Details of Grant 

EPSRC Reference:	EP/D061326/1
Title:	Direct measurement of salt-mineral repulsion: the key to controlling stone decay
Principal Investigator:	Hall, Professor C
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Department:	Materials and Processes
Organisation:	University of Edinburgh
Scheme:	Standard Research (Pre-FEC)
Starts:	01 November 2005	Ends:	30 June 2006	Value (£):	30,415
EPSRC Research Topic Classifications:	Materials Characterisation
EPSRC Industrial Sector Classifications:	Construction
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Summary on Grant Application Form
Stone buildings and archeological material throughout the world is subject to decay and degradation, and conservators face a continuing struggle to maintain the cultural heritage. A common cause of decay and damage is the crystallization of salts in porous stone materials. Although this is an old problem, scientists do not fully understand how the damage is caused. There is a paradox at the heart of the problem. If one imagines a salt crystal growing in a pore within a piece of stone, then one thinks that an outward stress develops when the crystal reaches the wall of the pore and pushes against it. But if the crystal is in intimate touching contact with the wall, then further growth should be impossible as it is no longer possible for further dissolved salt to reach the interface. If the salt and the wall of the pore make adhesive contact there should in fact be no stress. According at an important new theory by Scherer, the explanation is that in the acses where salt damage occurs, there is a short-range repulsive interaction between the growing salt crystal and the mineral forming the wall of the pore and a thin nanoscale liquid film exists between the two solid surfaces. This film allows further dissolved salt to diffuse to the crystal surface; as the crystal continues to grow it continues to push on the pore wall and eventually sufficient stress can be developed that the stone cracks. Stones are relatively weak and cannot withstand much tensile stress.We are proposing to use the techniques of atomic force microscopy and force probe measurement to measure this repulsive force directly and thereby to test the Scherer theory. These instruments allow us to take a tiny mineral crystal mounted on a minute flexible cantilever and bring this up to the surface a salt crystal immersed in a saturated solution. As the two surfaces approach, a short range interaction force begins to develop and this causes the cantilever to bend. We detect the bending of the cantilever with a laser beam. If independently we know the spring constant of the cantilever, we can calculate the forces acting between the surfaces. Mineral crystals of main interest are silica/quartz, the main constituent of sandstones, and calcite, the main constituent of limestones. An important implication of this idea is that if we can modify -- ideally if we can reverse -- the salt/mineral repulsion, then we could eliminate the stress which causes salt damage in stones. We may be able to do this by coating the walls of the pores with a carefully designed polymer which bonds to the surface and also makes an adhesive, rather than repulsive, contact with the salt. We propose to test this by using AFM to measure the change in the force in the presence of such a polymer.
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