Electro-desalination of sulfate contaminated carbonaceous sandstone – risk for salt induced decay during the process

Sodium-sulphate is known to cause severe stone damage. This paper is focused on removal of this salt from carbonaceous sandstone by electro-desalination (ED). The research questions are related to possible stone damage during ED and subsequently suction cycles are made in distilled water before, during and after ED. During suction in water the salts are concentrated in the upper part of the sandstone. After 2 days of treatment the average water soluble SO₄^2- concentration was half the initial and for this sample corners were damaged as was the case for the reference stone. After 4 days of ED the average SO₄^2- concentration was 15% of the initial, and here no stone damage was seen from the suction cycles. This result shows that the damaging salts are removed and that no new harmful salts are formed during ED in the actual case. Acid is produced at the anode during ED. The acid is buffered in the poultice with carbonate. The acid would be highly damaging to the carbonaceous sandstone as the binder-CaCO₃ is soluble in acid. From pH measurements of the poultice it seems as if the acid is buffered well, as pH is still slightly alkaline after ED, but this is a measurement of the average pH and thus it was decided to measure the compressive strength of the stones after ED. The lowest compressive strength was measured for the reference stone, which had not been treated by ED (but had the highest salt content). Thus from this investigation there is an indication, that dissolution of carbonates in the stone did not happen, though the data material is too scarce to make a final conclusion. In summary, this investigation did support that ED removes the salts without new damaging side effects in the stone.