Elektro-osmosis and capillary suction

Dampness in masonries is a major problem in many buildings and monuments worldwide. Techniques based on electroosmosis have been proposed as conservation methods to dehumidify masonry subjected to capillary suction. Although electroosmotic techniques have been applied for decades, the effect is still debated. This paper reports an experimental investigation on electroosmosis in single bricks in a laboratory setup originally designed for electro-desalination. Two types of bricks were used. The samples had different lengths and different initial water content. A poultice composition was designed with neglectable electroosmotic effect and effective buffering of the acid produced from the electrode process at the anode. Four poultices with different concentrations of CaCO3 and kaolinite were examined, and the poultice consisting of calcium carbonate:kaolinite:water with the ratio of 40:10:50 (wt%) fulfilled the requirement. It was seen that more the kaolinite in the mixture, more the electroosmosis. A constant voltage of 75V was applied to the electrodes in each end of the setup. The zeta potential of the bricks was comparable to the zeta potential in kaolinite, and thus electroosmosis in the bricks was expected. However, the water content reached saturation all through the brick specimens in every experiment, and there was no difference in water content in the two poultices. Thus, the capillary forces causing water suction into the bricks from the poultices were the strongest and overshadowed the possible electroosmotic effect. Therefore, this work shows that capillary forces need to be taken into account when designing setups for evaluation of electroosmosis in materials with strong capillarity. The same is valid for in real applications, in case a continuous source of water is present.