Thermodynamic modelling of changes induced by salt pressure crystallisation in porous media of stone

Crystallisation pressure of soluble salts in porous media of a stone is one of the most important mechanisms which contributes to the deterioration of porous ornamental stones. This mechanism may strongly modify porous media and in addition to this may also indirectly serve to quantify the pressure crystallisation efficiency as a function of supersaturation and porous size distribution. We establish a thermodynamic model, based on Young-Laplace and Fitter's models of pressure crystallisation: P-C = (Omega/Delta V-0) + (2y/r), where Omega is the degree of supersaturation, Delta V-0 is the increase of the molar volume, y the interfacial tension and r the pore size. This model explains the variation of porous media identified in the experimental samples, taking into account both the supersaturation and size distribution in porous media that former models use separately. Data of porous media variation obtained from tested samples and fresh samples are in agreement with the thermodynamic model. For the experimental study of the modification of porous media we used a 4 m Na2SO4 solution, taking into account the large temperature changes which occur in the values of the supersaturation. The modifications of the porous media were investigated using optical microscopy, SEM and mercury porosimetry. (C) 1999 Elsevier Science B.V. All rights reserved.