doubt that this will help in furthering applications of the SECM and thus contribute to the leading position of Uniscan Instruments in scanning electrochemical microscopy. A greater understanding of the fundamental carbonation mechanism of lime mortars will help promote the development of novel sustainable mortars. The UK construction industry will see immediate benefits from this work through a greater understanding of the carbonation mechanism in cement containing blast furnace slag, an additive used in the new Ty-Mawr products. Advances made during the project will allow materials to be specified for optimum performance providing long term durability with specific carbonation resistance. Greater confidence will be given to practitioners on the effects of adding low embodied energy and waste materials to their lime mixes for long term strength development following carbonation. The application of surface analytical techniques and electron microscopy will demonstrate influence of the experimental variables pH, film water thickness, ion concentration and composition on the structure and morphology of the calcium carbonate coating.
Description To summarize the project findings it is possible to highlight the fact that carbonation is a complex reaction, very sensitive to the conditions in which it takes place (i.e. temperature, pressure, evaporation rate of water, water condensation).

Contrary to the hydration of silicates that is considered a bulk reaction, carbonation is characterized by two mechanisms: an interfacial mechanism, at the beginning, and a diffusion controlled mechanism, in the later stages.

Carbonation can be seen as the result of a competition between two reactions occurring at the same time: the dissolution of calcium hydroxide in water and the dissolution of CO2 in water. The system where this competition takes place is formed from three phases: Ca(OH)2 (solid phase), H2O (liquid phase) and CO2 (gas phase). In this system, the water plays an important role: it acts as a medium in which the other two phases can dissolve and where the reactions can occur but it must be in the right amount. Too much water, in fact, reduces the carbonation rate because the diffusion of CO2 in water is 10,000 times slower than in air. In the absence of water, instead, the reaction cannot take place.

At the beginning, the Ca(OH)2 dissolution (an interfacial reaction) is the driving force of carbonation but, with the precipitation of the reaction products (the newly formed phases called hydrated calcium carbonate) on the dissolving surfaces, the diffusion mechanism becomes the driving force of the whole reaction. The new solid phases formed at the beginning of the carbonation reaction, in fact, produce a layer between the Ca(OH)2 and the water that reduces the dissolution rate of Ca(OH)2 (by reducing its interaction surface with water). This new layer is a gel phase, characterized by a lower solubility compared with the Ca(OH)2.

Once this new layer is fully formed the only reaction mechanism is the diffusion of H2O molecules, Ca2+ and CO32- ions within it. This layer tends to grow with a consequential reduction of the carbonation rate over time. In addition the hydrated calcium carbonate phases that form the gel layer tend to turn into an anhydrous calcium carbonate.

Within this mechanism the pH of the water plays an important role. In particular, in air lime where the only reaction is carbonation, high pHs allow dissolution of a higher amount of CO2 with a consequential increase in the amount of product reacted. In lime containing hydraulic additives (such as fly ashes) where carbonation is side by side with hydration of silicates, instead, high pHs increase the solubility of silicate phases thereby increasing the amount of hydraulic compounds formed (other than increases the dissolution of CO2).

One of the main objectives of this project was to exploit electro-analytical techniques to investigate the carbonation of lime in situ. The approach we proposed involved exploiting the properties of nanostructured Pd hydride (PdH) microelectrodes to measure the pH during the carbonation of Ca(OH)2. Another objective was to combine the nanostructured PdH electrodes with scanning electrochemical microscopy to monitor pH in situ. However the pH microelectrodes needed to be characterised further to assess their behaviour in very alkaline solutions and to improve their lifetime so as to have reliable measurements over the duration of the carbonation process.

Through a series of carefully designed experiments we demonstrated how the presence of dissolved oxygen affects the lifetime of the nanostructured PdH microelectrodes and showed that the electrode potential, and therefore the pH derived from the electrode potential, were affected by the presence of oxygen. We were able to quantify this effect and model the role of oxygen on the electrode behavior in terms of a mixed potential as found in corroding systems. These findings were successfully published in Analytical Chemistry. (M. Serrapede, G. Denuault, M. Sosna, G.L. Pesce, R.J. Ball, Scanning Electrochemical Microscopy: Using the Potentiometric Mode of SECM To Study the Mixed Potential Arising from Two Independent Redox Processes, Anal. Chem., 85 (2013) 8341-8346).

The electrochemical approach to the carbonation reaction used in this research showed that the potentiometric response of the microelectrodes used during the tests is Nernstian (the electrode potential is a linear function of pH) over a wide range of pH including alkaline conditions up to pH 14. This is the first time reliable operation of the electrodes in these highly alkaline conditions has been demonstrated. Results have been proved to be reproducible and stable over several hours. To our knowledge this is also the first report of pH microelectrodes working reliably in such alkaline conditions, most pH electrodes have a narrow pH range and never extend to pH 14.

We successfully exploited the properties of the nanostructured PdH microelectrodes to monitor pH in situ during the carbonation of saturated lime solutions confined in a porous medium. The experimental pHs recorded in situ with the nanostructured Pdh microelectrodes during the carbonation reaction were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. We modeled the solution pH with PHREEQC, a geochemical simulation software which accounts for bulk solution and interfacial processes. With PHREEQC, we were able to predict the pH for a range of CO2 partial pressures and for the different solid phases corresponding to the polymorphs of CaCO3. We analysed the solid phases produced during the carbonation with X-ray diffraction and with scanning electron microscopy and found that the microelectrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of anhydrous CaCO3. The methodology we developed to monitor the carbonation of lime confined in the porous medium is novel and we have recently submitted these results for publication in Analytical Chemistry.

