Dutch researcher Lourens Rijniers has discovered why William of Orange's grave, the monument on the Dam in Amsterdam and the Alhambra in Granada are all badly affected by salt damage. Salt can cause a lot of damage in materials with small pores, such as concrete and mortar. This is because the pressure which builds up during the formation of salt crystals causes cracks to develop in the surrounding material. Rijniers proved this with MRI scans of wet porous materials.
Rijniers used nuclear magnetic resonance, known in hospitals as MRI, to study salt crystallisation in model systems. The model system was a simplified porous material with pores of equal size. With this material Rijniers carried out the first experiments to demonstrate that the crystallisation of salts causes the build up of a pressure large enough to damage the material.
The applied physicist estimated which circumstances could cause damage. Salt damage varies from white spots on masonry and the carbonation of concrete to the erosion of stone and crack formation in statues. For materials with small pores, such as concrete, mortar and limestone, crystallisation was indeed found to result in damage. However, for materials with only large pores, such as brick, this damage mechanism was found to have no effect. It is not yet clear how the damage arises in these materials.
Rijniers wetted the model material with solutions of soda and sodium sulphate and studied the crystallisation process with the help of an MRI scanner. He calculated the pressure in the pores from the amount of salt that dissolved per volume of water. The Ph.D. student used theoretical models to explain how the pressure in the pores arose during the crystallisation process.
Salt crystallisation is an important cause of damage in building materials and stones. Although it is clear that salt from seawater and the environment is responsible for the damage, the mechanism behind this is still not understood. An improved understanding of this mechanism will make it easier to prevent possible damage.
This research was financed by Technology Foundation STW, the Priority Programme Material Research (PPM) and the Center for Building and Systems TU/e-TNO (KCBS).
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