36. Kinetics of Dissolution of Monument Building Materials

Built heritage consists to a large part of calcitic minerals (marble, limestone) which are prone to dissolution over a wide pH range. Indeed, chemical is solution is among the most significant factors contributing to the deterioration and damage of the built cultural heritage. Carbon dioxide dissolved in atmospheric water contributes to the development of acid conditions on the surface of the calcitic materials which accelerate dissolution of calcite crystals. These effects are enhanced in the case of polluted atmosphere which contributes to more aggressive environment for the dissolution process. The measurement of parameters that determine the kinetics of dissolution is very important for the understanding of dissolution mechanism and hence the design of efficient strategy for the prevention of it. As a result, precise and reproducible methods are necessary for drawing reliable mechanistic information. In the present work, the dissolution kinetics were investigated at conditions of constant driving force for the dissolution process. The methodology used to measure the rates of dissolution of calcitic marbles (>98% calcite) and sandstone was both accurate and highly reproducible. The rates of crystal dissolution measured were correlated with the corresponding 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 the same, i.e., surface diffusion controlled at alkaline pH values. The prevention of dissolution strategy therefore shall depend either on the alteration of fluid dynamics at the acid pH values or on the modification of the surfaces at the alkaline domain. The latter can be achieved by the addition of substances with functional groups, which may interact with the surface of the calcite crystals. Several inorganic ions (fluoride and sulfate) and one organic environmentally friendly compound, polycarboxymethyl inulin (CMI) (MW 15000), have been tested, and their effect on the rates of dissolution was discussed.