Controlling crystallization damage by the use of salt inhibitors on Malta’s limestone

Abstract

The main building stone in the Maltese Islands is the Globigerina Limestone, of which the Lower member is commonly used. This occurs in two types, the durable franka and the more easily weathered soll. Two types of fresh franka (bajda -white- and safra -yellow-), as well as fresh soll stone blocks, were obtained, based on the identification by quarry owners. Their designation was confirmed by geochemistry. Physical and mechanical properties of the three were investigated, including uniaxial compressive strength, water absorption by capillarity, permeability and porosimetry. Porosimetry results confirmed outcomes of previous research work. Soll was found to have a lower overall porosity, but a high percentage of small pores with practically no large pores. Some of the tested stones were then treated with a non-toxic phospho-organic compound containing carboxylic moieties as a salt inhibitor and the corresponding non-phosphorylated compound, as aqueous solutions at different concentrations. Both treated and untreated stones were then subject to salt crystallization tests, using sodium sulphate in different concentrations. For the untreated stones, even after only one salt cycle, faster and more pronounced deterioration was observed for the soll samples as opposed to the franka ones. In the case of the treated stones, less deterioration with almost no damage was observed as opposed to non-treated ones. The presence of even very low concentrations of the inhibitor thus helps crystallization to occur on the stone surface and not within the pores. These encouraging results led to the conclusion that salt inhibitors can be used to treat salt-infested stone. Further research in this respect, also using NaCl/NaHCO3, is continuing.