Deterioration of Monument Building Materials: Mechanistic Models as Guides for Conservation Strategies

Chemical dissolution and salt crystallization are very important factors contributing to the deterioration and damage of the built cultural heritage consisting of marble and limestone. Understanding of the underlying mechanisms of the calcitic materials damage is necessary for the design of efficient strategy for the prevention of deterioration can be done only through the appropriate kinetics measurements. Kinetics of dissolution of calcitic materials and of the growth of salts of interest should be measured, precisely and reproducibly. In the present work, the rates of dissolution of calcitic marbles (>98% calcite) and sandstone were measured at constant undersaturation and were correlated with the respective solution undersaturation. The dissolution kinetics measurements showed that calcitic limestone at pH 8.25 had a lower dissolution rate constant in comparison with the respective value for Pentelic marble, a calcitic material (ca. 98% calcite). The mechanism was surface diffusion controlled at alkaline pH values. In more acid pH values mass transport became more significant. Reduction of the rates of dissolution was achieved by the addition of substances with functional groups, which may interact with the surface of the calcite crystals. Several inorganic ions (including orthophosphates, pyrophosphates, phosphonates, fluoride and sulfate) and one organic environmentally friendly compound, polycarboxymethyl inulin (CMI) (MW 15000) were tested and their effect on the rates of dissolution was discussed. Phosphonates, were found to have a beneficial effect on the control not only of marble dissolution, but also on the crystallization of damaging soluble salts like NaCl and Na2SO4·10H2O (mirabilite).