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This recommendation is devoted to testing the resistance of natural stone and fired-clay brick units against salt crystallization. The procedure was developed by the RILEM TC 271-ASC to evaluate the durability of porous building materials against salt crystallization through a laboratory method that allows for accelerated testing without compromising the reliability of the results. The new procedure is designed to replicate salt damage caused by crystallization near the surface of materials as a result of capillary transport and evaporation. A new approach is proposed that considers the presence of two stages in the salt crystallization test. In the first, the accumulation stage, salts gradually accumulate on or near the surface of the material due to evaporation. In the second, the propagation stage, damage initiates and develops due to changes in moisture content and relative humidity that trigger salt dissolution and crystallization cycles. To achieve this, two types of salt were tested, namely sodium chloride and sodium sulphate, with each salt tested separately. A methodology for assessing the salt-induced damage is proposed, which includes visual and photographical observations and measurement of material loss. The procedure has been preliminarily validated in round robin tests., Materials and Structures, 56 (5), ISSN:1359-5997, ISSN:1871-6873

Water affects the susceptibility of stone to alteration by facilitating physical, chemical and biological weathering. Stone properties determine water transport and retention, but it is also expected that biofilms and extracellular polymeric substances could alter the water-stone relationship. A lot of research on this subject has been carried out on soils, but the effect on stones is understudied. For this reason, three sedimentary building stones, Ernzen, Euville and Savonnieres, each with a different pore size distribution, were biofouled with cyanobacteria. Their relationship with the stone material was investigated by optical and electron microscopy, and the effect of cyanobacterial biofilms on water transport and retention was studied. The results showed that the cyanobacteria primarily colonize the building stones on the outer surface and have a limited effect on the water transport properties. They slightly reduced the capillary coefficient and drying rate of the stones, but enhanced the water content in the stone and increased water vapour sorption. They induced (near) hydrophobic conditions, but had no measurable effect on the gas permeability and water vapour diffusion. Moreover, swelling and shrinkage of the biofilms were observed, which could potentially induce physical weathering. It is expected that these changes could influence other forms of weathering, such as freeze-thaw weathering and salt weathering.

Natural stones are essential for our built heritage. They are considered durable and permanent. However, they deteriorate through physical, chemical and biological action with water as the principal facilitator. Within a stone, water transport and retention depend on the stone's characteristics. It is expected that biofilms and extracellular polymeric substances change these properties and thus the water-stone relationship. To study the effect of the biofilms, three porous natural building stones: Ernzen, Euville and Savonnières, were biofouled with cyanobacteria Phormidium autumnale. The amount of biofilm was estimated by spectrophotometry, characterized by optical microscopy, and (environmental) scanning electron microscopy. Water transport and retention for biofouled and untreated samples were determined following European standards. Microscopy showed that the biofilms primarily covered the stone's outer surface. They experienced swelling and shrinkage dependent on the subjected relative humidity conditions, which could facilitate physical weathering. The biofilms slightly affected the water transport properties and induced a (near) hydrophobic surface. They reduced the drying rate and capillary coefficient. Moreover, biofouled samples absorbed more water vapor, had a higher water content, while no direct effect was measured on the water vapor diffusion and gas permeability. These changes could affect future weathering processes, including freeze-thaw and salt weathering.