2 1. INTRODUCTION

It has long been known that inorganic porous materials, such as rocks and masonry materials, are susceptible to deterioration when salts, in particular highly soluble salts or hydrate-forming salts, are found within them. J. I�iguez Herrero (1967) gives an excellent overview of the earliest papers, published around 1910, that clearly relate the presence of soluble salts to the deterioration of stone. Some years later another, more comprehensive review was published by I. Evans (1969–70). A. Goudie and H. Viles (1997) emphasize the influence that geomorphologists, notably H. Mortensen, had in developing the field of salt weathering.

In his ground-breaking paper, Mortensen (1933) contested the then-accepted view that the damage observed on rocks in the desert was caused by the crystallization of salts, a phenomenon compared to that of freezing water. He argued, instead, that crystals precipitating out of a saturated solution by evaporation occupy less volume than the original solution, and, therefore, no crystallization pressure can develop. He attributed deterioration to pressure developed upon volume increase when a salt hydrates. To illustrate the point, he advanced an equation to calculate hydration pressures for calcium sulfate, sodium sulfate, magnesium sulfate and sodium carbonate, typically salts found in desert areas. This theoretical approach was complemented by the work of C. Correns and W. Steinborn (1939), who studied both crystallization and hydration pressures in detail. However, as W. Duttlinger and D. Kn�fel (1993) pointed out, few and inconclusive attempts have been made to validate the hydration pressure theory experimentally. The last authors also give an excellent review of the literature on salt crystallization and theory.

Although there is a vast literature on this complex subject, or maybe because of it, the deterioration of materials is often simplistically attributed to “the presence of salts” or, at best, to the crystallization—or hydration—pressure of a particular salt. The present overview aims to present a coherent picture of the many mechanisms that are involved in the deterioration of porous materials by salts. For damage to occur, salts must move into and within porous bodies, a process that requires the presence of water (liquid) and/or moisture (water vapor). Thus, understanding how water moves is fundamental to elucidating the deterioration mechanism(s) that salts may have on porous materials.