Experimentally verified atomistic modelling of lime in construction materials

Since antiquity the construction industry has been using lime based binders to manufacture mortars, plasters and renders...Despite this history there is still a lack of fundamental understanding of the hardening processes and how these influence time dependent mechanical properties. In addition dolomitic limes, containing magnesium, exhibit enhanced properties when compared to their pure lime counterparts, however there is limited knowledge of the underlying reasons.

Lime based mortars are ideal candidates to replace cement mortars in many applications where lower strength is an advantage such as new build housing, forms of construction utilising organic fillers such as lime-hemp, and conservation and restoration applications. Indeed lime mortars offer many advantages over cement in terms of moisture permittivity, ability to accommodate movement, self-healing properties and ability to sequester carbon dioxide. Cementious binders are produced at much higher temperatures compared to lime and have large carbon dioxide emissions associated with their manufacture.

Atomistic modelling provides a unique opportunity to probe these mechanisms at a fundamental level thereby elucidating the processes responsible for developing the properties of industrial importance. Many existing and past studies of building lime binders have focused on bulk properties for instance through large scale bulk property testing, whilst not taking into account atom level processes. In recent years the cement industry has employed atomistic modelling of hydrated silicates as a means of understanding material behaviour.

Recent studies have demonstrated that the morphology and composition of a lime crystal can influence the carbonation process, and by association mechanical behaviour. In addition magnesium containing dolomitic limes show improved performance in many respects including strength development. Rate of carbonation is an extremely important issue as this can dictate the speed at which a building can be erected and therefore the associated costs. The ability to improve the carbonation rate and therefore hardening rate through control of composition and morphology will lead to enhanced products with better environmental credentials.

In the first instance this proposal seeks to develop atomistic models to describe the important aspects of lime binder behaviour and validate these against laboratory samples. Atomistic models will generate Raman spectra and X-ray diffraction patterns for direct comparison with experimental measurements. These initial models will then be developed further to investigate firstly carbonation and then time dependent and plastic mechanical properties.

Additionally the research will investigate the underlying reasons for the improved performance observed in magnesium containing dolomitic limes. The project is expected to bring long term benefits to the construction industry over the coming decades. In the shorter term industry will benefit through planned workshops and site visits which will showcase the application of atomistic modelling to lime manufacturers. The project will support the development of enhanced projects through the new knowledge gained.

The main thrust of this project is to use a combination of experiment and atomistic simulation to give an improved understanding of lime at a fundamental level and to link that understanding to the properties that make limes attractive to the construction industry. Academia, industry and society will benefit from this project primarily through a greater understanding of the way lime behaves in construction materials. In particular the knowledge gained will be essential for the greater exploitation of lime in the future through engineering the crystalline morphology and composition. Understanding of the important processes responsible for movement (creep and shrinkage) and hardening (carbonation) will impact the construction industry through better and more efficient use of lime.

Data from the project will impact the policy makers responsible for building regulations by allowing standards to be based on our scientific results for lime mortars. A greater understanding of the hardening and movement characteristic will impact thin wall construction by providing quantitative information to architects and engineers relevant to the prediction of long term behaviour from fundamental materials properties. The proposal will impact the conservation and restoration industries who exploit lime for its vapour permeable properties.

A significant impact of the project will be guidance for material specifications issued by the manufacturers. The greater depth of knowledge of lime carbonation behaviour will impact long term durability by promoting the optimisation of these materials. This will thereby support government targets to reduce the use of materials with high embodied energy such as Portland cement. The atomistic models developed during the project will provide a foundation upon which more complex models to address important issues such as the influence of ions leached from low embodied energy and waste materials can be based. Such materials include pulverised fuel ash, ground granulated blast furnace slag, silica fume and organic fillers such as hemp. Lime displays a propensity to react with airborne pollutants such as sulphur dioxide. The atomistic models developed will provide the foundation needed not only to model such interactions on the surface of buildings, for instance in a lime render, but also to inform upon surface reactions of industrial flue cleaning materials. Under polluted conditions dolomitic lime can display increased reactivity and this project will contribute towards elucidating the role of magnesium.

The post-doctoral researchers working on the project will gain valuable experience of the experimental and modelling techniques in addition to experience attending and presenting at conferences. The large amount of publishable data expected to be generated from the project will provide opportunity for the post-doctoral researchers to gain experience in writing technical reports and for journal publication. The project investigators who, between them, are professional members of the Institute of Materials, Minerals and Mining (IOM3) and the Royal Society of Chemistry (RSC) will provide relevant training and support on continuing professional development (CPD). Such activities will be appropriate for progression through these institutions, or similar. Provision to allow each PDRA, to attend post-doctoral training courses (as appropriate), will provide an enhanced research and development environment.