• Ottosen, Lisbeth M.
• Rörig-Dalgaard, Inge

Abstract

Salt induced decay of bricks is caused when salts exert internal pressures, which exceed the strength of the stone. The presence of aqueous electrolyte solutions in the capillary pores of brick materials can under changing climate conditions cause deterioration of wall structures. Ions move in brick depending on its water content and salts may be precipitated on the outer wall or concentrated under paint layers covering the surface of the brick. Different types of damage may appear in masonry walls due to these concentrating phenomena. Bricks themselves can be destroyed and the mortar can, too. If the masonry is covered with paint, the precipitating salts can affect the adhesion of the paint negatively. Furthermore the presence of the salts will increase the hygroscopic moisture content of the masonry meaning that the masonry will have a relatively high water content compared to a wall without increased salt content. The types and concentrations of salts found in relation to building stone vary greatly and depend on the stone type and the environment around the building. In general most common salts are sulphates, chlorides and nitrates. These include CaSO4, Na2SO4, MgSO4,KCl and KNO3.At present no method exists that can effectively remove salts from masonry. Some methods based on diffusion of ions into an external layer attached to the wall but it is a very slow process. In the present study it is investigated if electromigration can be used as transport process to remove the salt ions from brick masonry and also how much the removal rate can be increased by application of the electric field compared to diffusion alone. Some main differences occur between electrokinetic remediation of heavy metal polluted soil and electrokinetic removal of salts from brick masonry. The ions of interest in the brick are not adsorbed to a high extent, as it is often the case with heavy metals in soils. Bricks are made from baked clay, however during the baking process the cation exchange capacity of the clay is strongly decreased which affect the electric conductivity. The electric conductivity of bricks without increased salt content is very low compared to soils in general. Furthermore in a masonry wall there are boundaries with different chemistry (e.g. pH) that the ions must pass, brick-mortar boundaries.From initial experiments with electrokinetic removal of Ca2+ ions from bricks good results have been found. The bricks were spiked with Ca(NO3)2 to the brick before the current was applied and it was found that the Ca content of the brick after electrokinetic treatment was even lower than it was originally before the spiking. A series of different duration was conducted and it was evident that the Ca content decreased as the duration increased. More experimental series are now in progress where the mobility of other both cations and anions are compared. Furthermore the relation between removal rate and current density are studied.

Topics

• density
• impedance spectroscopy
• pore
• surface
• mobility
• experiment
• strength
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• current density
• concentrating