Electrokinetic removal of Ca(NO3)2 from bricks to avoid salt-induced decay

Salt-induced decay of masonry is a serious threat to our cultural heritage. In buildings near agricultural land or stables the masonry may suffer seriously from salt-induced decay from nitrates. It was investigated in laboratory scale with a single brick if Ca(NO₃)₂ could be removed by electromigration in an applied electric field. To relate the obtained nitrate concentrations to the danger for salt-induced decay the concentrations were compared with values from the Austrian ÖNORM B 3355-1. It was shown possible to even remove nitrate from bricks with initial concentrations in the range of no danger for salt-induced decay and this shows that the method can reach sufficiently low concentrations. It was also shown that the nitrate concentration could be reduced from a range where salt-induced decay can be expected to the range where there is no risk, though the experiments should proceed longer to obtain this low concentration all through the brick.Calcium removal was followed as well. On the contrary to nitrate, calcium was seen adsorbed in exchangeable sites to the internal brick surfaces and precipitation may also occur due to slight alkaline pore solution. Calcium was also removed during the experiments but the experimental setup did not successfully prevent acid from the brick surface in front of the anode. It was found that the brick itself had a buffering capacity against acidification, and involved in the buffering reaction may have been a release of calcium. This contact with acid to the brick should be avoided.A tendency for electroosmotic transport of liquid within the brick was seen. The volume of liquid in the anode unit was constantly decreased compared to in the cathode unit.