Crystallization modifiers applied in granite desalination: The role of the stone pore structure

Soluble salts of marine origin are one of the main agents in the weathering of granitic rocks used in buildings and monuments; they cause a loss of cohesion between the mineral grains, which results in granular disintegration. This deterioration is produced by crystallization–dissolution processes of salts, mainly sodium chloride, in the interior of the rock. Desalination procedures (mainly immersion and application of adsorbent poultices) are the more usual intervention in this circumstance, but complete extraction of the salts is generally difficult to achieve. In this paper we present the results of a study of the effectiveness of two NaCl crystallization modifiers, potassium ferrocyanide and sodium ferrocyanide, in desalinating two types of granitic rock with significantly different pore structure. Evaluations have been made of how these modifiers influence, first, the transport of NaCl solution inside the rocks; second, the susceptibility of the granite to salt crystallization cycles; and third, the effectiveness of desalination by immersion in distilled water. The results obtained are that the effectiveness of the two modifiers is completely different in each of the two granites studied. In one type of rock studied, both ferrocyanides increase the evaporation rate of the saline solution inside the structure of the rock by permitting NaCl to crystallize on surfaces as efflorescences; consequently, first, the intensity of sand disaggregation during the salt crystallization test is reduced, and second, when either of the modifiers is added to distilled water, desalination is more effective than extraction with distilled water only. For the second type of granite evaluated, neither of the modifiers was found to be effective. Therefore, we conclude that the network of fissures in the rock has a significant effect on the behaviour of the solutions with modifiers, since the pore structure of the stone determines the dissolution/evaporation kinetics of the solutions of salts. Thus, generalizations about the effectiveness of specific modifiers in granites are not possible: their effectiveness must be studied for each specific stone.