An Electrochemical Approach to Study Carbonation of Novel Lime Based Materials

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.

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.