Results obtained with Electrochemical Impedance Spectroscopy (EIS) on the reaction between Ca(OH)2 and hydraulic additives, instead, showed the potential of this technique as a non-destructive tool for real time in situ monitoring and study of the reactions.

Changes in the impedance response with time have been shown to be associated with reaction kinetics. Changes in bulk measurements at the very beginning of the tests (about 40 hours), instead, have been associated with the end of Ca(OH)2 dissolution. These results have been submitted for publication in Clay Minerals.

The research described above has proved invaluable in the study of lime carbonation in a variety of applications. An additional 12 publications in high impact academic journals have been published. These demonstrate the application of intellectual knowledge gained by the authors from the fundamental electrochemical studies conducted to inform upon 'practical' issues encountered by the construction industry. In particular the following have been addressed; Consolidation of weathered limestone using nanolime; Microstructural Changes of Lime Putty during Aging; Characterization of binders used in historic mortars and plasters (case study of samples sourced from 1A Royal Crescent, Bath); Impedance spectroscopy measurements of physio-chemical processes in lime-based composites; Factors affecting the water retaining characteristics of lime and cement mortars in the freshly-mixed state; Influence of carbonation on the load dependent deformation of hydraulic lime mortars; The application of electrical resistance measurements to water transport in lime-masonry systems; Environmental (wet and dry) cycling of hydraulic lime mortars; Radiocarbon Dating of Mortars and the identification, extraction, and preparation of reliable lime samples for 14C dating of plasters and mortars with the "Pure Lime Lumps" technique. These results have been presented at 7 international conferences and produced 1 book chapter.
Exploitation Route This research has proved that the techniques used during the tests (Electrochemical Impedance Spectroscopy and nanostructured PdH pH microelectrodes) can be exploited in the development of sensors for successfully monitoring a range of different processes such as those studied in this research.

Our research, in fact, demonstrated that Impedance Spectroscopy (a non-destructive technique) could be used in monitoring the hardening process of lime based materials in the construction industry. The ability of impedance spectroscopy to monitor setting reactions in lime based materials would allow the construction industry to estimate strength development based on quantitative data. This is particularly important in the early days and weeks after mixing when strengths are low and environmental vulnerability to processes such as frost damage is high. Its applications are of great interest to the construction industry because, unlike other techniques, the sensors used are inexpensive and simple to use.

Numerous chemical reactions involve pH changes but despite its importance pH still remains a difficult parameter to determine in many cases. On the one hand extreme pHs cannot be measured with most pH sensitive devices, including the conventional glass electrode. On the other hand pH is hard to measure in confined places with interfacial processes typically found in geochemistry, electrochemistry or biochemistry proving to be particularly challenging. Of all the pH sensors currently available the glass electrode is by far the most convenient for measurements in bulk solutions but it is unsuited for operations in localised environments. It is also unsuited to very basic media as the alkaline error (a phenomenon also known to worsen with temperature) affects the response for pH>9; even with alkali glass membranes its range does not extend beyond pH 12. The requirement for highly localised pH measurements in biological research and the need to overcome difficulties in fabricating miniaturised glass pH electrodes led to the construction of different types of pH micro-sensors such as the one used in this research. To our knowledge our research is the first that shows a practical example of reliability and precision at pHs>12. The nanostructured PdH pH microelectrodes open many possibilities for the development of sensors for the construction industry. Further applications in the medical field may also arise because unlike many other pH sensors, their potential - pH relationship is unique and very reliable in near neutral solutions typical of pHs encountered in biological conditions. Moreover the respond is characteristic of the Pd hydride and does not require calibration.
Over the past decades, chemistry of carbonation and its mechanism have been studied separately by chemists and engineers and the knowledge acquired in these two fields has been very often confined in their respective spaces.

This research has combined key knowledge from chemistry and engineering disciplines so that today it is possible to provide useful suggestions to the modern construction industry to improve lime based materials already available in the market and to develop new lime based materials.

A practical example of this is the application of knowledge acquired in this project to improve the consolidation effect of nanolime: a new lime based material mainly used in the conservation field. This material was introduced several years ago but its application immediately showed some limitations.

Nanolime has the potential to provide chemically compatible treatments with carbonatic materials (such as carbonatic rocks and lime plasters or renders) and to produce improved consolidation compared to that achieved with traditional methods such as limewater and milk of lime. However, the exact response of weathered stone to nanolime treatment is currently uncertain and widely debated. This is probably due to the fact that nanolime has been mainly developed for use on plaster, to conserve historic wall paintings, so much of the existing research has focused on this. Furthermore, some researchers also highlighted that different environmental conditions can affect the carbonation mechanism of nanolime producing less stable phases such as vaterite or a recrystallisation of portlandite . Owing to their instability, these phases change over time and this can affect the result of the consolidation treatment.

Within this framework the knowledge acquired in our research is currently helping to improve the development and the application of this material by studying the material itself and the in-situ carbonation processes.
Sectors Construction,Environment,Pharmaceuticals and Medical Biotechnology,Other

Description Experimentally verified atomistic modelling of lime in construction materials
Amount £646,152 (GBP)
Funding ID EP/K025597/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2013 
End 06/2016
Description UK India Education and Research Initiative (UKIERI) 
Organisation Maulana Azad National Institute of Technology
Country India 
Sector Public 
PI Contribution No Description Available
Start Year 2010
Description Use of lime in the modern construction industry 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Use of lime in the modern construction industry

greater awareness
Year(s) Of Engagement Activity 2010,2012