Preventing salt damage in porous materials

Abstract

Salt damage in porous materials may be prevented by appropriate control of the ambient relative humidity. Crystallization and hydration occur only at particular relative humidities, and damage can be prevented by keeping the ambient relative humidity above or below these critical values. Whilst the critical relative humidities for single salts are well known, the behaviour of salt mixtures is more complex. This paper examines some of the principles concerning crystallization from solutions containing more than one salt. It uses a computer program to calculate the relative humidity of air in equilibrium with any given mixed salt solution. It shows that crystallization does not occur at the equilibrium relative humidity of each component salt, but across a range of relative humidities. The implications for passive conservation are discussed, using a number of examples which demonstrate some unexpected features.