Electroosmosis in Masonry

Electroosmosis (EO) is the transport of a solution in porous materials such as soil, brick, and concrete in response to an applied electric DC field. This phenomenon is well-known for the consolidation of soil in geological engineering. However, there are still debates about whether this technique can be effective in drying masonry walls suffering from rising dampness. In this thesis, we have tried to see if EO is obtainable in bricks and provide a quantitative basis to measure the electroosmotic permeability coefficient.

Several theories have been proposed to explain EO. Each of these theories have limitations. A phenomenological approach based on non-equilibrium thermo-dynamics is used in this thesis to calculate the electroosmotic permeability coefficient, covering the sum effects by the characteristics of the samples.

Electroosmotic transport of water was obtained in all the investigated bricks. A unique cell equipped with four electrodes (two working electrodes and two reference electrodes) was used to measure the EO coefficients in the bricks. The cell was originally designed to measure EO coefficients in clay, so a new sample holder was developed. The brick specimens were water saturated (under vacuum) before the test. The cell design was designed based on the phenomenological approach and all other gradients (concentration, pH, pressure) than the applied electrical potential gradient were eliminated. This ensured, that the flow of water seen (in capillary tubes attached to the cetup) when applying the potential gradient, could only be due to electroosmosis. Electroosmotic coefficients (in the range of 10^-10) were measured in six Danish bricks (new from the factory or sampled at demolition sites), and the bricks were red or yellow as traditional Danish bricks. The phenomenological approach was a suitable tool to measure the electroosmotic permeability coefficient.

The electroosmotic flow velocity could not be assigned to a single brick characteristic. Two new bricks (Red and Yellow) were tested in the EO cell first. Here two constant currents, 1 mA and 2 mA, were applied. The Red brick with more narrow pores and lower porosity had a higher electroosmotic permeability coefficient than the Yellow brick. The value of the zeta potential in the Red brick was higher compared with the Yellow brick. Next four other bricks were tested, and they had different dates of manufacture and were extracted from old houses in Denmark. Here the potential gradient was kept constant (and tested at different levels). It was found, that a single brick characteristic linked to the zeta potential or porosity could not be used to describe the size of the EO velocity.

The EO velocity in the bricks increases at higher applied current or potential gradient. The EO flow under the application of 2 mA was higher than using 1 mA showing that the magnitude of EO was dependent on the magnitude of electric current in the two new investigated bricks. For the next four bricks four different constant electric potentials (60V-50V-40V-30V) were applied to the working electrodes, and the electroosmotic permeability coefficient was measured in every 2 cm of the capillary tube. It was evident from both series that a higher current or potential difference applied leads to higher EO flow velocity.

The electroosmotic flow cannot overcome capillary suction if a moist poultice is placed next to the brick specimen. A setup with poultices (kaolinite and calcium carbonate mixed in water, similarly to previous research on electrodesalination) was used. The poultices buffer the pH changes at the electrodes. A poultice mix design was experimentally developed so no EO was generated here (+80% Calcium carbonate per weight). One New and one Old brick were used in these tests. Some brick cubes were fully saturated (e.g., 12% %wt) with water, and some were partially saturated (e.g., 9% %wt). Two poultice containers were attached to the brick cubes. An electrical potential gradient was applied for 48h to measure the water content in different segments of poultice and brick at the end of the experiment. The results showed that the water content in the fully saturated bricks remained stable all through the brick specimen and the water content in the partly-saturated bricks increased after the experiment, i.e., the EO flow could not overcome the capillary suction forces in the bricks.

Electroosmosis is linked to the content and transport of ions in the brick. The effective diffusion coefficient of chloride was calculated using an introduced model that considered EO flow in four brick types. The electromigration coefficient was also calculated. There was a correlation between the effective diffusion coefficient, electromigration, and EO. The Brick with the highest electroosmotic permeability coefficient had the highest effective diffusion coefficient for chloride and the highest value of the electromigration coefficient. Furthermore, in kaolinite mixed in 0.1 M NaCl had a higher electroosmotic permeability coefficient compared with kaolinite mixed in 0.05 M NaCl showing that the content of free ions in the pore solution influences the electroosmotic flow.

In summary, an electroosmotic flow of water was obtained in all six investigated bricks under application of an electric DC field. However, a content of free water in the vicinity of the brick causes a counterflow of water due to capillary forces, which in the experiments conducted here were stronger than EO.