A very important class of technological porous materials is formed by building materials. Although these materials are widely used in practice they can be seriously damaged by moisture and especially salt. These enter by, e.g., driving (contaminated) rain, condensation, run off from roof and facade, and/or capillary rise of ground water, whereas concrete structures are often placed in salt water. Salts do also enter by the use of thaw salts in deicing of roads. If wet, the material can become  susceptible to freezing damage. It may also act as a substrate for the growth of bacteria, fungi, or algae with possible physical and chemical damages, but also possible health risks. Also chloride ions can penetrate, causing corrosion of steel reinforcement in concrete structures, which may result in structural damages. A large fraction of the absorbed water will leave a material again because of drying. While a material is drying salt crystallisation may occur at the surface, causing defacing (efflorescence), or just under the surface, where it may cause structural damages, e.g., delamination, surface chipping, or desintegration. Here a few examples are given of damages are given:.

MOISTURE                                                       SALT                                                      BIOLOGICAL